 |
PrecipitationMost precipitation falls as rain, but technically, it covers all forms of water - rain, freezing rain, sleet, snow, or hail - that can be coerced from the clouds. The official definition states that it "is the primary connection in the water cycle that provides for the delivery of atmospheric water to the Earth."How Do Raindrops Form?The clouds floating overhead contain water vapor and cloud droplets, which are small drops of condensed water. These droplets are way too small to fall, but they are large enough to form visible clouds. Water is continually evaporating and condensing in the sky. If you look closely at a cloud you can see some parts disappearing (evaporating) while other parts are growing (condensation). Most of the condensed water in clouds does not fall as precipitation because their fall speed is not large enough to overcome updrafts which support the clouds. For precipitation to happen, first tiny water droplets must condense on even tinier dust, salt, or smoke particles, which act as a nucleus. Water droplets may grow as a result of additional condensation of water vapor when the particles collide. If enough collisions occur to produce a droplet with a fall velocity which exceeds the cloud updraft speed, then it will fall out of the cloud as precipitation. This is no small task since millions of cloud droplets are required to produce a single raindrop. Precipitation Rates Vary Geographically |
