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How Wind Forms
Pressure differences make air flow
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Wind forms because the Sun heats Earth’s surface unevenly. Land, water, mountains, forests, and cities absorb and release energy at different rates. These temperature differences create differences in air pressure. Wind matters because it moves heat, moisture, storms, pollution, and energy around the planet.
Warm air becomes less dense and rises, leaving lower pressure near the surface. Cooler, denser air sinks and creates higher pressure. Air then flows from areas of high pressure toward areas of low pressure, creating wind. Local winds such as sea breezes, land breezes, and mountain valley winds follow this same pattern, while global wind belts are shaped by uneven heating and Earth’s rotation.
Key Facts
- Wind is moving air caused mainly by differences in air pressure.
- Air flows from high pressure to low pressure: high pressure -> low pressure.
- Warm air is less dense and rises, while cool air is denser and sinks.
- Pressure difference can be estimated by ΔP = Phigh - Plow.
- Wind speed increases when the pressure gradient becomes steeper: pressure gradient = ΔP / distance.
- Land heats and cools faster than water, causing sea breezes during the day and land breezes at night.
Vocabulary
- Wind
- Wind is the horizontal movement of air caused by differences in air pressure.
- Air pressure
- Air pressure is the force that air molecules exert on a surface as they collide with it.
- Pressure gradient
- A pressure gradient is the change in air pressure over a distance, and it helps determine wind speed.
- Sea breeze
- A sea breeze is a daytime wind that blows from cooler water toward warmer land.
- Global circulation
- Global circulation is the large-scale movement of air around Earth caused by uneven heating and Earth’s rotation.
Common Mistakes to Avoid
- Saying wind moves from low pressure to high pressure. This is wrong because surface air generally moves from high pressure toward low pressure.
- Forgetting that land and water heat at different rates. This is wrong because uneven heating is the main reason sea breezes and land breezes form.
- Assuming warm air sinks because it feels heavy or humid. This is wrong because warmer air is usually less dense than nearby cooler air and tends to rise.
- Ignoring distance when comparing pressure differences. This is wrong because the same pressure change over a shorter distance creates a stronger pressure gradient and usually faster wind.
Practice Questions
- 1 A high-pressure area has a pressure of 1020 mb and a nearby low-pressure area has a pressure of 1005 mb. What is the pressure difference between them?
- 2 Two towns are 300 km apart. The pressure changes from 1018 mb at one town to 1006 mb at the other. What is the pressure gradient in mb per km?
- 3 During a sunny afternoon at the coast, the land becomes warmer than the ocean. Explain which way the wind will blow and why.