Thread: Weather map.

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?

I think it's because deserts grow. All that dry hot wind blowing sand around tends to increase the size and ruin "normal" land and without any large mountains to stop the clouds they just blow right over or get evaporated by the heat...it's like a giant mirror reflecting light and heat. Millions of years ago, the Sahara was a swamp, but now it gets bigger every year. I'm sure there's some scientific stuff behind it but this is just what I remember from my schooling.

So it's mostly an issue of no change in the air temperature, such as when warm inland air hits cool ocean air and creates storms. In the arabian penensula, the air's hot and the water's hot, so the moisture in the air just stays in the air? That sounds plausible, which means that area will still be desert on my map.

I'll post my climate map soon.

Originally Posted by Xeviat

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.

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

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.

Desertification is a serious problem in most of the marginal regions of the world. The Sahara, for example, is expanding southwards through the Sahel due to human usage patterns. Plants respire water from the ground into the air, reducing the dryness of the air, which leads to more rainfall, which keeps the plants alive to respire more water, allowing marginal areas to support plant life in excess of what might be expected from other factors. When these conditions are upset by removal of plants through human usage or drought then these areas will quickly tend towards desert. Conversely, a few wet years or enforced establishment of plant cover can push them away from desert.

I think this is a great idea if done in a fantasy context. Lot's of fun to be had!!!

Deserts also don't hold onto water very well. They are not able to retain as much water as can evaporate off of them. By nature, when they do get storms, the storms pull the little water a desert has away. Once you get a bit of desert, it's hard to ever revitalize it without human intervention.