Cloud formation and properties
Clouds form when the invisible water vapor in the air condenses into visible water droplets or ice crystals. This can happen in three ways:
1. The air is cooled below its saturation point. This happens when the air comes in contact with a cold surface or a surface that is cooling by radiation, or the air is cooled by adiabatic expansion (rising). This can happen:
* along warm and cold fronts
* where air flows up the side of a mountain and cools as it rises higher into the atmosphere
* by the convection caused by the warming of a surface by insolation
* when warm air blows over a colder surface such as a cool body of water.
2. Clouds can be formed when two air masses below saturation point mix. Examples are breath on a cold day, aircraft contrails and Arctic sea smoke.
3. The air stays the same temperature but absorbs more water vapor into it until it reaches saturation point.
The water in a typical cloud can have a mass of up to several million tonnes. However, the volume of a cloud is correspondingly high, and the net density of the relatively warm air holding the droplets is low enough that air currents below and within the cloud are capable of keeping it suspended. As well, conditions inside a cloud are not static: water droplets are constantly forming and re-evaporating. A typical cloud droplet has a radius on the order of 1 x 10-5 m and a terminal velocity of about 1-2 cm/s. This gives these droplets plenty of time to re-evaporate as they fall into the warmer air beneath the cloud.
1. The air is cooled below its saturation point. This happens when the air comes in contact with a cold surface or a surface that is cooling by radiation, or the air is cooled by adiabatic expansion (rising). This can happen:
* along warm and cold fronts
* where air flows up the side of a mountain and cools as it rises higher into the atmosphere
* by the convection caused by the warming of a surface by insolation
* when warm air blows over a colder surface such as a cool body of water.
2. Clouds can be formed when two air masses below saturation point mix. Examples are breath on a cold day, aircraft contrails and Arctic sea smoke.
3. The air stays the same temperature but absorbs more water vapor into it until it reaches saturation point.
The water in a typical cloud can have a mass of up to several million tonnes. However, the volume of a cloud is correspondingly high, and the net density of the relatively warm air holding the droplets is low enough that air currents below and within the cloud are capable of keeping it suspended. As well, conditions inside a cloud are not static: water droplets are constantly forming and re-evaporating. A typical cloud droplet has a radius on the order of 1 x 10-5 m and a terminal velocity of about 1-2 cm/s. This gives these droplets plenty of time to re-evaporate as they fall into the warmer air beneath the cloud.
posted by JANET ROSE at 3:21 AM
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