Trade In Fantasy Ch. 4: Modes Of Transport, Pt 5

4.8.7.1 An introduction to Weather (oversimplified)

Take a look at the 8-panel illustration above.

★ In panel one I have quickly generated two landmasses, some islands, a volcano, and a lot of ocean. There’s also a ship in the middle of the panel, about 1/3 up, for no particular reason.

★ In panel two, I have added an equator (solid line), a couple of tropics, and an arctic region.

★ Panel 3 shows what most people expect to happen weather-wise – hot air rises in the tropics and moves north and south of the equator. Sorry to anyone who thought that – you have half of the story backwards. Hot air rises, true enough, but that lowers the air pressure, so cool air rushes in to the tropics from the cooler regions. Assuming that the planetary rotation is in the right direction, the arrows at the top of the panel should be at the bottom and vice-versa.

Is the planetary motion in the right direction? Well, the winds are moving west to east, so the planet is moving east to west, and if you picture that by rotating anything round, you’ll find that it means that the sun rises in the West and sets in the East.

Uh-oh. Either this planet rotates in the opposite direction to the Earth, or the top of the map is South and not North for some reason. The second choice is the simpler explanation, so some reason for the inversion has to be invented; I can think of several, but that’s neither here nor there.

★ In the daytime, both the land and the water heat up, as shown by panel 4. Convection currents carry some of the heated water down, replacing it with cooler water. The only places that don’t warm much at all are the snow-capped mountain peaks, which reflect more of the heat away.

That means that all day on a warm day, the air pressure over land is going down; when the sun dips low enough, the expansion of the air stops being enough to hold back the cooler air that wants to fill that partial vacuum, and you get a (relatively) cool sea breeze, something that I have enjoyed on many an occasion.

★ Panel 5 offers a heat-map of what then happens at night. The heat of the land and shallow waters is quickly radiated away (unless there’s significant cloud cover to reflect it back down). So the land cools off relatively quickly. But the oceans have been sequestering heat all day, becoming an enormous heat bank, but now the tables have turned and that heat is being given off, and taking evaporated water vapor with it.

When that water vapor hits land, it will condense into clouds. For that reason, most coastal cities experience early morning cloud cover most days; the sky doesn’t become clear until this burns off. How long this takes is a complicated question I don’t pretend to have an answer to; Here in Sydney, cloud cover burns off somewhere between 8AM and 10AM, but I once visited Los Angeles after they had their smog problems in hand, and the cloud cover still didn’t burn off until about 3PM local time!

My only hint is that I think local wind patterns have something to do with it, because I have read of this aspect of the weather being affected by Heat Islands. What’s a heat island, you may ask? Concrete and bitumen and asphalt absorb more heat than grass-covered natural areas, so every city absorbs more heat than the lands around it, which creates an island of high heat in a sea of moderate temperatures. This heat then impacts the local weather.

This is an extremely complicated field of study; another building blocking the light can have significant ramifications that aren’t recognized until it gets knocked down. And that’s without factoring in the concrete-canyon wind-tunnel effects!

But we still have the planetary rotation to take into account. The heat from a land mass will bloom outward to the West (that’s to the right on our map), and that will create low pressure systems off the western coast. As a general rule, though, there’s a lot of room out there for them to dissipate – unless they strike another landmass in the meantime.

Sometime overnight, then, the sea breeze will dissipate and be replaced with a breeze blowing from on-shore.

★ Panel 6 shows how complicated weather can get as a result of these simple contributors. Air flows(with moisture) from the middle of the ocean each night, carried along by sea breezes that head for the nearest land – but which are then twisted around by the planetary rotation, and pulled this way and that by other factors.

Who here remembers Biorhythms? This pseudo-science idea was popular in the late 70s because a computer was really good at calculating them and putting the results up on the screen. Ultimately, this was all about the simultaneous charting of three sin waves with different periods (23, 28, and 33, respectively), and summing them to produce a compound curve which can exhibit considerable complexity, like this:



Created using Desmos Tools and licensed under the Creative Commons “Attribution-ShareAlike” License (CC-BY-SA-4.0)

Well, weather is a bit like that – but the period (length of a cycle) and the amplitude of that cycle are both semi-randomly variable. Make a note of that point, it will become important when I get to the game mechanics.

The last thing to note before I move on is that there are already (unstable) indications of a circular airflow forming. If enough energy (in the form of heat) builds up in such a system – something that typically only happens in summer – and if there is enough moisture in the air to absorb and retain that energy, then you can end up with a hurricane or tornado or whatever you want to call it.

★ Panel 7 illustrates the complex compounding of movements of air masses that can result. It’s basically a Vector Sum but you don’t move in a straight line to the result, you move in a curve that touches the constituent vectors. The first one is the wind speed, the second one is the rotation of the world, and the resulting rotation of the air mass, and the third is the attractive properties of the nearest piece of land to the sum of the first two.

★ Finally, Panel 8 shows what happens as a result. A cold front – a mass of moist air in a low-pressure environment – has formed. It gets moved around by the wind along the path indicated. If more cold air joins the existing mass, the cold front gets bigger; if not, it gets smaller. And it’s motion is such that the direction of the wind Vector Sum is always at right angles to the cold-front – in this case, the bottom of the front doesn’t have to move as fast as the top, which causes the cold front to become more curved and to head back towards the source of the cold air, where it will have every chance of hooking up with more cold air.

But there is a landmass in the way, and that will cause it to lose a lot of the moisture (and the energy contained within it) as rain, hail, thunder, and lightning. The time of day and time of year are obviously critical – you don’t normally get offshore winds in the head of the day; if anything, you get the opposite, and that means that the cold front ‘stalls’ out to sea. That could be a bother to a ship, but it’s a desirable outcome for anyone living near the coast of the landmass in question. When the winds shift, the front will start to move very quickly toward land.

It’s not unreasonable to take the basic winds of panel 3 and add in the complications of other winds and pressure systems to get the basic track that a front will follow.

4.8.7.2 Accumulated Potential and Threshold

There are two variables at the heart of the Weather Mechanics that I am going to propose. The first is the Accumulated Potential for a change in the weather, and the second is the threshold that this potential has to reach before that change actually happens.

Potential will keep accumulating until something happens to discharge it. Once it starts, a random roll resets both potential and threshold.

Any weather event can be thought of as ‘the absence of the opposite condition’, at least in general terms. If a forecast indicates no cloud, there is no chance of rain, and the result is a sunny day. If a forecast is for heavy cloud cover, there is every chance of rain, and the result is a pronounced absence of sunshine. Any given day can either be ‘stable’ in it’s forecast or ‘transitional’ / changeable / unstable.

Some seasons are more prone to instability than others – the patterns depend on where you are but as a general rule, winter is more stable than spring and autumn, both of which are more stable than summer, because one of the key drivers of instability is energy, which is more plentiful in summer. However, this generally means that most weather events in summer are short and transitory; only a few will last more than a day, and most will last only a few hours (temperature trends excepted). However, summer is also prone to these disruptions being short and sharp – in winter, the miserable weather can be consistent for many days.

The situation is a bit different in tropical regions. They only really have two seasons – the wet season (which is Summer everywhere else, plus half of autumn and half of spring) and the dry season (the rest of the time). In the rainy season, the normal pattern is hot, humid, and sunny until sometime in the afternoon – the times will vary – when it will absolutely teem down for one or two hours. The earlier in the day this happens, the bigger the downpour will be the next day, simply because there’s been more time for the humidity to accumulate.

Disruptions to this norm still happen – you might get the rain event in the late morning instead of the afternoon, or the early evening, or even not get one at all for a day or two. Or it might rain all day for a day or two – not teeming down as heavily but still solidly wet weather. In the mountains, even in the tropics, in winter, you might get still snow or hail.

4.8.7.3 Process Overview
In writing this section, I came up with a completely new weather generation system. I’ve never used this in play, so it’s all theoretical at this stage but it promises to be more accurate and realistic without being a huge amount of work.

Here’s how it’s supposed to work – overview only, I’ll go into more detail afterwards:

     1. GM sets the climate baselines (once per general location).
     2. GM determines the seasonal weather trends (once per general location).
     3. GM rolls for the daily temperature variation.
     4. GM rolls for the change in Weather Threshold Change.
     5. Apply change to long or short threshold, whichever is operative..
     6. GM rolls d6 and d20 for the daily humidity variation.
     7. GM determines the Trend.
     8. GM totals Trend + Rolled Adjustment for both temperature and humidity.
     9. GM compares both temperature and humidity to the change threshold.
     10. GM interprets the weather forecast, adding the baseline and seasonal weather trends to the temperature and humidity to get daily peak values..

Even this makes it sound a lot more complicated than I think it is.

4.8.7.4 Climate Baselines

A year is divided into 4 seasons, each of which is divided into 2 parts, giving 8 baseline slots in a year. These are used to describe the climate, which is used – as the name suggests – as a baseline for the system.

If you can identify a real-life location with similar weather, you may be able to get climate information about the specific location – Wikipedia is often very useful for this. But, if not, this section will show you how to do it from scratch with total imagination.

Weather at Sea is actually more predictable than weather over land, because there’s not as much to complicate and confuse it – it’s all empty ocean except along the coasts. The dominant factors are therefore general location (the Atlantic has different patterns to the Pacific) and latitude. Longitude only really comes into it if you are near a continental coastline – the one to the east will have a different impact to the one on the western side of the ocean.

I mentioned the differences between the Atlantic and Pacific – water temperature is a big part of that. The pacific is comparatively warm, the Atlantic comparatively cool, for reasons far too involved to get into here. The Med is generally warm, the North Sea not. Without looking it up, from latitude alone, I would expect the Black Sea to be fairly Mediterranean in climate.

Start with Midsummer – what’s the average temperature and what’s the average humidity?

