2 Air Pressure and Wind

Pressure patterns on weather maps

Air pressure is the force exerted by the weight of the air above a surface. It generally decreases with altitude because less air lies overhead. On weather maps, pressure is often adjusted to sea level so pressures measured at different elevations can be compared.

Isobars connect places with equal pressure. Their spacing indicates how quickly pressure changes over distance: closely spaced isobars indicate a strong pressure gradient and usually stronger winds, while widely spaced isobars indicate a weaker gradient and lighter winds.

Forces that shape wind

Air tends to move from higher pressure toward lower pressure. This push is the pressure-gradient force, and a steeper pressure gradient generally produces faster winds. Wind direction and speed are also shaped by the Coriolis effect and friction.

• Coriolis effect: Earth’s rotation causes an apparent deflection of moving air. Winds turn to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The effect is weakest at the equator and stronger toward the poles.

• Friction: The ground, buildings, forests, and other features slow near-surface wind. Because the slowed wind is deflected less by the Coriolis effect, it crosses isobars toward low pressure rather than flowing nearly parallel to them.

Above the layer strongly affected by surface friction, winds often flow roughly parallel to isobars. Near the surface, they angle across isobars, inward toward lows and outward from highs.

Circulation around highs and lows

Around a low-pressure center, surface air converges and rises. Rising air cools, which can help clouds and precipitation form. Around a high-pressure center, surface air diverges and air tends to sink; sinking air warms, often making cloud formation less likely. These are common patterns, not guarantees: local conditions and the larger weather system also matter.

In the Northern Hemisphere, winds circulate counterclockwise around lows and clockwise around highs. In the Southern Hemisphere, the directions are reversed. These circulation patterns are called cyclonic around lows and anticyclonic around highs.

Sea breezes and land breezes

Local differences in heating can create small-scale pressure differences and winds. During the day, land heats faster than water. Warm air over land rises, and cooler air from over the water moves inland as a sea breeze. Sea breezes can change coastal temperatures and help trigger clouds or showers where incoming air meets warmer air over land.

At night, land cools faster than water. The wind may reverse and blow from land toward water as a land breeze.

Global wind belts

Uneven solar heating helps drive broad circulation patterns. Near the equator, strongly heated air rises; some of it flows poleward high in the atmosphere and sinks in the subtropics. Earth’s rotation shapes these circulations into prevailing wind belts:

• Trade winds generally blow from east to west in the tropics, toward the equatorial region where warm air often rises and produces clouds and rain.

• Westerlies generally blow from west to east across the middle latitudes and help steer many weather systems.

• Polar easterlies generally blow from east to west near the poles.

These belts are broad averages, not unchanging surface winds. Continents, mountains, seasonal heating, and shifting pressure systems modify them. The global pattern transports heat and moisture, influencing regional weather and climate. In the middle latitudes, movement of highs and lows and the winds that steer them often bring changing conditions from day to day.