GEOG 1 Lecture Notes - Lecture 18: Vertical Draft, Leading Edge, Cool Air
Document Summary
Precipitation - any form of water particles that falls from the atmosphere and reaches the. Air contains water vapour, which is the source of all precipitation. Ascending air can expand because there is less pressure on it. When heat from the lower air spreads through a larger volume in the troposphere, the mass of air becomes cooler. Cool air is less able to hold water vapour than warm air. Air is said to be supersaturated when it contains so much water vapour that it condenses (gas to liquid) and forms particles called condensation nuclei. At first, the tiny water droplets are usually too light to fall. As many droplets coalesce into larger drops, and become too heavy to remain suspended in air, they fall as rain. Large numbers of rain droplets or ice crystals form clouds, which are supported by slight upward movements of air. Descending air in high pressure zones usually yields cloudless skies.