Next, Midwinter – same questions.





Mid-Spring and Mid-Autumn are next, and where things can get interesting. Put a ruler down between the midsummer and midwinter points. You can either be on that line (a), above it (b), or below it (c) – you get to choose.

Next, we do the start of each season in exactly the same way, Draw a line connecting the points and hey presto! Job done!

I’ve done a blank chart that you can open in a new tab by clicking the thumbnail to the right. Because once you’ve done this once, you can use it again and again for that general location.



Here’s a fully worked example giving my perceptions of Sydney Weather after 40-odd years of living here.

     ★ Average Max Temp, Midsummer: 40 degrees Centigrade.

     ★ Average Daytime max, Midwinter: 18 degrees Centigrade.

     ★ Mid-autumn: Cooler than the average by a bit (nothing so extreme as the first example, which was deliberately exaggerated – a bit). 24°C.

     ★ Mid-spring: Both Spring and Winter seem to come early these days, and this is the turning point, so it’s right on the average between Midsummer and Midwinter.

     ★ Both Spring and Summer are a little warmer than the average, 24° and 32°, respectively.

     ★ The start of Autumn is even warmer at 36°C and the start of winter is noticeably cooler than the average at 20°C.

Note that it doesn’t matter whether you prefer °C, °F, or even Kelvins! You get to label the scale whatever you want (so long as your entire temperature range fits in the 40-division scale).

     ★ Humidity: typically fairly high in Sydney, midsummer peak of 40%, midwinter 24%, mid-spring and mid-autumn both way above the average of those two points at 36%.

     ★ Spring, also above the average of the points on either side of it at 33%; the start of summer is slightly down on the straight average, at 37%; the start of autumn is a very summery 39%; and the start of winter is well below the straight average at 26%.

4.8.7.5 Seasonal Weather Trends

Next, we need to count the differences and get a simple number for the change each N days, or the N days for a convenient rate of change. These are the seasonal weather trends.

     ★ Sydney Midsummer 40°C; Start of Autumn, 33°C; difference 7°C.

365 days in a year, / 8 = 45.625 days.

Always divide the bigger by the smaller. That will usually mean days / difference, but might not always do so when it comes to humidity or Fahrenheit temperature differences.

     ★ 45.625 / 7 = -1° every 6.5 days.

     ★ Start of autumn, 33°; mid-autumn, 24°; difference of 9°; 45.625 / 9 = -1° every 5 days.

One more:

     ★ Winter 20°; midwinter 18° – but just for the heck of it, let’s put both into Fahrenheit: 68°F to 64.4° (call it 64 1/2) – difference of 3.5°F. 45.625 / 3.5 = -1°F every 13 days.

But with a half-degree there, I’m tempted to take both of them to the half-degree mark.

To do this, I simply halve the time-span for a whole degree.

     ★ -1°F every 13 days becomes -0.5°F every 6.5 days.

Write these down on or below your weather chart. And leave a little room to write two more numbers underneath.

You don’t have to calculate them all right now, but you may as well.

Do the same thing for humidity and move on.

4.8.7.6 Long and Short Base Thresholds

The Long Threshold is how much buildup there has to be before there is a change in the weather.

Start by thinking about the length of the weather cycle – here in Sydney, it’s typically 7-and-a-bit days, at least in summer. If it rains Monday this week, it will probably rain on Monday next week, and the one after that – and then it will be Tuesday, and Tuesday again, and so on.

This pattern gets reset by the occasional more extreme weather event, but that’s the basic gist of it. And it means that the long threshold and short threshold will total 7-and-a-bit.

     ★ Maybe 7.3.

Important: For °F all thresholds should be doubled. This is because a °F is smaller than a °C, so you need more of them for the same change in weather. For high humidity locations, use the °F tables and double the threshold (for humidity) regardless of the temperature units being employed.

     ★ Again in midsummer, the typical short threshold is 1.5 days – sometimes it’s one day, sometimes 2, sometimes something in between.

     ★ 7.3 – 1.5 = 5.8. So that’s my base long threshold for midsummer and the 45.625 days that follow it.

     ★ In winter, the two are a lot closer. We’re still talking a 7-day cycle, but the short is more like 2.6 days (sometimes 2, occasionally 3 or 4), and the long is 7.5 – 2.6 = 4.9.

The higher the long threshold, the more stable and predictable the weather is. The longer the short threshold, the longer it takes to go back to that stable and predictable ‘normal weather’ after a disruption.

So go ahead and do all of these as well.

So, if I was using °F: the values I should use are cycle 14.6, long thresholds (summer) 11.6 and (winter) 9.8, short thresholds 3 (summer) and 5.2 (winter).

4.8.7.7 Daily Temperature Variation

With that prep out of the way, the system is now ready for use, believe it or not.

Let’s say that we’re doing a forecast for Sydney for 22 days into Autumn.

The relevant numbers from the chart, i.e. the work already done::

     ★ Autumn base temp = 33°C; Mid-autumn = 24°C; difference = 9; so -1 degree every 5 days.

     ★ 22 / 5 × -1 = -4.4°, so right now the long-tern average would be about 28.6°C.

     ★ Autumn base humidity = 39%; mid-autumn = 36%; difference = 3; so -1% every 15.2 days, or -0.5% every 7.6 days.

     ★ 22 / 7.6 × -0.5 = -1.447%, call it -1.4%, so the long term average is now about 37.6%.

     ★ Long Threshold: Midsummer + Midwinter = 5.8 + 4.9 = 10.7; average = 5.35.
     ★ Short Threshold: Midsummer + Midwinter = 1.5 + 2.6 = 4.1; average = 2.05.
     ★ But, given the season, long should be a little shorter and short a little longer. Use 5.2 and 2.5.


Roll a d6:
     1-2 = minus
     3-4 = same as yesterday
     5-6 = plus

But we don’t know what yesterday was, so if a 3 or 4 comes up, we may have to roll that, too. In fact, we may have to regress a number of days before a valid answer pops out.

The next part is a little trickier.

Roll a d6 for the adjustment column and a d20 for the adjustment amount on the table below:

	°C / Low Humidity
	d6	1	2	3	4	5	6
d20	1	0.1	0.2	0.3	0.4	<0.5	0.6
	2	0.2	0.3	0.5	0.6	0.7	0.8
	3	0.3	0.4	0.6	0.7	0.9	1
	4	0.4	0.5	0.7	0.8	1	1.2
	5	0.5	0.6	0.8	1	1.2	1.4
	6	0.6	0.7	0.9	1.1	1.3	1.5
	7	0.7	0.8	1	1.2	1.4	1.6
	8	0.8	0.9	1.1	1.3	1.5	1.7
	9	0.9	1	1.2	1.4	1.6	1.8
	10	1	1.1	1.3	1.5	1.7	1.9
	11	1.1	1.2	1.4	1.6	1.8	2
	12	1.2	1.3	1.5	1.7	1.9	2.1
	13	1.3	1.4	1.6	1.8	2	2.2
	14	1.4	1.5	1.7	1.9	2.1	2.3
	15	1.5	1.6	1.8	2	2.2	2.4
	16	1.6	1.7	1.9	2.2	2.3	2.5
	17	1.7	1.8	2	2.4	2.5	2.6
	18	1.8	1.9	2.2	2.5	2.6	2.7
	19	1.9	2	2.4	2.6	2.7	2.8
	20	2	2.2	2.5	2.7	2.8	3

	°F / High Humidity
	d6	1	2	3	4	5	6
d20	1	0.2	0.4	0.5	0.7	0.9	1.1
	2	0.4	0.5	0.9	1.1	1.3	1.4
	3	0.5	0.7	1.1	1.3	1.6	1.8
	4	0.7	0.9	1.3	1.4	1.8	2.2
	5	0.9	1.1	1.4	1.8	2.2	2.5
	6	1.1	1.3	1.6	2	2.3	2.7
	7	1.3	1.4	1.8	2.2	2.5	2.9
	8	1.4	1.6	2	2.3	2.7	3.1
	9	1.6	1.8	2.2	2.5	2.9	3.2
	10	1.8	2	2.3	2.7	3.1	3.4
	11	2	2.2	2.5	2.9	3.2	3.6
	12	2.2	2.3	2.7	3.1	3.4	3.8
	13	2.3	2.5	2.9	3.2	3.6	4
	14	2.5	2.7	3.1	3.4	3.8	4.1
	15	2.7	2.9	3.2	3.6	4	4.3
	16	2.9	3.1	3.4	4	4.1	4.5
	17	3.1	3.2	3.6	4.3	4.5	4.7
	18	3.2	3.4	4	4.5	4.7	4.9
	19	3.4	3.6	4.3	4.7	4.9	5
	20	3.6	4	4.5	4.9	5	5.4

NB>: Humidity cannot exceed 100%. Any excess: /10 and add to the conditions adjustment. That’s because humidity doesn’t drive changes in the weather in this system, temperature does.



I’ve put all 5 tables into a Free PDF for reader’s convenience. This includes the tables below for Threshold adjustment. NB: This includes a slightly older version of the above tables – the numbers are the same but some of the labels are incorrect.

Continuing the Sydney example:
     ★ d6 roll: 2 so “minus”.

     ★ d6 roll 4, d20 roll 14.
     ★ °C change = minus 1.9.

     ★ d6 roll 4, so “same as yesterday”
     ★ roll again for yesterday: 5 so “plus”

     ★ Yesterday’s change: d6 roll 6, d20 roll 10. % Humidity change = plus 1.9.
     ★ Today’s change: d6 roll 2, d20 roll 12. % humidity change today = plus 1.3.
     ★ 1.3 + 1.9 = 3.2. So, overall, the humidity is 3.2% higher than the average for this time of year.



