3 Pressure and Atmospheric Circulation
Learn how pressure differences and Earth’s rotation shape winds, global circulation cells, and local breezes.
Pressure differences drive wind
is the force per unit area exerted by the weight of the air above a location. Near sea level, average pressure is about , though actual pressure varies. Pressure generally decreases with altitude because less air lies above a higher location. Temperature differences and air movement also help create pressure differences.
A is a change in pressure over a given distance. The pushes air from higher pressure toward lower pressure. A steeper gradient produces a stronger force and, all else being equal, faster winds. On weather maps, connect places of equal pressure: closely spaced indicate a stronger than widely spaced ones.
How forces shape wind direction
Air does not usually travel directly from high to low pressure. Earth’s rotation deflects moving air, while friction near the ground slows it. Above the surface, the and often combine to make winds flow roughly parallel to . Near the surface, friction weakens the , so wind crosses toward lower pressure, spiraling inward around lows and outward around highs.
Rising air near a low can cool and form clouds, while sinking air near a high often discourages cloud formation. These are common patterns, not guarantees of a particular weather outcome.
The
The is the apparent deflection of moving air when its motion is viewed on rotating Earth. Air is deflected to the right of its path in the Northern Hemisphere and to the left in the Southern Hemisphere. The effect is weakest at the equator and stronger toward the poles. It changes the direction of moving air; it does not start the wind, which is driven by forces including pressure differences.
For example, air moving north in the Northern Hemisphere is deflected eastward relative to its path. This deflection helps shape large-scale wind belts and circulation around weather systems.
Uneven heating and global circulation
The equator receives more direct solar energy than high latitudes. This uneven heating helps drive large-scale atmospheric circulation: warm air tends to rise in some regions, while cooler air sinks elsewhere. Earth’s rotation, seasonal shifts in solar heating, and the distribution of land and ocean modify the resulting patterns.
The simplified three-cell model describes three circulation zones in each hemisphere. These cells are broad averages, not fixed walls or perfectly regular loops. Their positions and strength vary with season, and actual winds are affected by continents, oceans, terrain, and changing weather systems.
The three circulation cells
In the , strong heating causes air to rise near the equator. Aloft, it moves poleward and generally sinks in the subtropics, near latitude. Near the surface, air flows back toward the equator. Its deflection helps form the northeast trade winds north of the equator and the southeast trade winds south of it.
The lies between about and latitude. Its average surface flow is poleward and deflected eastward, forming the prevailing westerlies. This is an indirect circulation shaped by the neighboring cells and transient weather systems.
In the , cold air tends to sink over the poles and flow toward lower latitudes near the surface. The deflects this flow westward, producing polar easterlies. Air generally rises again near about latitude.
Sea and land breezes
Local winds develop when nearby surfaces heat or cool at different rates. They are most noticeable when large-scale winds are weak enough not to overwhelm the local circulation.
Along coasts, land usually warms faster than water during the day. Air over land warms and rises, and cooler air from over the water flows inland at the surface as a . At night, land cools faster; the near-surface flow may reverse, moving from land toward the water as a . Both circulations usually include a return flow aloft. A sea-breeze boundary can lift warm, moist air and help clouds or thunderstorms develop when conditions are favorable.
Mountain and valley winds
Daily heating and cooling of slopes drive mountain and valley winds. During the day, warmed air flows upslope and up-valley, creating a . At night, cooled, denser air drains downslope and down-valley as a . Terrain and larger-scale winds can alter these typical patterns.