Low pressure systems. Meteorologists are quick to point them out on the nightly surface map as a big red “L”. They also talk about them all the time. This is because low pressure systems are typically associated with bad weather.
In the northern hemisphere, low pressure systems rotate counter-clockwise (clockwise in the southern hemisphere). As the system rotates, air spins towards the center and converges, or comes together, at the surface. Since air cannot go through the ground, it has no choice but to rise.
Not only will the air rise, but moisture at the surface may get pulled upward into the atmosphere too. As this happens, the air cools and condenses, becoming more saturated. When the air becomes fully saturated, a cloud will begin to form. If this process occurs long enough, precipitation will begin to fall from the sky.
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