Several valid theories have been formulated in regard to the growth of raindrops. The theories most widely accepted today are treated here in combined form.

Raindrops grow in size primarily because water exists in all three phases in the atmosphere and because the air is supersaturated at times (especially with respect to ice) because of adiabatic expansion and radiational cooling. This means that ice crystals coexist with liquid water droplets in the same cloud. The difference in the vapor pressure between the water droplets and the ice crystals causes water droplets to evaporate and then to sublimate directly onto the ice crystals. Sublimation is the process whereby water vapor changes into ice without passing through the liquid stage. Condensation alone does not cause droplets of water to grow in size. The turbulence in clouds permits and aids this droplet growth pro-cess. After the droplets become larger, they start to descend and are tossed up again in turbulent updrafts within the cloud. The repetition of ascen-sion and descension causes the ice crystals to grow larger (by water vapor sublimating onto the ice crystals) until finally they are heavy enough to fall out of the cloud as some form of precipitation. It is believed that most precipitation in the mid-latitudes starts as ice crystals and that most liquid precipitation results from melting during descent through a stratum of warmer air. It is generally believed that most rain in the tropics forms without going through the ice phase.

In addition to the above process of droplet growth, simple ACCRETION is important. In this process, the collision of ice crystals with supercooled water droplets causes the droplets to freeze on contact with the ice crystals. As the droplets freeze on the ice crystals, more layers accumulate. This process is especially effective in the formation of hail.

There are other factors that explain, in part, the growth of precipitation, but the above processes are the primary ones.

Privacy Statement - Press Release - Copyright Information. - Contact Us - Support Integrated Publishing