Atmospheric Pressure The Earth's land surface is actually located at the bottom of a vast ocean of air. Like the water in the ocean, the air in the atmosphere is constantly pressing on the solid or liquid surface beneath it.

Atmospheric Pressure Atmospheric pressure. This figure depicts atmospheric pressure as the weight of a column of air. (a) Metric system. The weight of a column of air 1 cm on a side is balanced by the weight of a mass of about 1kg. (b) English system. The weight of a column of air 1 in. on a side is balanced by a weight of about 15 pounds.

Air Pressure Changes With Altitude Because atmospheric pressure decreases rapidly with altitude near the surface, a small change in elevation will often produce a significant change in air pressure.

Air Pressure Changes With Altitude Atmospheric pressure decreases with increasing altitude above the Earth's surface.

Wind Wind is caused by differences in atmospheric pressure from place to place. Air tends to move from high to low pressure until the air pressures are equal. For example, pressure was higher over Wichita than Columbus, a pressure gradient force would push air from Wichita to Columbus.

Wind Isobars and a pressure gradient. High pressure is centered at Wichita, and low pressure is centered at Columbus.

The Coriolis Effect An object in motion on the Earth's surface always appears to be deflected away from its course. This effect is a result of the Earth's rotation, and is called the Coriolis effect. *See movies on the Coriolis Effect in the geodiscoveries section of your text’s website.

The Coriolis Effect Coriolis effect direction and strength. The Coriolis effect acts to deflect the paths of winds or ocean currents to the right in the northern hemisphere and to the left in the southern hemisphere as viewed from the starting point.=

Surface Winds on an Ideal Earth A parcel of air in motion near the surface is subjected to three influences: the pressure gradient that propels the parcel toward low pressure; the Coriolis effect that deflects the parcel, and the frictional forces of the ground surface that slow the parcel down.

Surface Winds on an Ideal Earth Global surface winds on an ideal Earth. This schematic diagram of global surface winds and pressures shows the features of an ideal Earth, without the disrupting effect of oceans and continents and the variation of the seasons. Surface winds are shown on the disk of the Earth, while the cross section at the right shows winds aloft.

Global Atmospheric Circulation Wind and air pressure follow both global and local patterns. These patterns are affected by the level of the atmosphere, the hemisphere and the season in which they occur. *See movie on Global Atmospheric Circulation in the geodiscoveries section of your text’s website.

Rossby Waves The smooth westward flow of the upper-air westerlies frequently forms undulations, called Rossby waves.

Rossby Waves Development of upper-air Rossby waves in the westerlies of the northern hemisphere.

Ocean Currents Just as there is a circulation pattern to the atmosphere, so there is a circulation pattern to the oceans.

Ocean Currents January ocean currents. Surface drifts and currents of the oceans in January.

El Niño At intervals of about three to eight years, a remarkable disturbance of ocean and atmosphere occurs. This phenomenon is called El Niño. *See movie on El Niño in the geodiscoveries section of your text’s website.

El Niño Maps of pressures in the tropical Pacific and eastern Indian Ocean in November during normal and El Niño years. In normal year (a), low pressure dominates in Malaysia and northern Australia. In an El Niño year (b), low pressure moves eastward to the central part of the western Pacific and sea-surface temperatures become warmer in the eastern Central Pacific.