4. Air Pressure, Wind & Circulation

Air has weight, and the air pressing down creates air pressure. Differences in pressure are what make the wind blow.

When air is heated it expands, becomes less dense, and rises, leaving low pressure (L) below — air rising and cooling often makes clouds and stormy weather. Where air cools, it grows denser and sinks, creating high pressure (H) — usually clear, fair weather. Air then flows along the surface from high pressure to low pressure, and that moving air is wind.

A sea breeze is a perfect everyday example. On a sunny day the land heats faster than the water, so air rises over the land (low pressure) while cooler, denser air sits over the sea (high pressure). The result is a refreshing surface wind blowing from the sea toward the land.

On the scale of the whole planet, this rising-and-sinking sets up giant convection cells and global wind belts (like the trade winds). Because the Earth rotates, moving air is deflected and curves — the Coriolis effect — which is why large weather systems spin.

⚠️ Common misconception: wind does not blow from low to high pressure. Air always moves from high pressure to low pressure (think of air rushing out of a popped balloon — from high inside to low outside).

💡 Tip: rising air → low pressure → clouds/storms; sinking air → high pressure → fair weather; wind blows High → Low.