Standing on the ground and looking up, you are looking through the atmosphere. It might not look like anything is there, especially if there are no clouds in the sky. But what you don't see is air - lots of it. We live at the bottom of the atmosphere and the weight of all the air above us is called air pressure. A tower of air that is 1 inch square and goes from the bottom of the atmosphere is 14.7 pounds. That means air exerts 14.7 pounds per square inch (psi) of pressure at the Earth's surface. High in the atmosphere, air pressure decreases. With less air molecules above, there is less pressure from the weight of air above.
Pressure varies from day-to-day at the Earth's surface - the bottom of the atmosphere. This is, in part, because the Earth is not equally heated by the Sun. Areas where air is warmed often have lower pressure because the warm air rises and are called low pressure systems. Places where air pressure is high are called high pressure systems.
A low pressure system
has lower pressure at its center than the areas around it. Winds blow towards the low pressure, and the air rises in the atmosphere where they meet. As the air rises, the water vapor within it condenses forming clouds and often precipitation too. Because of Earth's spin and the Coriolis Effect, winds of a low pressure system swirl counterclockwise north of the equator and clockwise south of the equator. This is called cyclonic flow. On weather maps a low pressure system is labeled with red L.
A high pressure system
has higher pressure at its center than the areas around it. Wind blows away from high pressure. Winds of a high pressure system swirl in the opposite direction as a low pressure system - clockwise north of the equator and counterclockwise south of the equator. This is called anticyclonic flow. Air from higher in the atmosphere sinks down to fill the space left as air blew outward. On a weather map the location of a high pressure system is labeled with a blue H.
How do we know what the pressure is? How do we know how it changes over time? Today, electronic sensors are used to measure air pressure in weather stations. The sensors are able to make continuous measurements of pressure over time. In the past, barometers were used that measured how much air pushed on a fluid such as mercury. Historically, measurements of air pressure were described as "inches of mercury." Today, meteorologists use millibars (mb) to describe air pressure.
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