Step 5, rain patterns
So, here comes the rain.
This is the trickiest and longest task and it takes some time. Geoff's Cookbook has 8 different factors to include and we will cover them all in a layer upon layer technique to add them together. As before, this needs to be done twice, for the extreme months of January and July, and the previous layers of sea currents, atmospheric circulation and dominant winds will all be used.
Also, you need to have a good idea of elevation. Not necessarily an over-detailed map like the one I'm using in this example, but something with a little more detail than just the linear super-ranges. If you haven't got anything else than this, you can make a draft layer marking other ranges and elevated areas (plateaus, like southern Africa, East Africa, Iran and Turkey, Central Brazil, have climates specific to their elevation, so you can't ignore that sort of terrain completely)
Now, first thing to do is to create a few layers to work on, you can call them
- baseline rain
- ITCZ, extreme
- ITCZ, moderate
- onshore winds
- mountain rains
- warm coastal currents
- polar front
I am going to break down what to do for each layer, then we turn all this into a single layer mapping rain in colors ranging from dry to very wet, which we will relate later to Koppen's climates.
1. baseline rain
All the layers will be colored with one color only. I use a not-too-bright blue, RGB:100.100.150. Brighter colors will make the end result hard to read.
For the baseline rain you just select one very big area (the whole map if you want) and bucket-fill it with this color. Then toggle off the layer and toggle on atmospheric features. Make a selection (using the lasso tool) around the high pressure centers (stretch it further east and west than the actual centers drawn before). Don't loose the selection, toggle off that layer and toggle on the dominant winds. Adding to the selection choose coasts where the wind direction is offshore and the areas behind mountains that winds cross (normally called the rain shadows of those mountains). Still, keep the selection. Toggle off the winds. Go back to the baseline rain layer and just press delete. Now your initial baseline rain has gaps big and small. Toggle it off again so you can clearly see the land. Make a selection encompassing all land that is far away from the coast or protected from coastal influence by mountains. Back to the baseline rain layer and delete that as well. The end result, for me, was this:
Toggle it off, and leave it there...
2. ITCZ, extreme
Same process, but this time way faster. Toggle on your atmospheric features layer. Make a selection that covers the ITCZ you marked before. Bucket fill this with the same color as before, in the appropriate layer. It's done. Here's what it "could" look like:
In hindsight, I may have made it a little too narrow. Because it's a simple single color area, it's easy to adjust and you can always go back at any point.
3. ITCZ, moderate
The ITCZ isn't a very defined area and its effects may be both broader and stronger than any other effect, so we have a second layer for it - no other factor weighs as much.
We're already in the third layer. Make a much wider selection than before (I'd say, at least twice as wide) and bucket fill in this new layer.
4. Onshore winds
Obviously, you need to view your dominant winds layer for this, but also the elevation map. With those layers visible search for every piece of coast where the dominant wind blows inland from the sea. Select the area where these winds would carry clouds/moisture into. If it's a wide plain, the whole thing, if it's a mountainous coast, only up to the mountains, if they blow exactly onshore, further inland, if they are parallel to the coast, only a thin strip of land. Islands, it's plain to see, get onshore winds whichever direction they blow, so cover them in your selection as well.
My example works out this way:
5. Mountain rains
For this one, you need to see the dominant winds and the elevation map as well. Wherever winds meet mountains, select the areas just before the mountains. The extent of those areas will depend on the kind of mountains, but the amount of rain would also depend on the kind of slopes, so don't worry about this too much (we need to keep it practical to a certain level). If a dominant wind is coming from a particularly dry area (a hot inland region), ignore it in this step as it wouldn't create rain even against an everest style concrete wall).
Plateaus also work as barriers for this, although you need to extend the selection somewhat into them and not just "before".
If the layers are looking a little strange to you, don't worry. In the end they'll get added in a neat way into a rain patterns map.
6. Warm coastal currents
These influence the weather in a large area. Unlike cold currents which are narrow and stick in the coast, warm currents are more spread out and the moisture they carry gets transported by the dominant winds. So toggle on the sea currents and the dominant winds layers. Your selection will encompass the red arrows you drawn for currents but stretch it into the areas where warm currents meet cold currents and also in the direction the winds are blowing in the area. A warm current with onshore winds and a deep coastal plateau will generate a very large area of influence.
In the end, without loosing the selection, toggle off the current and winds layers and have a look at the elevation map. You may have to cut down the selection a bit if you went inland over mountains - those areas should not be influenced by the oceanic affairs.
This layer ended up like this for me:
7. Polar Front (lastly, but not least)
The Polar front has a broad effect as the low pressure centers that form in it are prone to move about and form in different locations. So all you need to do is to select a wide area around your previously drawn polar front (as seen in the atmospheric features layer). Now, these low pressure centers make an eastward movement between birth and death so they normally carry more moisture when they enter the west side of a continent than on the eastern coast. Because of this, make your selection wider in the West coast, stretch it inland but narrower over the distance and, eventually, non-existent on the east coast (this depends on continent width and on mountainous terrain in the way).
Because this continent is mainly tropical/sub-tropical, the PF has virtually no influence on its weather... less to worry about.
And so the different factors have been considered. Apart from the ITCZ, which we accounted twice, the assumption is that they all have the same weight, which is naturally wrong, but it's a compromise we can work with. I'll continue in the next post to break give you a break.
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