Meteorology is the study of weather and climate and encompasses the whole range of atmospheric science. The atmosphere is an envelope of air some 620 miles (1,000 km) deep, held by gravity to Earth, with which it mostly rotates.
Air is a mixture of gases, about 78 percent nitrogen, 21 percent oxygen, and traces of other gases with a variable amount of water vapor. If it were not for water, there would be no clouds and no visible weather. Radiation from the Sun provides the energy necessary for motion within the atmosphere. The weather that affects us occurs in the lowest 7 miles (12 km) or so of the atmosphere in a region called the troposphere. In this region, the temperature decreases with height by about 11°F per 100 ft. (30 m), up to the tropopause. Above this boundary is the stratosphere, where the temperature is nearly constant or increases with height. It is the troposphere that attracts the most attention from meteorologists.
Atmospheric energy
Some of the energy radiating from the Sun reaches Earth and its envelope of air as virtually parallel rays. Earth is approximately spherical, and the radiation reaching the surface is more concentrated in equatorial regions than toward the poles. If there were no atmosphere or oceans, the tropics would become continually hotter and the poles colder. The excess energy in the tropics drives air and ocean currents that, in simple terms, transfer heat energy toward the poles and draw cooler air at lower levels toward the tropics. The uneven heating of the surface, and hence the air, leads to unequal air pressures. The forces due to pressure differences cause the air to move.
The uneven distribution of sea and land, with irregular mountain ranges and friction at low levels, all contribute to complex interacting forces that move the air in complex patterns. The air moves at speeds varying from calm to well over 100 knots (170 ft., or 50 m, per second), often in ribbons of wind called jet streams in the upper troposphere. The atmosphere not only acts as a heat distribution system but also filters out some of the radiation that is harmful to life.
Water
In the atmosphere, water is a very important constituent that takes three forms: invisible gas (water vapor), liquid (cloud droplets, fog, drizzle, rain, and so on), and solid (ice—most usually, minute crystals but also as snowflakes or hailstones). The change of state from gas to liquid to solid releases latent heat into the atmosphere. Energy in the form of latent heat is absorbed during the reverse changes. The cycle of events starts with water evaporating from the oceans, lakes, and other wet surfaces, with the Sun providing the energy (latent heat) needed to convert liquid water to vapor. The invisible vapor in the moist air rises into the atmosphere and is transported by the wind. Later, it condenses as droplets or ice crystals to form clouds, releasing latent heat in the process. Droplets and crystals combine, often with further condensation, to form rain or snow that precipitates to Earth and ultimately replenishes the oceans and other surface water. The complex motion of the atmosphere means that both moisture and heat are transferred around the globe. As the temperature of air increases, its capacity to absorb water vapor increases more rapidly, hence the vigor of clouds and rain is generally greater in tropical regions.
