Note: Winds are parallel to the lines of constant pressure height

Equilibrium water vapor densities and vapor pressures Equilibrium water vapor densities and vapor pressures measured over both water and ice.

Why does air move the way it does? Up – Down (or vertical) motion: updrafts - downdrafts - Response to density (temperature) differences that produce or eliminate bouyancy. A warm parcel of air will rise in a cooler environment, and vice versa. - Lifted by mountains or complex terrain - Movement of frontal boundaries - Response to a convergence or divergence of surface air

North-South and Winds move horizontally East-West Primarily a response to temperature/density/pressure differences and Coriolis forces (caused by the rotation of Earth).

Warm air rises in cooler air because its density is lower than the envirnonment, e.g. a hot air balloon.

Thermal circulations are produced by temperature differences on the surface. Land surface becomes much warmer than nearby lake or ocean in the daytime. Therefore, air above the land is warmer and has a lower density. Because of the lower density, the air pressure above a warm surface decreases more slowly with height than air above the cool water surface.

A pressure difference develops in the air above the surface,

and air above the surface moves from the region of higher pressure toward the lower pressure.

When air above the surface moves from left to right, the surface pressure on the left goes down (from 1000 mb to 990 mb in the picture), and the additional mass of air (on the right) increases the surface pressure (from 1000 to 1010 mb). 

Thus, the movement of air aloft has created a pressure difference at the surface that moves air from the cool water toward the warmer land.

If the warm surface is a beach and the cooler water is an ocean, this type of circulation is called a “sea breeze.” At night, the beach cools faster than the ocean, and the thermal circulation reverses. This is called a “land breeze.”

Another example. Cities are often warmer than the surrounding countryside, especially at night.  This is referred to as an urban “heat island” effect.  The resulting wind is called a “country breeze.”

Asian Monsoon. In the summer, India and SE Asia are heated more than the oceans.  A surface low pressure forms over the warmer land, moist winds blow from the ocean toward land, and large amounts of rain follow.

In the winter, the winds change direction and blow from the land toward the oceans.

La Niňa Cold ocean currents along the west coasts of N. America and S. America normally converge at the equator and flow westward (top view).  As the water travels toward the west it warms.  Some of the highest sea surface temperatures are found in the western Tropical Pacific.  Note the thermal circulation.

El Niňo In an El Nino year the cold ocean currents don't make it to the Equator.  Warm water is carried from the western Pacific to the eastern Pacific. Thermal circulation is smaller or reversed.

Global Patterns of Surface Pressure

Winter Northern Hemisphere

Summer Northern Hemisphere

General Circulation of the Atmosphere

What actually happens? [Global animation of WV imagery]

Earth from the Moon video