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Lecture 13

ATMO 201 Lecture Notes - Lecture 13: Intertropical Convergence Zone, Cirrus Cloud, Nimbostratus Cloud

Atmospheric Sciences
Course Code
ATMO 201
Timothy Logan

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Lecture 13
Tuesday, March 7, 2017
3:50 PM
Classic atmospheric profiles
Note that raindrops can freeze on contact with cold ground while air
temperature is above freezing
Road surface, grass, objects cool faster than the air and water can easily
At 32 F you can still have rain
The temperature profile, once it gets above 32 F, you have to melt.
Once you melt from a snowflake, you never go back. You get either sleet or
freezing rain
oHigher up melting and the freezing again results in sleet and if it
freezes back once it makes contact with the ground, it is freezing rain
Precipitation Types
Precipitation in the form of frozen spheres or irregular lumps of ice, always
produced by convective clouds, ,nearly always cumulonimbus
Alternating clear and "frosty" layers
An individual unit of hail is called a hailstone
By convention, hail has a diameter of 5 mm or more, while smaller particles
of similar origin, formerly called small hail, may be classed as either ice
pellets or snow pellets
Hail formation and growth
Multiple theories and models
Embryo Stage (forming stage)
oGraupel - snow or ice pellets (less than 5 mm)
oDirt particles and insects
Translation and Accretion stage
oSimilar to cold rain process
oLateral movement (translation)(hail goes sideways) of ice particles -
more chance for riming and freezing (accretion)
oHailstones have layers
oSCLW (slow freezing, clear layer) or ice (quick freezing, frosty layer) -
see page 162!
Ejection stage
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