…Red Sky At Night, Sailor’s Delight!
Well, it might not be 100% reliable, but it’s better than nothing!
Image by Jody Davis from Pixabay

4.8.7.8 Weather Change Threshold

Since we’re assumed to be starting a cycle (now in it’s second day because I had to re-roll once for ‘yesterday’) we’re adjusting the long threshold, which is usually the case.

Same as above except that the original d6 roll remains – it said “minus” so the threshold change is a minus.:

	°C / Low Humidity
	d6	1	2	3	4	5	6
d20	1	0.025	0.07	0.15	0.3	0.425	0.72
	2	0.05	0.105	0.25	0.45	0.595	0.96
	3	0.075	0.14	0.3	0.525	0.765	1.2
	4	0.1	0.175	0.35	0.6	0.85	1.44
	5	0.125	0.21	0.4	0.75	1.02	1.68
	6	0.15	0.245	0.45	0.825	1.105	1.8
	7	0.175	0.28	0.5	0.9	1.19	1.92
	8	0.2	0.315	0.55	0.975	1.275	2.04
	9	0.225	0.35	0.6	1.05	1.36	2.16
	10	0.25	0.385	0.65	1.125	1.445	2.28
	11	0.275	0.42	0.7	1.2	1.53	2.4
	12	0.3	0.455	0.75	1.275	1.615	2.52
	13	0.325	0.49	0.8	1.35	1.7	2.64
	14	0.35	0.525	0.85	1.425	1.785	2.76
	15	0.375	0.56	0.9	1.5	1.87	2.88
	16	0.4	0.595	0.95	1.65	1.955	3
	17	0.425	0.63	1	1.8	2.125	3.12
	18	0.45	0.665	1.1	1.875	2.21	3.24
	19	0.475	0.7	1.2	1.95	2.295	3.36
	20	0.5	0.77	1.25	2.025	2.38	3.6

	°F / High Humidity
	d6	1	2	3	4	5	6
d20	1		0.1	0.3	0.5	0.8	1.3
	2	0.1	0.2	0.5	0.8	1.1	1.7
	3	0.1	0.3	0.5	0.9	1.4	2.2
	4	0.2	0.3	0.6	1.1	1.5	2.6
	5	0.2	0.4	0.7	1.4	1.8	3
	6	0.3	0.4	0.8	1.5	2	3.2
	7	0.3	0.5	0.9	1.6	2.1	3.5
	8	0.4	0.6	1	1.8	2.3	3.7
	9	0.4	0.6	1.1	1.9	2.4	3.9
	10	0.5	0.7	1.2	2	2.6	4.1
	11	0.5	0.8	1.3	2.2	2.8	4.3
	12	0.5	0.8	1.4	2.3	2.9	4.5
	13	0.6	0.9	1.4	2.4	3.1	4.8
	14	0.6	0.9	1.5	2.6	3.2	5
	15	0.7	1	1.6	2.7	3.4	5.2
	16	0.7	1.1	1.7	3	3.5	5.4
	17	0.8	1.1	1.8	3.2	3.8	5.6
	18	0.8	1.2	2	3.4	4	5.8
	19	0.9	1.3	2.2	3.5	4.1	6
	20	0.9	1.4	2.3	3.6	4.3	6.5

Continuing the Sydney example:
     ★ d6 roll: still “minus”, as noted above.

     ★ d6 roll 3, d20 roll 2. Threshold change = minus 0.25.

4.8.7.9 Trend

“Trend” only impacts accumulated change, not threshold. It is equal to 1/2 of yesterday’s change if any was recorded, or 0 otherwise.

Continuing the Sydney example:
No change was documented yesterday for temperature, which is what drives changes in the weather, so the trend is 0 at the moment.

4.8.7.10 Result Totals:

This is easier demonstrated than explained, so Continuing the Sydney example:

     ★ Temperature: Time-of-year average 28.6°C minus 1.9 = 26.7°C.
     ★ Humidity: Time-of-year average 37.6% plus 1.9 (yesterday) plus 1.3 (today) = 40.8%.
     ★ Change: Trend 0 em>plus 1.9 = 1.9
     ★ Long Threshold = 5.2 minus 0.25 = 4.95.
     ★ Short Threshold = 2.5, unchanged because it is not the active threshold at the moment.
     ★ Is the Change greater than the operative threshold? 1.9 is not greater than 4.95. Therefore, no change in the weather.

Write these down, and save yourself a bit of time by calculating the Trend for tomorrow while you’re about it:

     ★ Tomorrow’s Trend = -1.9 / 2 = -0.95

It should be recognized that the time-of-year correction and the Trend are both deliberate biases built into the system. One tracks with the flow of she seasons and is in the direction that seasonal averages would expect; the other tracks with previous changes so that they can accumulate until a change in the weather is triggered..

4.8.7.11 Interpretation

How these factors come together into a weather report / forecast is up to the GM, but generally follows logic.

Take the Sydney example: temperature dropping (but still high), humidity rising (but still relatively low) – what can account for that? There are only two things: cloud cover and winds.

Well, winds are a whole separate deal, given how important they are to a sailing ship. But it’s fair to say that if they aren’t enough to justify that drop in temperature, the only remaining option is cloud cover. This could be partial, broken clouds, or it could be threatening rain – except that we’re nowhere near the threshold, and the humidity isn’t really high enough, so it’s more likely to be the first than the second.

4.8.7.12 Severity

A further tool at your disposal is the change total itself – this can be looked at as the fraction of a cycle or as the intensity of the change. They are both the same thing in this context.

     ★ 1.9 / 7.3 is a bit under two sevenths. Maybe three fourteenths. That says that the weather on this particular day is going to be appreciably mild – strong winds being a possible exception, but they aren’t very likely.

     ★ Today’s forecast: Warm with broken cloud and light winds. There is a remote chance of a brief thunderstorm in the later afternoon which will have no significant weather consequences. Similar conditions are likely to persist for several days to come.

4.8.7.12.1 Extending The Example

Just to show readers how it works, and because it provides a lead-in to the next sections, let’s extend that forecast for a while.

Day 2:

     ★ Time-of-year average Temperature: 28.6°C – (1 / 5) = 28.6 – 0.2 = 28.4.
     ★ Time-of-year average Humidity: 37.6% – (0.5 / 7.6) = 37.6 – 0.066 = 37.6 – 0.07 = 37.53%.

     ★ Change Direction Temperature (d6): 1, so minus.
     ★ Change rolled (d6, d20) = (2, 9) = 1 degree.
     ★ Change Direction Humidity (d6): 5, so plus.
     ★ Change rolled (d6, d20) – (4, 6) = 1.1%

     ★ Temperature: Time-of-year average 28.4°C minus 1 = 27.4°C (actually slightly warmer).
     ★ Humidity: Time-of-year average 37.53% plus 1.1 = 38.63%.
     ★ Change: Trend 0.95 + Change 1.9 (yesterday) + 1 (today)= 3.85.

     ★ Threshold Change Direction (d6): 6, so plus for the first time
     ★ Threshold Change Rolled (d6, d20) = (4, 14) = 1.42
     ★ Long Threshold = 4.67 plus 1.42 = 6.09.
     ★ Short Threshold = 2.5, unchanged
     ★ Is the Change greater than the operative threshold? 3.85 is not greater than 6.09. Therefore, no change in the weather.

     ★ Tomorrow’s Trend = 3.85 / 2 = 1.925, round to 1.93.

Interpretation: Slightly warmer but with increasing cloud cover. Light winds.

Day 3:

     ★ Time-of-year average Temperature: 28.4°C – (1 / 5) = 28.4 – 0.2 = 28.2.
     ★ Time-of-year average Humidity: 37.53% – (0.5 / 7.6) = 37.6 – 0.066 = 37.6 – 0.07 = 37.46%.

     ★ Change Direction Temperature (d6): 2, so minus.
     ★ Change rolled (d6, d20) = (5, 8) = 1.5 degrees.
     ★ Change Direction Humidity (d6): 5, so plus.
     ★ Change rolled (d6, d20) – (1, 18) = 1.8%

     ★ Temperature: Time-of-year average 28.2°C minus 1.5 = 26.7°C
     ★ Humidity: Time-of-year average 37.46% plus 1.8 = 39.26%.
     ★ Change: Trend 1.93 + Change 1.5 + 3.85 (previous change) = 6.28

     ★ Threshold Change Direction (d6): 1, so minus
     ★ Threshold Change Rolled (d6, d20) = (6, 16) = 0.28
     ★ Long Threshold = 6.09 minus 0.28 = 5.81.
     ★ Short Threshold = 2.5, unchanged
     ★ Is the Change greater than the operative threshold? 6.28 is greater than 6.09. The weather changes – from sunny to rain.

     ★ Tomorrow’s Trend = 0 because the weather has turned.

Interpretation: Cloudy & cooler, showers becoming likely in the evening.

Day 4:

     ★ Time-of-year average Temperature: 28.2°C – 0.2 = 28.0.
     ★ Time-of-year average Humidity: 37.46% – 0.07 = 37.39%.

     ★ Change Direction Temperature (d6): 5, so plus.
     ★ Change rolled (d6, d20) = (6, 12) = 2.1 degrees.
     ★ Change Direction Humidity (d6): 6, so plus.
     ★ Change rolled (d6, d20) – (2, 5) = 0.6%

     ★ Temperature: Time-of-year average 28.0°C plus 2.1 = 30.1°C
     ★ Humidity: Time-of-year average 37.39% plus 0.6 = 39.86%.
     ★ Change: Trend 0 + Change 2.1= 2.1

     ★ Threshold Change Direction (d6): 2, so minus
     ★ Threshold Change Rolled (d6, d20) = (2, 17) = 0.63
     ★ Long Threshold resets to base value of 5.2
     ★ Short Threshold = 2.5 minus 0.63 = 1.87
     ★ Is the Change greater than the operative threshold? 2.1 is greater than 1.87. The weather changes – from rain to sunny.

