High and low pressure

Air is pressing down on us! Air pressure refers to the force the atmosphere exerts on the Earth. Remember, the atmosphere is not a void, but packed with molecules in the form of gas. Pressure varies with height and temperature. At sea level, more air is above you than at high altitudes. Because of this, air pressure is lower on the top of a mountain than at the beach.

The Sun does not heat Earth uniformly. Warm areas tend to have lower air pressure than cool areas. Why? Hot air has more energy, meaning molecules in the air move around more, and thus move farther apart. The result is that the air takes up less space, and the pressure decreases. The reverse is true of cooler regions. As air cools, a greater number of molecules take up the same amount of space.

You probably have heard that hot air rises, and cool air tends to sink. This is key to understanding high and low pressure areas. A low pressure area is a section of warm air that is rising. As warm air near Earth’s surface rises, it cools off. This creates clouds, because cool air cannot hold as much moisture as warm air. Low pressure areas are associated with rain.

A high pressure area is a section of cool air that is sinking. As the air sinks, it warms up and is able to contain more moisture. This prevents clouds from forming. That is why high pressure areas are associated with good weather. Fog forms overnight when the temperature of Earth’s surface cools. The air can no longer hold the moisture, so water droplets condense to form a low cloud. If you live in a large city, watch out! Pollution can become trapped in the sinking air, creating smog.

Additional Resources:
http://kids.earth.nasa.gov/archive/air_pressure/index.html
http://www.bbc.co.uk/weather/features/basics_airpressure.shtml