You may have seen a weather map with a red L on it. This red L means there is a low pressure system over that area of the map. Just what does that mean?
Windows to the Universe original image
There are no exact measurements that would make a system a Low. It’s all relative! If a certain system is a Low pressure system, that means it has lower pressure then the areas around it.
The differences in pressure from one place to another on Earth are what cause wind. You see, air wants to move from areas of high pressure to areas of low pressure. Away from areas of high pressure and into areas of low pressure…
The Coriolis Force also adds rotation to the movement of air. In the Northern Hemisphere, air moves clockwise around a High and counter-clockwise around a Low.
Air moves into a Low pressure system. It pushes any air that was there up. As the air rises, water vapor condenses into clouds that can bring precipitation. That’s why many people see a red L on weather chart and know there is a good chance of “bad” weather – cloudy skies and possibilities of rain or snow.
source: windows to the universe