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    1.    Compare and contrast how precipitation forms in warm clouds and in cold clouds.

    2.    Describe how rising air produces condensation.

Key Terms:

    precipitation         sleet        freezing rain        hail

 Notes: (18-3)

Precipitation refers to any form of water that falls from the sky.   It can be measured using a simple rain gauge.  No matter what type of precipitation that you are talking about, it all starts with the condensation nuclei.
the size of the condensation nuclei determines the size of the initial droplet
larger droplets capture smaller droplets on the way down
the longer the droplet is in the cloud the larger it grows

The type precipitation depends on the conditions surrounding its formation and delivery.

Rain and Drizzle

separated by size
rain = > 0.5mm & drizzle < 0.5mm
freezing rain occurs when rain is super-cooled on the way to Earth and freezes on contact with Earth's surface


Higher altitude rain falls through a lower freezing layer of air 
Forms into ice crystals before reaching Earth's surface



Forms much the same way as sleet
is kept aloft by wind as it accumulates more frozen water mass
falls to the ground once its mass is heavier than the force of the air currents

Cross-section of thunderstorm showing warm updrafts (red), cold downdrafts (blue), and feezing level (black line)(www.erh.noaa.gov/er/ cae/svrwx/hail.jpg)

A large irregular hailstone
Photo from
National Center for Atmospheric Research

usually 1-5cm in diameter (13.97 cm is largest on record)


is an aggregate of ice crystals that does not thaw on its way to Earth's surface


(www.nwac.noaa.gov/ CMT_Powder.gif)




Last modified: March 19, 2003