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I am trying to fill my world with climates as realistically as possible. In addition to climate science stuff I read on this site, I've been using these two maps of the Earth as a comparison for where climates appear in relation to the ITCZ: Pressure Systems in July

Pressure Systems in January

One thing is super confusing to me, though - the Sahara/Arabian deserts. Unlike western Australia, which is under a high-pressure area and receives no rain year-round, the Sahara and Arabia are both under the ITCZ during half the year. And, according to these maps they should be getting some rain from Atlantic winds. Why do they receive so much less rain than the African savanna?

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  • $\begingroup$ I believe it is something to do with wind cells and rain shadow but I'm not certain. $\endgroup$ – Bellerophon Oct 6 '16 at 21:44
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The short answer is that your maps aren't that accurate. The ICTZ never really gets beyond ~15N, and at that latitude, its only there for a month or two. Here is a map that might be more accurate.

The ITCZ reverses direction basically as soon as it gets to its northernmost point. You can see this in the climate of N'Djamena, the capital of Chad. ITCZ rolls up that far north (12N in that case) in July, sticks around for about two months, then the place goes back to being a desert. So, while it gets a decent amount of rain for two months, it is also super hot and sunny. That is called the Sahel. So even though some areas of a map may show ITCZ over them, they might only get a month or two of rain; not enough to support savannah grass (though grass will grow in the short rainy season) making them more of a semi-desert scrubland.

Farther north, the rains never really get there at all and you get your giant ergs and wandering Tuareg.

Northern Australia has the ITCZ directly over it, but another factor is at work. It is right on the edge of a warm ocean, so when the ITCZ does show up, there are hot wet winds full of rain. So it ends up getting a lot more rain in the period that the Sahel, and then it gets cyclones as the ITCZ retreats bringing even more rain.

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  • $\begingroup$ Saying that the ITCZ map is wrong is not an answer to the question. It's a comment to the question, somewhat explaining why the OP's expectations do not conform to reality.ITCZ is wrong, but whether it's wrong or right has nothing to do with why a desert can form outside of it, which is the question and even if it were, a correct map of the ITCZ doesn't help with the ultimate goals of the unless they can figure out how the correct version maps onto their map to use to get their actual answer. $\endgroup$ – Durakken Oct 7 '16 at 0:42
  • $\begingroup$ I hate to contradict you, but it actually was quite helpful. Both of your answers were. $\endgroup$ – Merkava120 Oct 7 '16 at 3:33
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The simple gist of it is that ocean, wind, and magma currents along with physical features of a plate combined with the rotation speed, angle of the axis, and distance from the orbited star cause high and low pressure zones that are triggered when they interact to release rain in one place rather than another meaning that rainfall isn't random and doesn't fall everywhere equally. Because rain falls in some places a lot it must not fall in other places much and likewise because rain has just fallen it cannot fall again so even if the conditions are perfect in a given areas, if another area comes before it the area will have little or no rain...

Now if you want to know how to figure out how to map all this, that's a slightly different question that cuts out some of the details of that stuff so that we can generalize...And to answer that question I can either give you copy and pasted answer/tutorial from some place else or just give you the links that will help you and that's what I'll be doing, because it really is too much to ask someone to copy all the information... and also cutting it down might make you think you shouldn't go to the link to get it all right...

Also I don't fully understand all of it myself.

Have fun
- An amazing tutorial by Azelor
- Another good tutorial by Pixie
- A simple helpful explanation from an unfortunately now defunct site

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    $\begingroup$ The links are generally useful, but this doesn't really answer the specific question. $\endgroup$ – kingledion Oct 7 '16 at 0:20
  • $\begingroup$ @kingledion His goal to get accurate climates which is what is done with those links. The answer to his question of "why does this happen" is only answered by explaining how the climate of a certain place has that climate. Those links provide that answer because they are about replicating accurate climates on maps. The answer is "several intersecting variable causes it to be that way, not just the single thing you posted" which is the factual answer. So I answered both, the actual question, and the underlying reason for asking. $\endgroup$ – Durakken Oct 7 '16 at 0:32

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