|MadSci Network: Earth Sciences|
On a typical sunny day, a parcel of moist air will rise. The air will cool as it rises in adiabatic ascent. Since the vapor pressure of water decreases with a decrease in temperature, the relative humidity of the parcel will gradually increase until it hits about 101.5% In this supersaturated state, cloud formation often occurs on atmospheric aerosol particles. A typical cloud droplet has a radius on the order of 1 x 10-5 m. If we assume a spherical droplet, then on free fall, this droplet will have a terminal velocity of about 1-2 cm/s. As a typical cloud droplet falls back down at this velocity, it will enter warmer and warmer air. The warmer air with lower relative humidity, combined with the slow velocity, will allow the cloud droplet time to evaporate. In addition, the small size of the droplet decreases the likelihood of the droplet colliding with another droplet, and thereby forming a larger particle. Cloud particles with radii larger than 1.8 x 10-5 m will likely form rain droplets due to cascading collisions.
The dynamic rise and fall of cloud particles in an equilibrium results in a stable collection of water particles that we witness as clouds. Please note there are many different types of clouds and this explanation only holds for one of them.
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