Because of the way atmospheric circulation works. In particular, because the equator is the hottest part of the world, there tends to be an updraft at the intertropical convergence zone. This draws air in from the north and south near the surface and sends it away in the upper atmosphere. As it rises, it loses the moisture it picked up from evaporation in the tropics, so the area near the equator gets a bunch of rain dumped on it.
Conversely, when the upper-atmosphere air gets to the horse latitudes (roughly the dividing line between the tropics and the temperate zone, 25-30° north or south), it is now dry and sinks back to ground level so it can start the circulation over again. That means those are the latitudes where deserts tend to form, because they get a lot less rain.
There’s still more to the story because of things like the Coriolis effect and mountain rain shadows, but if you look around the world, a lot of the major deserts – the Sahara, the Middle East, the American Southwest, the Kalahari in southern Africa, Australia – are right around those horse latitudes.
Sendbeer@lemm.ee 1 year ago
Someone smart is going to have to answer but it is the lack of rain that defines a desert rather than the temperature. I imagine this is determined by closeness to water, currents, and a million other things. Temperature I am sure is a factor, but far from the only one.
Now hopefully someone smart who actually knows what they are talking about answers. ᕕ( ᐛ )ᕗ
kryptonianCodeMonkey@lemmy.world 1 year ago
You’re correct. In fact the largest desert in the world is actually extremely cold. It’s not the Sahara Desert, it’s the Antarctic Desert. A lack of precipitation defines a desert, not the temperature. The image you have in your head of the sand dunes, oases, mirages and a relentless sun are aspects of desert in hot climates. But those are not features that define all deserts.
You are also correct that the deciding factors for precipitation involve proximity to water sources, air currents, etc. If there is no water to evaporate to turn to rain nearby, or no air flow from water sources to be blown in, or if the temperature is so low that the humidity remains essentially zero at all times, then little to no precipitation can happen in a region. That’s a desert.
kambusha@feddit.ch 1 year ago
When the rain leaves the sky, like a burnt oven pie, that’s a-deee-sert.
kersploosh@sh.itjust.works 1 year ago
Many of the world’s deserts exist - at least in part - because of rain shadow effects from nearby mountain ranges. The Andes range creates the Atacama desert. The Himalayas create the dry Tibetan plateau. The Sierra Nevada and Cascade ranges create a lot of desert in the western US.