Chapter 18

Force exerted by the weight of the air above Atmospheric pressure decreases with increasing altitude.

Barometer A barometer measures the weight of the air.

Horizontal movement of air Wind moves out of areas of high pressure into areas of low pressure.

• Pressure gradient force • Isobars • Pressure gradient • Coriolis effect • Friction The Coriolis effect causes apparent deflection in wind direction due to Earth’s rotation.

Apparent deflection in the wind direction due to Earth’s rotation In the Northern Hemisphere, deflection is to the right.

Only important near the surface Friction acts to slow the air’s movement.

Jet stream The jet stream is a ‘river’ of air at high altitude with high velocity (120-240 kilometers per hour).

Low pressure, associated with rising air, often brings clouds and precipitation Cyclones are typically linked with stormy weather.

High pressure, pressure increases toward the center, associated with fair weather Anticyclones lead to stable and clear conditions.

Occurs over continents, during warm months, air flows onto land Monsoons bring warm, moist air from the ocean.

• Speed • Direction Winds are labeled from where they originate (e.g., North wind blows from north to south).

Descending air, divergent winds, dry conditions Examples include the Sahara and Kalahari deserts.

Ascending air, converging winds, ample precipitation Examples include the Amazon and Congo basins.

• Large landmasses in middle latitudes have less precipitation toward their centers • Mountain barriers alter precipitation patterns • Windward slopes receive abundant rainfall • Leeward slopes are usually deficient in moisture Orographic lifting contributes to rainfall on windward slopes.