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The Challenge: Predict the Weather

Why is air pressure measured?

Air pressure is the weight of all the air molecules above you pushing down. Although air molecules are invisible to the naked eye, they still have weight, and because of their thermal energy they keep colliding with each other. Because there is a lot of "empty" space between the air molecules, air can be compressed to fit into a smaller volume.

When air is compressed, the number of air molecules in a given space increases and so it weighs more: the air is said to be under 'high pressure'. The exact pressure is not very informative but the relative changes in pressure from day to day are important indicators of changes in weather. For example, a large area of warmer air being blown in from another part of the world will cause the air pressure to change many hours, perhaps days, before any indication can be seen or felt. A drop in pressure usually predicts foul weather, while a rise in pressure usually means fair weather ahead.

Weather map