     ★ Tomorrow’s Trend = 0 because the weather has turned again.

Interpretation: Very warm, showers clearing.

Before I can continue with the example, I need to explain how rain affects humidity.

4.8.7.13 Clouds & Rain

If humidity goes over 70%, cloud cover is pretty close to inevitable. The daily base weather changes to include this without the weather pattern itself changing (unless the threshold for such a change is exceeded).

Rain events can be savage downpours or constant precipitation or a thunderous deluge or a continuous drizzle or passing showers.

     ★ Calculate the difference between 4 x the humidity before the event and half the time of year average for the day.

     ★ Divide by the number of days that this event is going to last, as determined in previous sections.

     ★ Roll a d8 and consult the tables below (these have also been included in the free PDF):

EG1: Base 40% humidity, currently 30%, Duration 1 day

     ★ 4 × 30 – (40 / 2) = 120-20=100; 100 / 1 = 100
     ★ d8 roll = 5
     ★ Type 3 weather event
     ★ one or more thunderstorms, snow & possible hail if cold enough

EG2: Base 25% humidity currently 35%, Duration 2 days

     ★ 4 × 35 – (25/2) = 140 – 12.5 = 127.5 / 2 = 63.75
     ★ d8 roll = 4
     ★ Type 2 weather event
     ★ Continuous Drizzle or Snow for several hours

EG3: Base 25% humidity, currently 45%, Duration 2 days

     ★ 4 × 45 – (25 / 2) = 180 – 12.5 = 167.5; 167.5 / 2 = 83.75
     ★ d8 roll = 8
     ★ Type 4 weather event
     ★ Constant soaking rain or heavy snow (possible sleet) if cold enough

EG 4: Base 25% humidity, currently 45%, Duration 3 days

     ★ 4 × 45 – (25 / 2) = 180 – 12.5 = 167.5; 167.5 / 2 = 55.8
     ★ d8 roll = 8
     ★ Type 3 weather event again
     ★ As above

EG5: Base 25% humidity, currently 45%, Duration 1 day

     ★ 4 × 45 – (25 / 2) = 180 – 12.5 = 167.5; 167.5 / 1 = 167.5
     ★ d8 roll = 8
     ★ Type 5 weather event
     ★ Massive downpour lasting 1-3 hours

EG6: Base 40% humidity, currently 55%, Duration 4 days

     ★ 4 × 55 – (40/2) = 220-20 = 200; 200/4 = 50
     ★ die roll = 3
     ★ Type 1 weather event
     ★ One or more light showers each day, passing

EG7: Base 40% humidity, currently 55%, Duration 2 days

     ★ 4 × 55 – (40/2) = 220-20 = 200; 200/2 = 100
     ★ die roll = 3
     ★ Type 2 weather event
     ★ Continuous Drizzle most of the day

On the table, note the “Daily Loss”. That’s how much of the current humidity gets lost in the course of 1 day of this weather event.

If this takes the total below the Base for this time of year, all humidity adjustments are +5%/day until this is no longer true. Long Thresholds increase by 2 while this is taking place.

Let’s look at those 7 examples again:

EG1: 1-day Type 3 event starting at 30% humidity vs a Base of 40%:

     ★ 50% daily loss
     ★ 50% of 30% is 15%
     ★ Humidity after event = 30 – 15 = 15%
     ★ 15% is less than 40% so +5%/day for next 5 days, maybe more.

EG2: 2-day Type 2 event starting at 35% vs a base of 25%

     ★ 40% daily loss
     ★ 40% of 35% is 14%
     ★ Humidity after event = 35 – 14 = 21% after day 1
     ★ 40% of 21% is 8.4%
     ★ Humidity after event = 21 – 8.4 = 12.6% after day 2
     ★ 12.6% is less than 25% so +5%/day for next 3 days or so.

EG3: 2-day Type 4 event starting at 45% vs a base of 25%

     ★ 60% daily loss
     ★ 60% of 45% is 27%
     ★ Humidity after event = 45 – 27 = 18% after day 1
     ★ 60% of 18% is 10.8%
     ★ Humidity after event = 18 – 10.8 = 7.2% after day 2
     ★ 7.2% is less than 25% so +5%/day for next 4-5 days.

EG4: 3-day Type 3 event starting at 45% vs a base of 25%

     ★ 60% daily loss
     ★ 60% of 45% is 27%
     ★ Humidity after event = 45 – 27 = 18% after day 1
     ★ 60% of 18% is 10.8%
     ★ Humidity after event = 18 – 10.8 = 7.2% after day 2
     ★ 60% of 7.2% is 4.32%, round to 4.3%
     ★ Humidity after event = 7.2 – 4.3 = 2.9% after day 3
     ★ 2.9% is less than 25% so +5%/day for next 5-6 days.

EG5: 1-day Type 5 event starting at 45% humidity vs a Base of 25%:

     ★ 75% daily loss
     ★ 75% of 45% is 33.75%, round to 33.8
     ★ Humidity after event = 45 – 33.8 = 11.2%
     ★ 11.2% is less than 25% so +5%/day for next 3 days or so.

EG6: 4-day Type 1 event starting at 55% humidity vs a Base of 40%

     ★ 33% daily loss
     ★ 33% of 55% is 18.3%
     ★ Humidity after event = 55 – 18.3 = 36.7% after day 1
     ★ 33% of 36.7% is 12.2%
     ★ Humidity after event = 36.7 – 12.2 = 24.5% after day 2
     ★ 33% of 24.5% is 8.167%, round to 8.2%
     ★ Humidity after event = 24.5 – 8.2 = 16.3% after day 3
     ★ 33% of 16.3% is 5.433%, round to 5.4%
     ★ Humidity after event = 16.3 – 5.4 = 10.9% after day 4
     ★ 10.9% is less than 40% so +5%/day for next 6 days, give-or-take.

EG7: 2-day Type 2 event starting a 55% humidity vs a base of 40%

     ★ 40% daily loss
     ★ 40% of 55% is 22%
     ★ Humidity after event = 55 – 22 = 33% after day 1
     ★ 40% of 33% is 13.2%
     ★ Humidity after event = 33 – 13.2 = 19.8% after day 2
     ★ 19.8% is less than 40% so +5%/day for approximately the next 5 days.

Tropics add 2 to the Event Type in the rainy season and +10% per day recovered.

4.8.7.14 Winds

Given that we’re talking about sailing ships, winds are pretty close to all-important. For that reason, I’ve gone more deeply into the subject than I otherwise would have.

4.8.7.14.1 Wind Speed

This is based on a divided die roll. These are useful because they permit extreme but improbable results while maintaining a low that is bunched up toward the low end of the results.

     ★ Step 1: 5d10 =
     ★ Step 2: + 1/2 × d% =
     ★ Step 3: × 4.84 / 4d6, round down =
     ★ Step 4: + 2d6

This has a minimum of 0, an average of 22.5, and a maximum of 133. The lowest result with a significant chance of occurring is 11 (0.58%) and the highest is 44 (0.51%). So it’s far to say the result is usually 11-44, with 22-23 the most likely, but with a slim chance of results outside of this.

I’ve done a graph using AnyDice, my favorite tool for this sort of thing, and am presenting it below in two ways. First, the usual orientation of these things:



And now, turned on it’s side (so that I can make it far bigger):



These show the two constituents. Note the way the “+2d6” smooths the probability curve out!

The results are significant. Once again: Usually 11-44, with 22-23 the most likely, but with a slim chance of results outside of this, down to 0 or up to 133:

Beaufort Scale Table 1
km/h		mph		knots		ft/sec		m/sec		Beaufort Number
low	high	low	high	low	high	low	high	low	high
2	5	1	3	1	3	1	4	0.4	1.3	1
6	11	4	7	3	6	6	10	1.8	3.1	2
13	19	8	12	7	10	12	18	3.6	5.4	3
21	29	13	18	11	16	19	26	5.8	8	4
31	39	19	24	17	21	28	35	8.5	10.7	5
40	50	25	31	22	27	37	45	11.2	13.9	6
51	61	32	38	28	33	47	56	14.3	17	7
63	74	39	46	34	40	57	67	17.4	20.6	8
76	87	47	54	41	47	69	79	21	24.1	9
89	101	55	63	48	55	81	92	24.6	28.2	10
103	121	64	75	56	65	94	110	28.6	33.5	11
122+		76+		66+		111+		34+		12

…they can be used directly on the km/h column.

Conversions: 100 km/h = 62.1371 mph = 53.9957 knots = 27.7778 m/s = 91.1344 f/s

but, it’s usually close enough to say:

100 km/h = 62 mph = 54 knots = 28 m/s = 91 f/s

Okay, so what does the “Beaufort Number” mean, in real terms? I’ve more or less covered this before, but we’re talking events at sea, and that requires additional information.

