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Water
Water is a necessary component of the convection cycle. Water exists in three states: solid, liquid and gas. Water in gaseous form is called water vapor. Moist air is air heavy with water vapor. Water vapor forms droplets by coalescing around cloud condensation nuclei, which are tiny salt particles, sulfate or nitrate aerosol or particles in smoke. Frozen droplets become ice crystals. A cloud may be composed of water droplets, ice crystals or a combination of both. Droplets join to form rain drops. Ice crystals join to form snowflakes.
Thermals
Earth absorbs heat from the sun, then radiates that heat back into the atmosphere. This long-wave radiation forms convection cells that transfer the heat primarily in a vertical direction, which forms a buoyant, rising column of air called a thermal. Thermals carry water vapor upwards with them by means of evaporation.
Temperature
As long as the temperature within the thermal is higher than the surrounding air temperature, the thermal continues to rise. As the air in the thermal rises, however, it expands due to decreasing atmospheric pressure, and the temperature drops a specific amount for every foot of ascension. This is known as adiabatic cooling. Buoyancy is lost as temperatures fall.
Cloud Formation
As the temperature in a thermal falls, the kinetic energy of the water vapor molecules slows. Water vapor begins to precipitate into tiny water droplets. Clouds, composed of millions of these droplets, begin to form. A pot of water boiled on the stove demonstrates this process. The water is heated by the burner, as the air is heated by heat radiating from the earth. The hot water rises, like a thermal. When it cools as it escapes the pot, the water vapor condenses to become steam, like a small cloud.
Cloud Types
Once clouds begin to form, evaporation at their edges cools the air, causing it to subside, or sink. Convection and vertical cloud growth is limited by this process. Cumulus clouds form, which are typical of fair weather. An unstable air layer, fueled by the latent heat released by water vapor condensing into water, allows more convection, continued vertical cloud growth and the formation of cumulonimbus clouds, which are typical of thunderstorms.
Cumulonimbus Clouds
The vertical towers that form in cumulonimbus clouds reach 10 kilometers tall at higher latitudes and as much as 20 kilometers near the equator. They don't exceed these heights because that is where they bump up against the tropopause. The tropopause is the boundary between the troposphere and the stratosphere, the point at which air gets no colder as altitude increases. Tops of cumulonimbus clouds flatten against the tropopause into characteristic anvil shapes. Large aggregations of cumulonimbus clouds formed from deep rotating updrafts are called supercells. They produce lightning, hail and strong winds.
Precipitation
Precipitation occurs when the temperature high in the atmosphere is below freezing, as is typical in cumulonimbus clouds. Ice crystals form and fall towards Earth, collecting to form snowflakes. When the temperature rises above 32 degrees Fahrenheit, the snowflakes melt and become raindrops.
Global Convection
Convective cells generally move air from the equator toward the poles. Three types of convective cells cooperate to keep air circulating world-wide. Hadley cells govern tropical and sub-tropical climates. They move low-latitude air toward the equator, where it rises and moves toward the poles in the upper atmosphere. Ferrell cells are mid-latitude cells that move toward the poles and easterly near the surface, and toward the equator and westerly at high levels. Polar cells rise and travel toward the poles, where they sink, forming areas of high pressure, and diverge outward. Polar winds at the surface are easterly.