When you have finished this page, try the Air Pressure Quiz.
Although they are invisible, the air is filled with tiny molecules. We feel these molecules press against us at times - this is air pressure. Since the molecules in the air are so tiny and are very active and spread out, we do not see them. However, they do play a significant role in weather as well as many other parts of our lives.
As the air is heated on a warm day, the molecules in the air move faster and further apart. When the molecules are cooled down, they move slower and move closer together.
Differences in air pressure help cause winds and affect air masses. They are also factors in the formation of storms such as thunderstorms, tornadoes and hurricanes.
Differences in air pressure are shown on a weather map with lines called isobars. The map below from NASA illustrates isobars marking areas of high and low pressure. High pressure areas generally have dry, good weather and areas of low pressure have precipitation.
Air pressure is measured with a barometer.
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