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Clouds form when the water vapor condenses into small particles. The particles
in clouds can either be liquid or solids. Liquid particles suspended in the
atmosphere are referred to as cloud droplets and the solid particles are often
called ice crystals. As a volume of unsaturated air cools, its relative humidity
increases. If sufficiently cooled, the relative humidity becomes 100%, the
temperature equals the dew point. Here's a fun hands-on
activity (teaching
applet) to help you explore the relationship between temperature,
dew point temperature and relative humidity.
The potential for cloud formation (and precipitation) depends on the amount of water vapor in the atmosphere. As a parcel of air rises, the moisture it contains cools and condenses out onto small particles of dust called cloud condensation nuclei until a cloud forms. Lifting, also referred to as adiabatic cooling, is the most common method of humidification of air to form clouds. As air rises it expands because pressure decreases with altitude. Kinetic energy is converted to potential energy and the parcel temperature decreases, and the relative humidity increases. The method of vertical lifting (orographic, convective, convergence, or frontal) and the stability of the atmosphere determines the type of cloud. Cumulus clouds tend to form in unstable atmospheres. Layered clouds form in more stable environments in which large layers of air are slowly lifted. The two main large scale lifting processes that result in cloud formation are convection and advection of air. Convection refers to air rising vertically in the atmosphere due to heating. Advection is the horizontal transfer of air that usually results in warmer air being forced up over cooler air. Both advection and convection results in the formation of clouds.
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