Dear Tahta,

    I wanted you to know how pleased I was to read your e-mail. I'd like to tell you what I've learned about precipitation; Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail. It is the primary part in the water cycle that provides for the delivery of atmospheric water to the Earth. Most precipitation falls as rain.

    The clouds floating overhead contain water vapour and cloud droplets, which are small drops of condensed water. These droplets are much too small to fall as precipitation, 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 speed is not 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 vapour when the particles collide. If enough collisions occur to produce a droplet with a velocity which exceeds the cloud updraft speed, then it will fall out of the cloud as precipitation.  This is not a trivial task, since millions of cloud droplets are required to produce a single raindrop. A more efficient mechanism (known as the Bergeron-Findeisen process) for producing a precipitation-sized drop is through a process which leads to the rapid growth of ice crystals at the expense of the water vapour present in a cloud. These crystals may fall as snow, or melt and fall as rain.

    Well, that's all I know. But you may find more information about it from other resources. I hope that you do well in your final exam.

Best of luck!

Source: Adapted from water.usgs.gov 

Complete the statement with the information from the text.

Precipitation-sized drop mechanism is caused by _____________________________________