Step 3, outline of atmospheric features
From now on, all steps will come in pairs. That is to say, you need to make maps for Winter and for Summer. But best call it July and January - they represent the extremes in terms of how far south or north one can find the hottest areas on a planet and not the typical temperate seasons.
So, new layer. And you can toggle off the currents one. This new one could be called "January weather outline" and will contain the major features in the atmosphere.
The main features are the areas where air rises, causing rain, and where cold air from the upper atmosphere sinks, causing blue skies and moisture free air. In simple terms, here's what matters for the map:
- Close to the equator is the Intertropical convergence zone (ITCZ). Humid air from both tropics meets in the equator and rises, releasing it's water content. Draw this as a very blurry line as it isn't a well defined place. Inland, the ITCZ moves up and down according to the seasons (it is at is northernmost point in july and at its most southern in january). In the oceans it hardly moves because the surface temperature of the oceans varies much less. The ITCZ has a year-round bias towards the hemisphere with large continents, if you have those - this happens on Earth, with Eurasia. On the other hand, if you have large oceans, the ITCZ might split into two, with a clear sky area just over the equator - this happens on Earth in the Pacific.
- Close to 60º we have the Polar Fronts (PF). This is even less defined than the ITCZ and moves southward/northward during the year in the same manner. Here, the cold air from the poles meets temperate air and again, clashing air masses will rise and make rain. You should now have three East-West lines.
- Around the latitude of 30º, on every ocean, a high pressure center will form. This is where the dry air sinks. Those high pressure zones are more or less stable around the year but they also move up and down as the seasons progress. They tend to stick to the eastern side of any given ocean, not far from where the cold currents moving to the equator appeared on the previous map.
- Contrary to the high pressures at the oceans, the warmest areas of a large land area will be indeed very hot and that causes the local atmospheric pressure to drop. This generates low pressure centers, which only form if the land mass is pretty big, at latitudes close to the tropic lines but maybe as far as 45º (equal latitude to the tilt of the planet, I'm assuming 23º as on Earth).
- In this particular map we can't spot these ones. But, usually, terrific high pressure centers are found at the center of the continents when at their coldest - which means, during Winter. On Earth this happens only in the core of Eurasia, north of the Tibetan plateau.
... January
New layer for July...
I struggled a bit to envision the movement of the ITCZ in this continent because of the huge mountain range on the eastern side. There were two options: either it shifts between north of that range and south of that range during the year or it is an area where it is stable throughout the year. Given the proximity of the ocean, I went for the second.
... July
Note how the PF's have moved up a little, like the ITCZ in same parts. Also, the high pressure centers moved to the north, only slightly, except for the Western Danip High Pressure Center, which was able to stretch eastwards with the change of position.
Have a look at how atmospheric pressure and wind formation happens on Earth with this awesome link, for comparison:
Global Climate Animations Circulation and Winds
... and once you're happy with your work...
Task #3 is done.
(EDIT: corrected January map and added extra instructions - to add low pressure centers in continents, when warmest and high pressure centers, when coldest)
(EDIT 2: added link to Global Climate Animations)
(EDIT 3: added further instructions for placing the ITCZ)
(EDIT 4: made clearer instructions for continental High Pressure Centers during Winter time)
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