The earth's atmosphere thins rapidly with increasing altitude. Pure, dry air is mostly nitrogen, oxygen, and argon, with traces of carbon dioxide and other gases. Atmospheric air also contains a variable amount of water vapor. Water vapor cycles into and out of the atmosphere through evaporation and precipitation.

Atmospheric pressure is measured with a mercury barometer. At sea level, the atmospheric pressure will support a column of mercury about 76.00 cm (about 29.92 in) tall. This is the average pressure at sea level, and it is called the standard atmospheric pressure, normal pressure, or one atmosphere of pressure.

Materials on the earth's surface absorb sunlight, emitting more and more infrared radiation as they are warmed. Carbon dioxide and molecules of water vapor in the atmosphere absorb infrared radiation, which then reemit the energy many times before it reaches outer space again. The overall effect warms the lower atmosphere from the bottom up in a process called the greenhouse effect.

The layer of the atmosphere from the surface up to where the temperature stops decreasing with height is called the troposphere. The stratosphere is the layer above the troposphere. Temperatures in the stratosphere increase because of the interaction between ozone (O3) and ultraviolet radiation from the sun.

The surface of the earth is not heated uniformly by sunlight. This results in a differential heating, which sets the stage for convection. The horizontal movement of air on the surface from convection is called wind. A generalized model for understanding why the wind blows involves (1) the relationship between air temperature and air density and (2) the relationship between air pressure and the movement of air. This model explains local wind patterns and wind patterns observed on a global scale.

The amount of water vapor in the air at a particular time is called the absolute humidity. The relative humidity is a ratio between the amount of water vapor that is in the air and the amount needed to saturate the air at the present temperature.

When the air is saturated, condensation can take place. The temperature at which this occurs is called the dew point temperature. If the dew point temperature is above freezing, dew will form. If the temperature is below freezing, frost will form. Both dew and frost form directly on objects and do not fall from the air.

Water vapor condenses in the air on condensation nuclei. If this happens near the ground, the accumulation of tiny water droplets is called a fog. Clouds are accumulations of tiny water droplets in the air above the ground. In general, there are three basic shapes of clouds, cirrus, cumulus, and stratus. These basic cloud shapes have meaning about the atmospheric conditions and about the coming weather conditions. Water that returns to the earth in liquid or solid form falls from the clouds as precipitation. Precipitation forms in clouds through two processes: (1) the coalescence of cloud droplets or (2) the growth of ice crystals at the expense of water droplets.

Weather changes are associated with the movement of large bodies of air called air masses, the leading fronts of air masses when they move, and local high- and low-pressure patterns that accompany air masses or fronts. Examples of air masses include (1) continental polar, (2) maritime polar, (3) continental tropical, and (4) maritime tropical.

When a location is under the influence of an air mass the location is having air mass weather with slow, gradual changes. More rapid changes take place when the front, a thin transition zone between two air masses, passes a location.

A stationary front often develops a bulge, or wave, that forms into a moving cold front and a moving warm front. The faster moving cold front overtakes the warm front, lifting it into the air to form an occluded front. The lifting process forms a low-pressure center called a cyclone. Cyclones are associated with heavy clouds, precipitation, and stormy conditions because of the lifting action.

A thunderstorm is a brief, intense storm with rain, lightning and thunder, gusty and strong winds, and sometimes hail. A tornado is the smallest, most violent weather disturbance that occurs on the earth. A hurricane is a tropical cyclone, a large, violent circular storm that is born over warm tropical waters near the equator.

The general pattern of the weather that occurs for a region over a number of years is called climate. The three principal climate zones are (1) the tropical climate zone, (2) the polar climate zone, and (3) the temperate climate zone. The climate in these zones is influenced by four factors that determine the local climate: (1) altitude, (2) mountains, (3) large bodies of water, and (4) ocean currents. The climate for a given location is described by first considering the principal climate zone, then looking at subdivisions within each that result from local influences.