Weather map.

[waldronate]

I'm actually working on a climate map for my world right now. My setting is earth in the future, so I get to use Earth for examples. Right now, I'm having a hard time figuring out why Arabia and Somalia are so arid when they're on the equator and the air currents move from the ocean to land. At similar latitudes are jungles worldwide. Anyone know why this is?

The deserts around 30 degrees north and south latitude is caused by Hadley circulation, where warm moist air rises at the equator and moves poleward. As it rises, it cools and loses its moisture. When it comes back down it is very dry.

There are other causes of deserts including rain shadows (where mountains extract rain from clouds), cold ocean currents, and just plain distance from the ocean. Places where more than one of these things happens tend to be very dry indeed. The Atacama desert, for example, has descending dry air masses combined with the rain shadow from the Andes that combine to make it one of the driest places on Earth.

[Gamerprinter]

Threadjack -- regarding rain shadows. The weirdest climate I'd ever witnessed was on vacation on Kuau'i, Hawai'i a couple years back. The island is only 45 miles across (roughly) north to south and east to west. The dormant volcanoes at its center are about a mile high.

The north side of Kuau'i is lush tropical rainforest/jungle, gorgeous. The east side has palm and monkeytree groves, but is largely grassy. The south side is rather dry and as you travel west along the shore it turns into something out of Australian outback or the Arizona desert. The west side is barren eroded volcanic material with some stream fed palm forested valleys. Oh, and on top center near the volcano caps is both the wettest place on Earth (450 to 700 inches of rain per year), and the highest swamp on Earth, called the Alakai Swamp, really its a dwarf tropical rainforest with tropical trees only 10 to 20 feet high, drenched in water everywhere (planked walkways allow human movement), cold and misty all the time (50 degrees F), almost never sees sunlight - maybe 1 or 2 hours / day.

Extreme biospheres all within one tiny island.

Sorry for the Threadjack - just had to say that!

GP

[Midgardsormr]

Also, continuous human occupation has converted some areas of Arabia from fertile to desert. Three thousand years ago, much of that land was farmable, but a combination of irrigation dropping toxic minerals into the soil and never letting the fields lay fallow transformed a lot of that land into desert.