Beaufort Scale Table 2
Beaufort Number	Unofficial Label	Description
	Calm	No Wind, Sea is like a mirror. Sails useless.
1	Light Air	Barely moves tree leaves. Water ripples but no foam crests. Waves under 0.5 ft.
2	Light Breeze	Wind felt, leaves & weather-vanes Move. Small wavelets with glassy tops crests that do not break, 0.5-1 foot height. Light flags fully extended. Insufficient wind for anything larger than a Sloop.
3	Gentle Breeze	Leaves in constant motion, Wavelets 2-3 ft in height, crests begin to break, foam of glassy appearance, occasional whitecap.
4	Moderate Winds	Small Branches move, dust and light objects blown about. No noticeable sound from the rigging. Flags extended, heavy flags flap limply. Waves 3-5 ft high, frequent whitecaps. Slack halyards curve and sway.
5	Fresh Winds	Wind felt strongly. Large Branches & small trees sway, Crested wavelets on inland lakes & rivers. Waves tending to be long 6-8 ft tall at sea, some spray. Slack halyards whip while bending continuously to leeward, taut ones maintain slightly bent positions. Heavy flags do not extend but flap over entire length.
6	Strong Winds	Large Branches in continuous motion, Umbrellas used with difficulty. Large Waves 9-12 ft in height begin to form, some spray likely, white caps extensive. Below 35°F (22°C), wind stings face. Slight effort to maintain balance against the wind. All halyards whip slightly. Low moaning from the rigging. Heavy flags extend fully and flap more vigorously.
7	Dangerous Winds	Whole trees in motion, hard to walk Against the wind and you have to lean into the wind to stand up. Waves 13-19 ft, white foam from breaking waves begins to be blown in streaks along direction of wind. Low whistling from rigging. Oilskins & loose clothing catch the wind and puff out. Heavy flags fully extended and only flap at the ends.
8	Gale	Wind breaks twigs & small branches, and impedes walking. At sea, creates waves 18-25 ft tall. The edges of crests begins to break, blown in clear streaks following the line of wind. Loose clothing blows out strongly, loose limbs/head blown back if resistance is relaxed. Loud whistle from the rigging, Heavy flag flies straight out fully extended. Masts noticeably bend and creak and sail must be reduced to 50%.
9	Strong Gale	Structural damage to buildings, tree branches downed. Waves 23-32 ft with dense streaks along direction of wind. Crests of waves begin to topple. tumble, and roll over. Spray may affect visibility. Sails must be reduced to 25% or less and masts are still at risk. Wind cannot be safely quartered without risk of capsizing.
10	Extreme Gale	Considerable structural damage, weak roofs torn apart, small trees uprooted. Large tree limbs may break. Glass windows may shatter from wind alone. Waves 29-41 ft high with long overhanging crests. Foam in great patches is blown in dense patches in the line of the wind. Sea is overall whitish from foam and tumbling becomes heavy. Visibility down to approx 15-25m. Sails must be taken/cut down or masts will break. Wooden vessels can be felt flexing underfoot, must be anchored or will be turned sideways and then capsized. Anchor chains taught and may break Impossible to walk into the wind without ropes or similar assistance.
11	Weak Hurricane	Widespread damage, roofs may be completely torn away and large trees uprooted. Masts break even without sails. Waves 37-52 ft tall, small & medium ships completely lost to view behind waves. Sea completely covered with long patches of foam along the direction of the wind. Edges of wave crests are blown into froth. Visibility of 5-15m. Anchor chains at risk, ships assume extreme angles, turning to broadside the wind; chains may have to be cut / released. Impossible to stay upright in the wind and even crawling is difficult.
12	Hurricane	Severe and extensive damage, Vehicles overturned, timber houses leveled. The air at sea is filled with foam and spray. The sea is completely white. Visibility is 2-5m only. Cannot crawl into the wind. Small buildings may be leveled or torn away whole. Anchor chains snap, ships capsize unless steered directly away from or into the wind and most can’t respond to slight variations in wind direction quickly enough. Masts lost / broken, and it is often safer to chop them down. Ships may break in two. People are lifted up / blown away.

We’re not quite done yet – there are a couple of biases to throw in.

Step 5: If there is a significant change in temperature, add 4.
Step 6a: If there is an actual change in conditions, add 2d6+5 when moving to short threshold; or
Step 6b: divide by 2 and add 2d6+5 if moving to or in long threshold.
Step 7: (optional): divide by 2 and add half of yesterday’s result.

Personally, I wouldn’t go with the Step 7 option; one day doesn’t have to look like another and there’s enough variability built into the system that it’s probably a wasted effort, anyway.

EG1: 2.5° drop in temp, long threshold conditions. Yesterday’s wind at this time of day was 26 km/h..

     ★ 5d10 roll = 25
     ★ + 1/2 d% roll = +46 / 2 = +23; total 48
     ★ x 4.84 / 4d6 = × 4.84 / 7 = 33
     ★ + 2d6 roll = +9, total 42.
     ★ 42 is Beaufort Scale 6, strong winds.

Commentary: because the 4d6 roll was well below average, the result is right at the top of the results spread that should be expected, most of the time. If the 5d10 had been higher, the wind speed would be well outside the normal range.

     ★ 2.5°C is not a huge temp drop, but it is noteworthy. The GM rules it significant.
     ★ 42 + 4 = 46.
     ★ Step 6a does not apply.
     ★ Divide by 2 = 23
     ★ add 2d6++5 rolled =+8 = 31.

     ★ 31 is right at the bottom limit of Beaufort Scale 5, Fresh Winds.

Steps 5 and 6 definitely make a difference.

     ★ Optional: 31/2 = 15.5; + 1/2 of 26 = +13, total 28.5.
Commentary This would be enough to drop to Beaufort Scale 4, Moderate Winds – for no better reason than there were Moderate Winds yesterday, too. I don’t think shifting from one Beaufort Number to one higher is too big a step for a day. But it does make changes more gradual and progressive, less abrupt.

I could even see GMs deciding that Step 7 applies in some specific waters and not others. The Mediterranean is fairly sheltered by land; step 7 might apply. The Atlantic is notoriously more changeable than the Pacific; step 7 might also apply to the latter. Heck, there’s nothing stopping anyone who wants to from working out what the impact of Step 7 will be on any particular day and then deciding whether or not it’s in effect!

EG2: 1.2°C temperature change, weather becoming unsettled; yesterday at this time was 15 km/h.

     ★ 5d10 roll = 29
     ★ + 1/2 d% roll = +53 / 2 = +26.5; total 55.5
     ★ x 4.84 / 4d6 = × 4.84 / 23 = 11.68, round to 11.
     ★ + 2d6 roll = +7, total 18.
     ★ 1.2°C is a significant change.
     ★ Step 6a: +2d6+5 = +14; total 32.
     ★ Step 6b does not apply.
     ★ 32 is Beaufort Scale 5, Fresh Winds.

If ever there was a time to ignore Step 7, it’s when the weather changes.

One more: EG3: 2.5° Temperature Change, weather already unsettled becoming stable, yesterday at this time =28 km/h.

     ★ 5d10 roll = 30
     ★ + 1/2 d% roll = +72 / 2 = +36; total 66
     ★ x 4.84 / 4d6 = × 4.84 / 16 = 19.965, round to 19.
     ★ + 2d6 roll = +5, total 24.
     ★ 2.5°C is a significant change, so +4 = 28.
     ★ Step 6a: +2d6+5 = +7; total 35.
     ★ Step 6b: / 2 = 17.5, + d6+5 again = +7 (as already rolled) = 24.5.
     ★ 24 is Beaufort Scale 4, Moderate Winds.

I’ve added a worksheet for these calculations to the free PDF. It has room for 9 calculations on a page (I wanted 10 but couldn’t quite fit them in).

4.8.7.14.2 Ship Speed vs Wind Speed

The trigonometry is fairly obvious – the speed of the ship is the amount of wind that is pushing the ship forward. By changing the angle of the ship relative to the wind, you can change the proportion of the wind that’s pushing you in the forward direction.

That’s far too complicated for a game. So, instead, I’ve employed the concept of Favorable Winds. I’ll go into this in much more detail in 4.8.7.14.5 below, but, in a nutshell:

Wind from directly behind is 70% efficient. Every ship has a defined angle of Favorable Winds – the base angle is ± 30° to either side.

Beyond that is a ‘near favorable’ wind, usually another 30° arc. Winds from this direction are 50% efficient.

After that comes ‘tolerable wind’, usually another 30° arc. Winds from this direction are 30% efficient.

‘Intolerable wind’ is blowing in completely the wrong direction. Some vessel types can nevertheless make forward progress at 10% efficiency. The rest need to rely on rowers.

Adding extra masts adds 5% efficiency to favorable winds, 7.5% to near-favorable and 3% to tolerable wind efficiencies. Extra masts also increase the ranges of Favorable Winds and Tolerable Winds by 10° for the first and 5° each after that. Note that all this is a simplification of a much more complex situation, and generalized, and even a bit exaggerated.

If the resulting speed is greater than the vessel’s limit, there is a risk of the wind uprooting the masts, which could break a ship in two. Find/estimate the % over maximum and add 35 – that’s the percentage chance per 30 minutes of this happening. If the % is over 100, halve both the % and the time scale between checks. Round % up and time down.

EG: 260% of safe speed: 35+260=295% every 30 minutes; becomes 147.5% every 15 minutes, becomes 73.75% every 7.5 minutes, becomes 74% every 7 minutes.

To avoid this, it is normal practice to ‘trim the sails’ until the speed is the maximum safe plus whatever margin the captain wants (just in case of a gust of greater force).

Another way to look at all this: divide the ship’s maximum rates speed by the % shown and you will find out at what wind speed you need to start trimming sail for safety.

4.8.7.14.3 Wind Direction

There are two ways of specifying wind direction: Wind From and Wind To. And endless confusion awaits unless you are clear which one you are talking about. Unless stated otherwise, or obviously otherwise (as in Favored Winds), I will consistently try to use Wind To.

Clearly, wind direction is just as important as wind speed. The base direction is always away from (day) or towards (night) the nearest significant land mass.

The GM decides what’s a significant land mass, and what’s not, but in general, peninsulas and small islands won’t cut it. A small island with a 10,000 foot mountain, on the other hand…



I threw the above image together to illustrate this point. Essentially, I put a whole heap of possible locations on the map and then guesstimated by eye where the closest land would be. There’s even one rogue point (bottom row, third from the right) that in retrospect should probably be a little more westerly, like those on either side of it.

