I am nearly done with the system as it stands so far. It isn't finished. All that I've designed so far does work, however; and there is room for adding the little details like tornado and such - though I need to think on the best method for that. Right now, we can finish up the fundamentals; for what it does, the system stands on its own at this point.
Step 6: Storms
There are four kinds of storms from the list of conditions described yesterday: condensation, thunderhead, wind storm and rain clouds.
If there is a weak point in this system right now, I would say that it is that thunderheads occur more often than rain clouds, due to the pair system I've designed. I can think of ways I might tweak that - but for today, let's just go with the system as is. Rain is rain. Let's deal with each type of storm one at a time.
Storms produce precipitation or, in the case of wind storms, particulate matter. The wind 'storm' is the downburst, remember, so it is responsible for throwing up dust and sand, for the kind of storms characters don't want to be in. Most of the time, however, a wind 'storm' will exist in fair conditions - which in this system means that the air is clear because it isn't dry enough.
These storms are connect to the amount of precipitation and the wind storm is no different. If the average rainfall for the month is below 15 mm, the ground is considered very dry and the wind storm will include blowing grit. These are the kind of conditions where the visibility is relatively clear but it is difficult to see comfortably due to particles in the air.
If the rainfall is half as much, 7.5 mm, the blowing grit will be upgraded to a dust storm. This is even less pleasant. Full on dust storms will never occur in many parts of the world. At some point, there would be a way to adjust the average rainfall for a given region, so that a given month of a given year wasn't a static number, but that's a future notion. For now, if the players aren't in a part of the world where it is very dry, a dust storm won't happen.
If we half the rainfall again, so that it is no higher than 3.75, then we have a sandstorm. In some parts of the world, like the upper Arabian peninsula (such as around Palmyra), there isn't enough sand to create a legitimate sandstorm, but we can treat it as an intensified duststorm. There are months of the year when Palmyra and other nearby places get zero rain for two or three months. Those times of the year would be full of sandstorms.
I mentioned yesterday that I was treating 'fog' as a storm. That is simply convenient. Fog is a kind of precipitation and for our purposes covers a wide range of possibilities.
The trickiest thing about precipitation is accounting for the temperature, for at certain temperatures fog and rain manifest as other conditions. Nor is it as simple as saying that drops falls as either rain or snow; there is a mid-point there where it falls as sleet. Moreover, there is room for describing the types of snow, based on the total amount of precipitation . . . but I have decided not to include that yet. That design, too, is for another day.
To determine if fog occurs when 'condensation' conditions are indicated, we roll a percentile die under the amount of rain in mm. This is the reason for using mm and not inches. I am judging that any part of the world that drops more than 100 mm in the space of a month will produce precipitation if the conditions are right (LH, HL, etc.). I could as easily make it a d120 roll, as this is excel, but 100 is a nice, round number.
If the number is below the amount of rainfall that occurs, then fog will happen. Palmyra in February, remember, has a precipitation of 18.6 mm. That means an 18% chance of fog when condensation occurs. Since condensation as a condition occurs rarely, fog is equally rare in Palmyra (and would likely last just a few minutes in the morning when it did occur - again, time not included in the system yet).
However, if the temperature is less than -35F, something that never happens in Palmyra, then the fog is frozen fog. Between -35 and 27 above zero (Fahrenheit), the fog never condenses in the air but it does produce hoar frost. A little bit warmer, up to 33F, and this becomes rime. Finally, above 33F we get fog.
But if fog doesn't occur at all, there is a chance we will get mist. Mist has an equal chance of occurring if fog has failed to appear. If the temperature is 33F or less, then mist manifests as simple frost.
If we get neither fog nor mist, there is an equal chance again for a thin mist (typically just above the ground, having no effect on visibility. Again, if this happens in 33F or less, this thin mist manifests as a 'freezing,' which describes where frost is so thin it is merely a slight layer of crystals that have frozen over a surface, making it cold to the touch but producing a layer that is relatively invisible.
Finally, if all of the above fail to occur (and in Palmyra that is any roll above 55), the result is dew; in 33F or less, this translates as a thin cold surface that will melt from body heat.
Like condensation, the results here are greatly dependent upon the temperature and the likely amount of rainfall. Primarily, however, it is the type of cloud determines the kind of precipitation.
Altostratus clouds, which are quite high, will produce falling ice crystals below 33F and spattering rain otherwise. The chance of this happening is again a d100 calculated against the mm of rain at that station in that month. A failed roll produces no rain at all. Some might remember that altostratus are the only kind of rain clouds that occur in desert conditions.
Stratus clouds always produce rain if the conditions are right. However, the type of rain will depend on whether or not the d100 is less than the number of average mm for that station for that month.
If the roll is positive for the stratus cloud, then the rain will be enough to wet the environment. If it is 33F or less, then this will fall as a snow flurry, medium sized snowflakes that fall quickly or are blown. Otherwise, the status cloud will provide pelting rain, being hard, small drops.
If the roll is not positive for the stratus cloud, then it will either be tiny, hard snowflakes or spattering rain, depending on the temperature.
Remaining rain clouds will be cumulus clouds, or cumulonimbus on yesterday's cloud image. To determine the intensity, the d100 is rolled against the average mm x2. (For Palmyra, that would equal a 37% chance - 18.6 x2).
A more intensive precipitation will either yield a snowfall or a shower, each of which will either lay 1-4 inches of thick snow or thoroughly drench the environment. A less intensive precipitation will either produce a softly falling snow (large flakes, typically an inch of snowfall) or a drizzle.
These are typically nimbus clouds (either stratonimbus in cold climes or cumulonimbus in warmer climes), producing lots of rain. First, we again roll a d100 to determine the intensity, with the average monthly mm x2 (37% again for Palmyra). A success would indicate one of three types of precipitation, depending on the temperature.
If less than 31F, this is a blizzard, a full on snowstorm with low visibility; thunderheads tend to include a high wind as well (generated by other means, the reader will remember). Between 31F and 33F, this falls as sleet - wet, slashing snow-rain that is very unpleasant. Above 34F but below 69F, this is a quiet downpour (it isn't warm enough for thunder and lightning), which will drop more rain than a rain cloud shower but won't look much different from the ground. Finally, at 70F and above, we have a full thunderstorm.
An unsuccessful roll against the average monthly rainfall will indicate that there are dark storm clouds overhead, but no rain will fall before these clouds move on.
In a desert climate, thunderheads manifest as cumulus clouds. Like with rain clouds, these will either produce ice crystals or spattering rain (if a d100 is successful, dependent on temperature) or merely pass over as dark clouds.
This covers storms for the time being. I can and will expand this, probably one feature at a time, but for the present it is clear that different conditions/temperatures will produce distinctly different precipitation. This is all programmed in, so if I change the temperature the excel does all the work for me (no tables to look up!).
Ultimately, I'd like to write a list of effects for each kind of precipitation (and a lot of other features as well), to make this more detailed and precise in-game. Last night I did a little work on pleasant temperatures and warm temperatures. Worth a look; the beginning of a long series of wiki pages.
The last post in this series will be up in not very much time.