Air pressure differences drive wind. A steeper pressure gradient usually means stronger wind; surface friction slows it.
On a clear day, land warms faster than the sea. Air above the warmer land rises, and surface pressure becomes lower over the land than over the sea. Air moves inland from the higher-pressure sea: a sea breeze. Air returns towards the sea higher up, completing the simple circulation.
At night, land cools faster than the sea. The sea stays relatively warmer, so air may rise there instead. Surface air can flow from land towards sea: a land breeze, often weaker than the daytime breeze. Strong prevailing winds can alter or overwhelm either local circulation.
Wind is driven by air pressure differences. The pressure gradient measures how much pressure changes over a distance. The same pressure difference over a shorter distance gives a steeper gradient and usually stronger wind. On a map, closely spaced isobars, lines joining places with equal pressure, show a steeper gradient.
Surface friction slows wind. Trees, buildings and rough ground usually create more drag than open water, though gaps and valleys can funnel and speed up air locally. Earth's rotation also affects direction over larger areas. A wind is named for where it comes from: an easterly blows from east towards west.
Step by step
Land warms faster
By day, sunlight heats the land faster than the sea.
Air rises over land
Air above the warmer land rises. Surface pressure is lower over land than over the cooler sea.
Sea air moves inland
Surface air flows from higher pressure over the sea to lower pressure over land: a sea breeze.
Worked example: Pressure difference needs distance
In this invented comparison, pressure falls by 8 hPa over 100 km: 8 / 100 = 0.08 hPa per km. The same fall over 400 km gives 0.02 hPa per km. With similar friction and other conditions, the steeper first gradient usually produces stronger wind.
Watch out for this
A sea breeze means wind blowing out to sea.
Winds and breezes are named by where they come from. A sea breeze comes from the sea and blows inland.
Check your understanding
Two areas have the same pressure difference. Area A spans 50 km and Area B spans 200 km. With other conditions similar, which supports stronger wind?
- B, because the air has farther to travel.
- A, because its pressure gradient is steeper.
- They must have identical winds.