There’s actually a lot to unpack in the image. The coast curves southwest from the start of the peninsula. This has a significant effect if you are trying to head north. From just below the point of the peninsula, the indicated nearest land-mass is north of most positions until you get to the right-hand side of the map – where the headlands between beaches bulge outward, causing more westerly deviations. That headland would be a great place for a trading port, with the beaches to either side for wharves.

A brief mention of scale: the map doesn’t have one. All that can be said is that there is no comparable landmass closer than the one shown. If that means that the map is 100 km to a side, or 1000, it doesn’t matter.

Significant (there’s that word again) levels of overgrowth can reduce the significance of the landmass. This is all about how hot the ground gets.

More than 50% cloud cover can also impact significance.

If there are no significant land masses in reasonable proximity, then the base direction is directly away from the equator and opposing the direction of motion of the planetary body. The closer to either equator or poles you are, the less significant the equatorial contribution and the more significant the rotational contribution:



But even this is sporting more precision than we really want. N,NE, E, SE, S, SW, W, NW – those eight directions are almost good enough. Throw in one more subdivision – NNW, etc – and we’re good to go. This divides the circle into 16 pieces of 22.5° each. That’s convenient, too, because the tropics are almost exactly 22.5° from the equator (the correct figure is 23.5°, but this is close enough).

Next, we have the familiar d6 roll:

d6
     1-2 minus angle (counterclockwise)
     3-4 as yesterday
     5-6 plus angle (clockwise)

And then a d20 (included in the Free PDF):

d20	Interpretation
1-6	0 steps from base direction
7-11	1 step from base direction
12-14	2 steps from base direction
15-16	3 steps from base direction
17-18	4 steps from base direction
19	5 steps from base direction
20	6 steps from base direction

There are two adjustments to the resulting direction:

     +1 step away from next greatest heat source
     +1 step towards west (if normal planetary rotation).

…. and that’s it. The hardest part is visualizing the environment and determining the base wind direction.

I realize that almost everyone will be completely familiar with it, but on the off chance that someone is not:



I had some room left, so I threw in figure 2 (actually, it ended up taking 4x as long as figure 1 to complete, but it seemed like a good idea at the time). This illustrates the probability of wind coming from a given direction relative to the base direction.

If the wind has a base direction from the Southwest, i.e, to the Northeast, then almost all of the time it will be 1-2 steps away at most, i.e. blowing to the North, North-North East, North-East, East-Northeast, or East. 70% of the time, one of those directions will be the wind direction, and 57% of the time it will NOT be the indicated Northeast, but off to one side or the other – but that’s before the adjustments above are taken into account.



This is what can easily result: A Shipwreck In Stormy Seas by Claude-Joseph Vernet, Public Domain, courtesy Wikimedia Commons

4.8.7.14.4 Wind Duration

Another critical question is how long the wind will blow before it shifts. Note that the strength usually doesn’t change from such a wind shift, it will stay steady – unless you are in short threshold mode.

This is a straight d% roll. As usual, also in the PDF.

d%	Wind Duration			d%	Wind Duration
01-06	(6%)	1 hour		75-84	(10%)	8 hours
07-14	(8%)	2 hours		85-87	(3%)	9 hours
15-26	(12%)	3 hours		88-93	(6%)	10 hours
27-40	(14%)	4 hours		94-95	(2%)	11 hours
41-54	(14%)	5 hours		96-99	(4%)	12 hours
55-66	(12%)	6 hours		0 0	(1%)	12 hours + roll again
67-74	(8%)	7 hours		Odd Numbers:		– 1/2 hour

     ★ The first Wind event of the day has a rolled duration threshold. It generally starts at around 4AM.

     ★ The second wind event lasts from the end of the first until 4 PM or for a new duration roll, whichever is later.

     ★ The third starts at 4PM+ and has a rolled duration.

     ★ The 4th starts at the end of the third and has a rolled duration or until 4AM, whichever is sooner.

     ★ The 5th event, if any, lasts until 4AM.

Let’s do an example or two.

EG1:
     ▪ Wind 1: 25 km/h North. d% roll = 74. Rolled duration 7 hrs. Starts approx 4AM, lasts until approx 11AM.

     ▪ Wind 2: still 25 km/h, now to the North-north-west. d% roll = 58, duration = 6 hrs i.e. to 5PM. This is later than the 4PM cutoff but it still applies.

     ▪ Wind 3: base direction has changed from NNW to SSE. Wind is 18 km/h and to the South. d% roll = 96. Duration = 11 hours. Starts at 5PM so that’s until 3AM.

     ▪ Wind 4: At 3AM, there is a wind shift to the ESE. Wind remains 18 km/h. It doesn’t matter what the d% roll is because at 4AM it will be the start of a new day with a new wind roll – according to the RAW.

       But if I were the DM, I’d rule that the day started an hour early (in terms of wind), just as the night started an hour late, and roll a duration starting from 3AM: d% = 49. Duration 5 hours, less 1/2 hour for the odd number result. Starting at 3 AM so lasting until 7:30 AM. And I would list this as Wind 1 of the next day.

EG2:
     ▪ Wind 1: 11 km/h ESE. Started at 4AM. d% roll = 77, duration 7.5 hrs, until 11:30 AM.

     ▪ Wind 2: still 11 km/h, shifted clockwise 0 steps relative to the adjusted base direction (East) – so wind shifts to the East. d% roll 40, duration of 4 hrs, from 11:30 AM to 3:30 PM.

     ▪ Wind 3:
still 11 km/h, shifted clockwise 5 steps from the base – so a big shift to the SSW! d% roll = 16, so Duration is 3 hrs, from 3:30PM to 6:30 PM. This is past the 4 PM wind change, which is delayed accordingly.

     ▪ Wind 4:
31 km/h. Baseline shifts from East to West. Wind direction is 2 steps counter-clockwise of that, to the Southwest. In effect, this is a 1-step change in wind direction. d% roll =53, so duration is 4 1/2 hrs from 6:30 PM to 11 PM.

     ▪ Wind 5: From 11 PM until 4AM, when the next baseline shift takes place, the wind is from the South, i.e. at right angles to what would normally be expected. It continues to be pretty strong at 31 km/h – officially “Fresh”. This is fast enough to cause considerable windchill, but that’s a subject for another day.

4.8.7.14.5 Favorable Winds

Favorable Winds are winds that move a vessel in the direction it wants to go. They don’t have to be perfect; just good enough for the sails to function properly.



Above are 5 panels. The first one illustrates what was described earlier – the favorability of the wind and the impact of extra masts on those values.

Once you get into the red / dark colors, you are (at least in part) trying to sail into the wind. That… doesn’t really work (see 4.8.7.14.7 unfavorable winds, below).

Again, the ratings are:
     Favorable 30°+ (70%+)
     Near Favorable 30°+ (50%+)
     Tolerable 30°+ (30%+)
     Intolerable 90°- (0 or 10%).

Extra Masts (each):
     +5% Favorable efficiency
     +7.5% Near-favorable
     +3% Tolerable efficiency

     +10° Favorable (to either side), first additional mast
     +5° Favorable (to either side), other additional masts
     +0° Near-favorable
     +10° Tolerable (to either side), first additional mast
     +5° Tolerable (to either side), other additional masts

The figure depicts a two-masted ship, so let’s work some numbers:
     ▪ Favorable: 70%+5% = 75% efficiency, Arc 30°+10°=40° to either side.
     ▪ Near Favorable: from 40° to 70° on either side, efficiency 57.5%
     ▪ Tolerable: 30+10=40° arc from 70° to 110° on either side; efficiency 30%+3%=33%.
     ▪ Intolerable: forward 70° to either side.

Here’s a quick comparison between 1 and 2 masts, showing exactly how much a vessel gains:



Here’s the fun part: Our wind direction system uses increments of 22.5° – or, more accurately, ±11.25°. The ship operations subsystem does not – people might consider that an oversight, or a mistake. This paragraph is here to correct any such impression – this is a deliberate choice because it makes the results a bit rubbery and flexible. The GM decides where in that ±11.25° the wind is actually blowing – and hence, at times, how favorable the winds are.

Some things are covered automatically – 22.5° to either side is less than 30°, but 33.75° is not – but unless the GM is deliberately nasty, players would reasonably expect both winds from directly behind the ship and any from one step to either side to be “Favorable”. It’s when winds stop being Favorable that this flexibility comes into its’ own.

Getting back to the main illustration, panel 2 shows a ship attempting to reach a port along line A1. The problem is that – as shown by line B1 – the closest significant land is West of the ship, and not in the desired direction of travel. And that means that the base winds are from the direction arcs shown.

Now, with 2 masts, there’s a 50-50 chance that these will still be at least Tolerable, and possibly even Near-Favorable, though that’s unlikely. So progress would be slow, but possible.

But time is money, and there is a faster approach. Note the dashed line – that’s the dividing line between the land to the North being the nearest significant landmass, and the land to the West. If the captain knows what he’s doing, he will try to position his vessel on the right-hand side of that dashed line, as shown in the third panel.

A2 is a shorter distance to land than B2, so the base winds are now oriented north and south. During the daytime winds, no progress might be possible, but when the wind shifts in the later afternoon, the captain can ride them all the way into harbor.

But time is money, and a smart captain will realize the situation shown in panel 4, in which the zone of maximum efficiency of his particular vessel has been mapped onto the harbor entrance, creating a zone between that and the dashed line. So long as he ends up anywhere in that zone when the winds shift, he can reasonably expect them to be favorable.

Panel 5 shows the results of what can be done by a captain who is smart enough – even starting from the user-unfriendly initial position. Starting the previous night, the captain steers Northwest. If he can manage a 45-degree course, half his movement is in the direction he wants to go in Leg 1.

