Atmospheric pressure is a term used to describe the amount of force that is exerted on the surface of the Earth by the weight of the air above it. An air pressure system is a description of the atmospheric pressure for a given area. Air pressure varies wildly across the vast expanses of the planet; it can even vary greatly from one city to another. Anyone who has watched their local news can attest to seeing the red L’s and blue H’s shown by the meteorologist and describe where the high and low pressure systems are developing for the day. But, what exactly does that mean and how does it happen?
Both high and low-pressure systems are caused by the differences in temperature in the atmosphere as well as the differences between the water on the ground and the surrounding air. Air pressure systems are the cause of the daily weather that everyone experiences. A low-pressure system means that there is a greater likelihood of clouds and rain and so the temperatures for that day will not be as high. A high-pressure system means that the weather will be dry and warm with few clouds.
A high-pressure system is defined as one where the atmospheric pressure is greater than the pressure of the surrounding area. They are normally caused by a weather phenomenon known as subsidence. The air in the high-pressure system is either being cooled from below such as off the coast of California where the oceans cool the air, or the cooling happens from above when the cooling of the air by the sun exceeds the amount of warming the sun is providing. As this mass of air is cooled it shrinks and allows air from the surrounding area to occupy that space. This creates more pressure from the air at the Earth’s surface and a high-pressure system is created.
Since the atmospheric pressure is different between a high and low-pressure system, this creates a wind at the surface that flows out of the high-pressure area and into the low-pressure area. If this wind direction is added to the rotation of the Earth, it creates a clockwise rotation around the high-pressure system in the Northern hemisphere and a counter clockwise rotation in the Southern Hemisphere. These winds will cause the air pressure to subtly change and move, which is why those H’s and L’s during the weather report change position as well.
While the small air pressure systems move and change on a continual basis there are several areas of the Earth that have consistent high or low pressure. For example there is what is called the Subtropical High-Pressure Cells that is located between 20 degrees N/S and 35 degrees N/S. This is an area of the Earth that consistently has dry, warm air that forms high in the atmosphere and as it descends it becomes more dense and hot.
When meteorologists study these patterns of high and low pressure, it helps them to better understand the Earth’s weather patterns and allows for more accurate weather predictions. Predictions for daily weather can affect everything from whether or not to take a morning stroll to making cross-country shipments. Air pressure is an important part of Earth’s weather and the better it is understood the better society can understand their local weather and how it may be effected by the rest of the world.