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ความคิดเห็นที่ 41 |
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When the Sahara was a forest, the axis of the earth was different and so were the seasons.
Here is why the Sahara is a desert today:
Air rises over the equator because it is warm. Air sinks over the poles because it is cold.
When air rises, it creates a low pressure and the rising moisture condenses in clouds that fall back as rain. This is why we have the equatorial rain forests!
Over the poles, the air sinks, moisture dissolves as the air warms up and the sky is clear. The two poles are desert regions with very little precipitations.
If the earth wasn't spinning and the Coriolis effect not working, the air would then move from equator to the poles and back.
But the Coriolis effect makes any air mass to turn to the right hand side in the northern hemisphere and the opposite in the southern.
Because of that, the air rising from the equator never gets a chance to reach the pole and falls back at roughly latitudes 30 north and south, creating high pressure zones with dry climate.
Between those two high pressure zones, at roughly latitude 60 north and south, a zone of low pressure exist along the front between cold and dry polar air and, mild and moist temperate air.
It works like this:
Latitude 0 (equator): low pressure
Latitude 30: high pressure
Latitude 60: low pressure
Latitude 90 (Pole): high pressure.
Now, go and look at a world map and notice how, everywhere around the world, places that are at the same latitude as the Sahara are deserts.
Of course, deforestation has increased the size of the sahara to some extend. But it is its latitude that makes it a desert.
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