When the wind shifts, the captain turns 90° to port, so that he is now heading North-east. Again, about half his movement is in the direction he wants to go, and so far, everything has been at maximum efficiency, i.e. the best speed possible.

That stops being the case when the ship crosses the dashed line, because the winds shift to a base north-south configuration, and the daytime is in the wrong direction. If he has enough masts, though, he can continue going a little bit north of east. or he can turn to head due East until the afternoon change in base wind direction. The whole purpose of the day’s maneuvers is to ensure that when this happens, the ship is within that shaded zone – and favorable winds will then generally put the ship into port in short order.

If you can predict the wind, however vaguely and uncertainly, you can manipulate the situation to your advantage.

4.8.7.14.6 Becalmed

Beaufort Scale 0 and 1 is not enough wind to get any ship moving. You are Becalmed, and the only thing you can do is wait and hope.

Beaufort Scale 2 is enough for Pinnaces and Sloops. Everything else is still Becalmed, and not going anywhere.

Beaufort Scale 3 is enough to get most vessels moving, however slowly – but ‘most’ is not all. Vessels of significant inherent weight, or that are fully laden with cargo, may not quite get going – that’s up to the GM. They have to wait for at least Scale 4 winds.

4.8.7.14.7 Unfavorable Winds

The only thing worse than being becalmed is having the wind come from completely the wrong direction. With Barques and Pinnaces, they have sail configurations that can overcome this problem (see 4.8.7.15.7 below) – everything else has a choice: drop anchor and furl sails and wait it out, or furl sails and deploy oars. Lots of oars – see 4.8.7.15.6.

Sometimes (but not always) it is possible to position the ship so that the winds are at least Tolerable, enabling you to keep moving, even if only slowly. If the winds are strong enough, you might even get a decent rate of speed up. If even a little of the motion is in the desired direction, you can tack back and forth, zigzagging your way to where you want to go.

Some vessels are better at this than others – sloops and Light Frigates and Extreme Clippers, for example, are very good. Beyond those three, it’s all about turn mode – if you ship is not very maneuverable, you can’t tack very well. As a general rule, the Turn Mode doubles the number of lengths of the ship that it takes to execute a 90-degree turn (1/3 this for a 30 degree turn, 1/10th for a 9-degree turn – you get the idea).

So a turn mode of 5 is twice as bad as a turn mode of 4, which is twice as bad as a turn mode of 3, and so on.

On top of that, the GM can add penalties for a ship being large / over-sized, or for a ship being fully laden (+1 turn mode for either), and for any sail damage. (A 50% damaged sail has only half the usual efficiency).

4.8.7.14.8 Violent Winds

Did I really say that the only thing worse than being becalmed was Unfavorable Winds? Well, from a plain English point of view, that’s a valid statement – but ‘Unfavorable’ has been given a specific, restricted, meaning within the system, and by that usage, even worse than winds coming from the wrong direction are winds strong enough to damage the vessel.

This is where being big helps instead of hindering; the presumption is made that heavy ships are stronger. This alone can be enough to drop the effective Beaufort number by 1 for the purposes of dealing with the effects of violent winds.

     ★ Square the Beaufort Number, multiply by the number of masts, then divide by the ship class number to get the number of dice of damage that the wind inflicts every 30 minutes.

     ★ Ships can ignore 1.5 points of damage for each class number.

EG1: Beaufort 5, three masts, Class 23: 5^2 = 25, × 3 = 75, / 23 = 3.26 d6, round down to 3. Maximum result of 18, vs Resistance of 23 × 1.5 = 34.5 (round down to 34). This is not going to damage the ship.

EG2: Same ship, Beaufort Number 7. 7^2 = 49, × 3 = 147, / 23 = 6.39, round down to 6. Maximum result of 36, so there is a possibility of slight damage.

EG3: Same ship, Beaufort Number 9: 9^2 = 81, × 3 = 243, / 23 = 10.56 d6, round down to 10. Maximum result of 60, average result of 35; vs resistance of 34 – the ship will take damage, and enough damage can sink it.

Furling sails reduces damage to sails and masts to 10%.

4.8.7.14.9 Extreme Winds: Hurricanes etc

Beaufort Number 10 Winds are when the trouble really starts.

Instead of squaring the Beaufort Number, Square double it (or simply multiply the number of dice by 4).

EG Same Merchantman, Beaufort Number 10: 10^2=100, x4 = 400, x3 = 1200; / 23 = 52.1 d6, round down to 52. Minimum result 52 vs resistance of 34 – ever 30 minutes, the ship will take damage. Average result, 148 pts over resistance. Maximum result 278 points over resistance. Furling sails reduces actual damage to 27.8 points, maximum – so no damage. Don’t expect that to be the case at Beaufort numbers 11 and 12, however.


4.8.7.15.6 Unfavorable Winds / Currents – Oarsmen Requirements

     ▪ Divide the total weight of the ship and cargo by the number of oarsmen.
     ▪ Divide their average Carrying Capacity by the result and multiply by 264 to get it in feet / minute.

     ▪ Divide that by 3.281 to get m / min.
     ▪ Divide m / min by 26.82 to get mph.
     ▪ Divide mph by 1.609 to get km/h.

Stop whenever you have a unit of speed that is appropriate to the scale of your maps.

Another way of looking at it: Total Carry Capacity of the Oarsmen has to exceed the total weight of the ship and cargo or it’s not going anywhere under oar power.

4.8.7.15.7 Unfavorable Winds / Currents – Sail Solutions

As mentioned, Pinnaces and Barques have a sail configuration that lets them sail straight into the wind – albeit slowly.

But I did once hear about an enterprising if desperate crew in a pirates game who cut down two of their three masts on a merchantman so that they could sew the sails into an over-sized version of this solution, improvising the rigging required. I forget what the GM in question rules, but in my book the inconvenience and loss of efficiency caused by the loss of the two masts was quite enough to make up for permitting this solution to work. But, given that the ship now had less than the usual minimum number of sails, efficiency would have been compromised accordingly.

Still, desperate times…

4.8.8 Currents & Waves

Being becalmed may not be the end of the world. Currents alone can provide motive power, although far less speedily than wind power.

Currents flow not only horizontally but vertically, cycling up and down as they travel. To a ship on the surface, this effectively means that they wax and wane.

The Wikipedia page on Ocean Currents has a long list of currents that you can draw upon for further information, and there are more detailed maps available, too.

I’m not going to go into all the effects that they have; all I’m concerned with here is their direction and speed.

It’s important to note that there can be different currents at different depths, just as there can be different winds at different altitudes at the same time.

Surface currents are predominantly caused by wind, and – where possible – will be in the same direction. However, they have to turn away from that direction if there is land in the way, and the direction they turn is away from the equator – unless there’s a land mass between the wave and that geographic feature. But the shape of the land and all sorts of other factors can complicate this simple principle.

     ★ Look for the largest, strongest current in the vicinity, even if it’s not directly affecting the vessel’s location.

     ★ If it IS affecting the vessel, job done.

     ★ if it isn’t, one of two things will happen: a side current, or a reverse current.

A Side Current is basically a smaller curve looping off a larger one. They are a LOT smaller and weaker than the main current.

A Reverse Current interacts like two gears, with one gear being a side current:

Well, actually what happens is that variations in the major current creates a short-lived low pressure zone which gets filled with water from surrounding layers – but more tries to rush in than will fit, and this movement of water has momentum, and as the variation travels around the main current, additional water pushes against the water already moving in the other direction, and you end up with a much weaker current going in the reverse direction of the main current.

You can also get side-currents within a reverse current:

And, anywhere where the land varies greatly in its proximity to the main current, you can also get reverse currents:

… so it’s complicated.

The strongest current on earth is the Antarctic Circumpolar Current, which averages 10m / second (600m / min = 36 km/h), 5 times the strength of the far better-known Gulf Stream. Lesser currents range down to a couple of knots – refer to the Beaufort Scale tables to see just how low that is. The Gulf Stream and the Kuoshio Current are fast currents, moving about 2m per second (120 m / min or 7.2 km/h).

Tidal currents are caused by the positions of Earth and Moon and vary greatly in strength, even from one hour to another. Some locations experience much stronger / greater tides than others and stronger tidal currents are one of the results. Strong Tidal Currents can reach 8 knots or more.

Deep water currents are driven by density differences, and variations in water temperature and salinity (both of which trigger density differences in water). They move much slower than Tidal and Surface currents (but I couldn’t find an exact number, so I’m going to assume from the “much” that we’re talking about more than the Gulf Stream to Antarctic Current ratio and assume a factor of 10).

     ★ Using the above as a guide, estimate the strength of the major current in whatever units are most convenient.

     ★ A side-current will have about 1/4 the speed / power.

     ★ A reverse current will have about 1/10th the speed/power.

     ★ A side-current of a reverse current will have about 1/25th the speed/power.

     ★ A reverse current powered directly from the main current will have about 1/4 the speed/power of the main current.

     ★ A tidal current can add to another current, subtract from it (possibly overwhelming it), or be off at an angle to it, producing a compound current:

The diagram above demonstrates 12 different compound currents.

Figure 1 shows a Tidal Current adding to a Surface Current in the simplest possible configuration, exactly the same line.

Figure 2 shows the opposite, with the tidal current opposing the surface current. In this case, the tidal current is not enough to completely cancel out the surface current.

Figure 3 makes the Surface Current weaker and the tidal current stronger – now the tidal current is diminished but not overcome by the surface current.

Figure 4 shows a slightly more realistic situation in that the two currents are now in different directions. The results are our old friends vector sums.

Figure 5 uses a stronger tidal current for comparison to figure 4.

Figure 6 shows a slightly weaker surface current and a tidal current of the same strength but at an acute angle.

Figure 7 has the same configuration but a much stronger tidal current – so much so that the compound current is more than at right angles to the surface current.

