I know it evaporates but shouldn’t it it form clouds the will eventually rain back down?
In: Planetary Science
Meteorologist here.
So, it turns out that major weather patterns are caused by waves in the atmosphere…HUGE waves. You know when you see a weather map and they show the jet stream waving over the continent? That’s how big these waves are.
Now, these waves usually move, west to east. And like water waves, you have parts of that wave which are rising, and parts which are falling. Rising air cools, and then the water vapor condenses, forms clouds, and (if conditions are right) rains. There are differences depending on terrain and water sources, but that’s the gist.
Areas where the air is descending warms…which means that you don’t get cooling and thus you don’t get clouds. These are your “ridge of high pressure”.
However, you often get a situation where the waves go stationary. When this happens the ‘rising’ and ‘descending’ parts don’t move. For a week, two weeks, months even. And even if you get a bit of a break down in that pattern, a week later it sets up again. This is frequently caused by El Niño and La Niña over North America.
So you get situations where those descending air regions stay over the same spot. No clouds, no rain. Additionally you can then get secondary effects because the lack of clouds increases the temperature, which can dry out the soil and it gets hotter.
You can get puffy cumulus clouds in these high pressure areas, if there is enough moisture, and you get some small bits of instability (especially over mountains). But because the air is descending, you can’t get deep convection which is needed to form thunderstorms.
Edit: for some reason I thought you were asking why droughts happen. But it’s 3am and I really should go to bed. 😆
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