• Water vapor, an odorless, colorless gas, changes from one state of matter (solid, liquid, or gas) to another at the temperatures and pressures experienced near Earth's surface. The processes involved in changing the state of matter of water are evaporation, condensation, melting, freezing, sublimation, and deposition.

• Humidity is the general term used to describe the amount of water vapor in the air. Relative humidity, the ratio (expressed as a percent) of the air's water vapor content to its water vapor capacity at a given temperature, is the most familiar term used to describe humidity. The water vapor capacity of air is temperature dependent, with warm air having a much greater capacity than cold air.

• Relative humidity can be changed in two ways. One is by adding or subtracting water vapor. The second is by changing the air's temperature. When air is cooled, its relative humidity increases. Air is said to be saturated when it contains the maximum quantity of water vapor that it can hold at any given temperature and pressure. Dew point is the temperature to which air would have to be cooled in order to reach saturation.

  

• The cooling of air as it rises and expands due to successively lower pressure is the basic cloud-forming process. Temperature changes in air brought about by compressing or expanding the air are called adiabatic temperature changes. Unsaturated air warms by compression and cools by expansion at the rather constant rate of 10°C per 1000 meters of altitude change, a figure called the dry adiabatic rate. If air rises high enough, it will cool sufficiently to cause condensation and form a cloud. From this point on, air that continues to rise will cool at the wet adiabatic rate which varies from 5°C to 9°C per 1000 meters of ascent. The difference in the wet and dry adiabatic rates is caused by the condensing water vapor releasing latent heat, thereby reducing the rate at which the air cools.

• Three mechanisms that can initiate the vertical movement of air are 1) orographic lifting, which occurs when elevated terrains, such as mountains, act as barriers to the flow of air; 2) frontal wedging, when cool air acts as a barrier over which warmer, less dense air rises; 3) convergence, which happens when air flows together and a general upward movement of air occurs; and 4) localized convective lifting, when unequal surface heating causes localized pockets of air to rise.

• The stability of air is determined by examining the temperature of the atmosphere at various altitudes. Air is said to be unstable when the environmental lapse rate (the rate of temperature decrease with increasing altitude in the troposphere) is greater than the dry adiabatic rate. Stated differently, a column of air is unstable when the air near the bottom is significantly warmer (less dense) than the air aloft. When stable air is forced aloft, precipitation, if any, is light, whereas unstable air generates towering clouds and stormy conditions.

  

• For condensation to occur, air must be saturated. Saturation takes place either when air is cooled to its dew point, which most commonly happens, or when water vapor is added to the air. There must also be a surface on which the water vapor may condense. In cloud and fog formation, tiny particles called condensation nuclei serve this purpose.

• Clouds are classified on the basis of their appearance and height. The three basic forms are cirrus (high, white, thin, wispy fibers), cumulus (globular, individual cloud masses), and stratus (sheets or layers that cover much or all of the sky). The four categories based on height are high clouds (bases normally above 6000 meters), middle clouds (from 2000 to 6000 meters), low clouds (below 2000 meters), and clouds of vertical development.

• Fog is defined as a cloud with its base at or very near the ground. Fogs form when air is cooled below its dew point or when enough water vapor is added to the air to bring about saturation. Various types of fog include advection fog, radiation fog, upslope fog, steam fog, and frontal (or precipitation), fog.

  

• For precipitation to form, millions of cloud droplets must somehow join together into large drops. Two mechanisms for the formation of precipitation have been proposed. One, in clouds where the temperatures are below freezing, ice crystals form and fall as snowflakes. At lower altitudes the snowflakes melt and become raindrops before they reach the ground. Two, large droplets form in warm clouds that contain large hygroscopic ("water seeking") nuclei, such as salt particles. As these big droplets descend, they collide and join with smaller water droplets. After many collisions the droplets are large enough to fall to the ground as rain.

• The forms of precipitation include rain, snow, sleet, hail, and rime.

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