Pressure, Winds, Storms, and Cyclones
Wind is air moving from high pressure to low
Find out what makes the wind blow.
Some days the wind blows hard, some days it is still, and sometimes it is strong enough to damage homes and hurt people. What makes the wind blow at all?
You've seen air rush out of a balloon left open, and a punctured bicycle tube go flat. In both, air moves out of a place where the pressure is high to where it is low.
Try it (Activity 6.5). Take two similar thin balloons and a drinking straw. Push one end of the straw into the first (empty) balloon and tie it with thread. Blow up the second balloon and pinch its neck shut. Push the other end of the straw into its neck and tie it, without letting air leak.
Guess first, then watch. The full balloon shrinks and the empty one grows. The air in the full balloon was at a higher pressure, so some of it moved across to the lower-pressure balloon.
After a while both are about the same size and the flow stops. Air flows only while one pressure is higher than the other. Once they are equal, nothing is pushing it across: it's like water levelling itself out.
So wind is air moving from high pressure to low pressure. And the bigger the difference in pressure, the faster the air moves.
This explains the breezes you met in Grade 7:
| Sea breeze (by day) | Land breeze (by night) | |
|---|---|---|
| which is warmer? | the land (it heats up faster than water) | the sea (water cools more slowly) |
| where the warm air rises, leaving low pressure | over the land | over the sea |
| which way the wind blows | from the sea to the land | from the land to the sea |
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