High Pressure
When air molecules exert a lot of pressure on the surface of Earth, meteorologists call this a high pressure area. Meteorologists then use that information to predict the weather. How do they do this? Well, to fully understand this, you have to think about the water cycle. Remember in the water cycle, water on Earth evaporates into the sky, condenses into clouds, and eventually falls back to Earth as precipitation. So, in order for it to eventually rain, you need water to evaporate into the sky.
Let's think about that hand again. In a high pressure area that hand would be exerting a great amount of pressure and would hold everything down towards Earth. If air pressure is holding everything down, then water would not be able to evaporate into the sky. If you don't have evaporation, you don't have condensation. Bye, bye clouds! If you don't have clouds, you don't have precipitation. Hence, if a meteorologist uses a barometer, a tool used to measure the air pressure, and sees that it will be a high pressure day, he or she can predict that it will be a sunny, clear sky day.
Low Pressure
A low pressure area is the opposite of a high pressure area. Since air pressure, or the hand in our illustration, is not exerting a lot of pressure, air molecules are able to float high into the sky. They're not being held down. If air molecules are able to float up, then water can also evaporate and float high into the sky. With evaporation, you have condensation, and eventually precipitation. Hence, in a low pressure area, you will have cloudy skies, and more than likely, rainy weather.
Windiness
High and low pressure areas exist all over the world, sometimes right next to each other. Did you know that the wind is tied to these pressure systems? When air is under great pressure, it wants to escape. Just like us! Therefore, it will always try to move to a low pressure area. This movement of air from a high pressure area to a low pressure area is what causes wind.
Lesson Summary
Let's take a few moments to review what we've learned. Air has weight, and because of that, exerts pressure. Meteorologists are scientists who study and attempt to predict the weather and they measure pressure to predict the weather. In a high pressure area, air molecules are held down resulting in clear, sunny days. In a low pressure area, air and water molecules are able to float high into the sky resulting in cloudy, rainy days. So what do you like better, high or low pressure areas?