Figure 8 has the same surface current but this time the small tide is at an oblique angle. The compound current is stronger as a result.

And Figure 9 shows an increased Tidal Current but is otherwise unchanged. The compound current shifts direction but only by a little, but it becomes much stronger.

Figure 10 is the first of three multi-hour compound charts. In this example, I’ve kept the surface current the same and varied the tidal current – again at right angles to the surface current. It starts strong, weakens, then the tide starts to go out, and finally, it becomes stronger again.

Figure 11 has the same configuration but with a surface current that is also varying – probably a bit more than normal, to be honest.

Finally, Figure 12 varies the angle of the surface current as well as it’s strength, and is the most realistic demonstration of the compound current that results.

     ★ Use the information provided to estimate the average strength of the respective surface and tidal currents.

     ★ Select the next smallest die size and roll it; subtract the result from the current’s strength. EG 7 knots minus 1d6 because 6 is the next smallest result from 7.

     ★ Using the diagrams, visualize the resulting compound tide’s direction and strength. Don’t worry too much about accuracy.

     ★ The longer the ship, the greater the effect side-on to the current, maybe as high as 25% efficiency. The wider the ship, the greater the effect when running with the current, maybe as high as 10% efficiency. In both cases, it can dip to as little as 3% going the other way.

     ★ The faster the ship, the more sleek and narrow it is. The larger its cargo capacity relative to its length, the broader it is. Using this information, guesstimate the proportions of the ship, and from them, the amount of speed that the currents will impart to it.

I’ve saved the above image until close to the end of this article for a very good reason: take a look at the surface of the sea depicted, and you can tell that the current and wind are operating in different directions from the shape of the foam caused by the breaking waves ahead of the storm. Initially, you can’t tell which one is from the left and which one is coming rear-to-front or vice-versa, but look at the sails and the way they have billowed out and it becomes clear – the ship is running with the wind and across the current, exposing the broad side of the ship to that current, which will be pushing it just a bit left-to-right.

4.8.8.1 A brief note on Sea Anchors

A lot of people think that sea anchors work by digging into the sea floor. Such people have no appreciation for the depth of the sea, and how much chain would be required to lower the anchor that far, and how long that would take, and how much it would weigh.

Nope, not happening. It’s the weight of anchor and chain dangling in the water that slows a ship’s movement (it doesn’t stop it) – because the subsurface currents are so much weaker, and potentially in a completely different direction to the surface currents.

That means that the anchor can’t move as fast as the surface current tries to move the ship, and so it holds the ship more-or-less in place, the chain snapping taut. If the surface current is too strong – which can happen in a hurricane, say, or anytime wind and currents combine – it can even snap the anchor rope.

Anchors can and do drag ships, and if there’s a hazard close by in that direction, and the current in which the ship is anchored is strong enough, captains may be forced to cut away the anchor and let it – and most of the chain – fall to the bottom in a bid to save his vessel.

4.8.8.2 Favorable Currents

As with winds, favorable currents are currents that push the ship in the desired direction. Currents are often best viewed as a modifier to the direction of wind – the stronger the push on the ship, the stronger the impact, but it all has to be relative to the wind speed.

More accurately, the speed that the current imparts to the ship has to be assessed relative to the speed that the winds impart to the ship, so both are a question of the efficiency of this conversion. The effective force of the wind depends on the amount of sail being presented to the wind and the relative angle of the wind to the ship; the more unfavorable the wind, the more scope there is for currents to have an effect.

4.8.8.3 Still Currents

Still currents – the equivalent of being becalmed – are fairly rare, but they do happen – especially if a ship finds itself in a situation of current shear, where two currents ‘rub shoulders’ but flow in opposite directions. Take another look at the illustrations of side currents, and you can see that where two side currents exist side to side, this can be the case. However, it requires that the main current also be weak (because it moves those side currents with it).

Unless you are also becalmed, this won’t bother captains too much. When the winds have also dropped, though, it’s time to think about which of the maritime gods you may have offended, and what you can do about it.

4.8.8.4 Unfavorable Currents

Obviously, UNfavorable currents are those that push the ship into danger, or that oppose the winds, in whole or in part. A ten-knot current – very strong – can (if it’s in the wrong direction) slow your forward speed by… a couple of knots. Keep things in perspective – currents are a minor influence compared to winds, unless and until you become becalmed or furl the sails.

4.8.8.5 Violent Waves

Big waves, on the other hand, can pose an existential threat to the ship. The density of sea water varies from 1020 to 1029 kg / m^3. So, a wave 20m long and 1m high is essentially a shape like one of those below:



Wave shape #1 is low-velocity, gentle, rolling. It has a volume of 1 × 20 × 1/2 × 2, 3, 4, or 5m – so 20-50 m^3. It weighs 20500-51250 kg. Perhaps tonnes? 20.5- 51.25 of them. Or short tons? 22 – 56.5 is the number.

But the heavier the wave, the slower it’s moving, so the less damage it will do. We’re talking nothing more than Beaufort Scale 3, and probably less. That’s a wind speed of 5.4 m/s, tops, and 1/2 m v^2 gives kinetic energy of 1/2 × (20.5-51.25) × 5.4^2 = 10.25-25.625 × 29.16 = 299-747 whatevers – I’ve mixed units terribly for convenience, but that doesn’t matter so long as I use the same ones for the other shapes.

Wave shape #2 shows a wave not yet breaking but being pushed by the wind. It has a volume of 1/2 × 1 × (2.5-3.5) × 20 = 25-35m^3 and a weight of 25625 – 35875 kg or 25.625-35.875 Tonnes. These are the waves you get from Beaufort scale 5 winds, maybe 6, so 13.9 m/s. Kinetic energy = 1/2 × (25.625-35.875) × 13.9^2 = (12.8125-17.9375) × 193.21 = 2475.5 – 3465.7 whatevers – so the wave weighs about as much as the middle value for wave 1, but hits with 5-8 times as much energy.

Wave shape #3 shows breaking waves, and 1m tall is rather on the small side for them. It is presumed that the two shaded areas are roughly equal in size, so this is still (effectively) a simple triangle. So the volume is about the same as shape #2 and so is the mass. Here, we’re talking Beaufort Scale 7 or maybe 8, and 17 m/s winds. Kinetic Energy is 1/2 × (25.625-35.875) × 17^2 = (12.8125-17.9375) × 289 = 3702.8 – 5184 whatevers – almost 1 1/2 times as much force as wave #2 and 7.5 – 12 times what the gentle waves were doing.

But if I scale that up to what the wind speed says the waves should be, in height: 13-19′ waves = 3.9624 – 5.7912 m in height = 4 – 5.8 times as much volume = 4 – 5.8 times as much mass (102.5 -208 Tonnes) = 4 – 5.8 times as much kinetic energy, or 6 – 8.7 tomes as much as Wave #2 – and 30 – 70 times as much as the gentle waves.

That’s a Blue Whale at 38 mph hitting the side of the ship – every 2 or 3 seconds.

And that’s just the edge of the dangerous winds part of the Beaufort Scale. A full-scale hurricane, with 34 m/s winds, would be 4 times as bad even if the waves were no larger, which they would be, by a factor of almost 2.75. Between them, around 11 × the striking power of Wave #3.

And such waves would almost certainly be longer than the 20m used here – potentially as long as the ship, which is as long as it gets.



Some pictures have been included for purely decorative, evocative, reasons!
This image is by Elias from Pixabay

4.8.8.6 Extreme Waves: Tsunamis & Walls Of Water

But the dangers at sea don’t end there. Tsunamis are really only a problem at port – the wave as it travels across the ocean is really quite gentle but extremely fast moving and containing a LOT of displaced water, which (in turn) is being pushed by the water behind it, and so on.

Once it hits shallows, though, there is nowhere for all that water to go but up into the air, and that’s when you get an appreciation of how much power it has. Whatever isn’t nailed down – including apartment blocks – is up for grabs. Ships will find themselves bottoming out on the sea floor as the normal water retreats out to see, and then being crushed beneath thousands of tons of water, being carted miles inland if they manage to survive it – before being swept back out to sea as the water retreats.

But Tsunamis don’t frighten most sea captains.

Rogue Waves are a whole different story.

These are like two sets of percentiles and getting 00 on all of them. Every little thing adds up to create a wall of water more than twice as tall as the waves around them. Like Tsunamis, they can also move at ferocious speed and in directions other than the prevailing winds and currents.

The biggest one ever recorded was 58 feet tall.

In most parts of the world, rogue waves are roughly 3 in 10,000 – but in some places, they can be 3 in every 1,000, because the shape of the land controls the shape of the water and focuses the energy of the waves.

They travel at 500 mph (800 km/h). At that speed, they can strike a wooden vessel and leave nothing but kindling without even noticing. They do slow down when approaching land – which only makes the water coming behind the wave front pile up on top of it, producing an even bigger wave.

It’s often speculated that, like several other weather phenomena, there is a wind equivalent of a rogue wave. Anyone who has seen footage of an Australian Dust storm hundreds of kilometers long, a wall of brown in the sky, has plenty of reason to believe it’s true.

Rogue Weather can and does scare any captain that knows about them.

4.8.9 Other Applications of the Weather System

The main weather system – thresholds and change accumulators – can be used to simulate a vast range of other phenomena, both natural and unnatural. Everything from meteor strikes to stock markets to the price of coconuts.

In fact, anything that has a cyclic nature of some kind, where trends accumulate until something has to give or a tipping point is reached, can be modeled in this way.

That’s because it’s built on the concept of variable length cycles operating concurrently – sooner or later, you’ll ht the jackpot.

Most of these phenomena are ferociously difficult to simulate, so if you need to track one, you could do a lot worse than carving out a variation of the base system offered here.