Pressure Systems: Types & Effects

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

Learn about the types of pressure systems we observe when forecasting the weather, and what their effects tend to be. Discover how the motion of the air causes those effects.

What are Pressure Systems?

Have you ever wondered why one day it's dreary and pouring rain, but the next day it's perfect for a trip to the beach? A lot of factors go into the often fickle weather system, which is why meteorologists sometimes give inaccurate weather forecasts. One of these factors is the pressure system. If you ever watched a weather report, you may have heard about high and low pressure systems, often marked with L or H on a weather map. But what is a pressure system?

Weather map showing high and low pressure
Weather map showing high and low pressure

A pressure system is an area of the Earth's atmosphere that has a particularly high or low pressure compared to the air around it. Pressure simply refers to how far apart the air molecules are. When the air is squeezed and packed together, that's high pressure. And when the air is spread apart, that's low pressure.

What causes these pressure systems? That's the more complicated part and can relate to the motion of air currents in the area, the terrain (whether it is flat or mountainous), the presence of the sea, and other factors. In any case, high-pressure air is denser than low-pressure air. Now, let's explore how pressure systems affect weather.

Effects of Pressure Systems

If you watch enough of the weather forecast, you've probably noticed that high-pressure air tends to lead to good weather: warm temperatures and clear skies. Low-pressure air, on the other hand, is associated with cloudy and rainy conditions. These weather conditions occur, in part, due to the way that pressure systems move.

Clear skies are associated with high pressure.
Clear skies are associated with high pressure

Have you ever taken a bath with a rubber duck or went swimming with an inflatable tube? What happens when you try to push it under the water? You'll find that it bobs back up the surface. That's because the water is denser than the rubber duck. (The particles in the water are closer together than the particles of air inside the rubber duck.) Lighter, less dense things tend to rise, and heavier, more dense things tend to sink. The same happens with air.

Low-pressure air is lighter and less dense than high-pressure air. This means that low-pressure air tends to rise, and high-pressure air tends to sink. This happens naturally, but if two different pressures of air collide with each other, it speeds up the process. An area of low pressure on a weather map is, therefore, an area where the air is generally rising, and an area of high pressure on a weather map is where the air is generally sinking.

A low-pressure system near Australia
A low-pressure system near Australia

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