What is the difference between the
various types of precipitation? I.e. snow, sleet, freezing rain and rain
Precipitation can be divided
into three types dependent on whether it falls as liquid water, liquid water
that freezes on impact with the surface, or ice. Different types of precipitation
can fall simultaneously. Rain is an example of liquid precipitation, freezing
rain is an example of liquid that freezes on impact, and snow is an example of
frozen precipitation.
Rain, and all precipitation,
occurs when a portion of the atmosphere becomes saturated with water vapour (the
‘gas phase’ of water) and the water condenses. Two processes can lead to the
air becoming saturated; cooling the air and adding water vapour to the air.
Freezing rain is where a
droplet starts as snow, and melts as it falls through the layers of the
atmosphere. This melted snowflake then hits a below freezing layer of the
atmosphere near the Earth’s surface. This quickly cools the droplet, meaning
that on impact with a surface it will freeze; turning into ice and coating what
it hits with clear ice. The cold layer which the melted snowflake hits is thick
enough to cool the drop below 0cº but not thick enough to freeze it to sleet.
Sleet is similar to freezing
rain except that it freezes before it hits the Earth’s surface. A snowflake
falls, enters a warm layer and melts. This melted snowflake then enters a cold
layer near the surface of the Earth and freezes again. When this frozen droplet
hits the Earth, it is usually in the form of ice and it does not freeze on
impact.
Snow, on the other hand, can
only form when the atmospheric temperature is below 0cº and won’t settle if the
ground temperature is above 4cº. Snow will form when the atmospheric
temperature is below 0cº and when there is a minimum amount of moisture in the
air. Snow is basically made up of crystals of frozen water; ice crystals have 6
points, however one snowflake can consist of multiple crystals. Snow will reach
the Earth’s surface so long as it does not pass through any layers in the
atmosphere that are above freezing.
Emma Barkley, 14th
February 2012
Thanks for a clear and helpful summary - for those of us who learn by pictures, I am wondering if there are any animations out there to tell the same story???
ReplyDeleteDuncan
Hi Duncan, I posted it separately, but I found an interesting youtube video which could help visual learners.
DeleteEmma