FLOOD05189

• County: Statewide
• Community: State of Colorado
• Stream Name: All
• Basin: Statewide
• Title: Survey of Thunderstorm Rainfall Characteristics - 1979 to 1988
• Date: 9/1/1989
• Prepared For: CWCB
• Prepared By: Henz Kelly Associates
• Floodplain - Doc Type: Educational/Technical/Reference Information

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2.0 General Storm Development Ingredients

What is needed tD produce precipitation? The answer is moisture and
lift. In general, precipitation is associated with storm systems
which include cODler weather, low pressure centers and upper air
troughs, Non-precipitatinB weather is associated with wsrmer
weather, high pressure centers and ridges. These relationships
result from horizontal and vertical flows Df temperature and
mDisture fields.

The earth's ongoing attempt to balance beat and energy frDm tbe sun
on its spherical surface is the weatber macbine. The Dngoing
energy balancing act is done largely by wind circulations and
different air masses. Air masses derive tbeir names from their
source regions. Continental Polar air masses bave their origins
over relatively dry land and relatively colder polar regions.
Maritime Polar air masses are cool and wet witb tbeir origins over
oceans in tbe polar regions. Continental Tropical air masses are
warm and dry witb origins over land in tbe tropical regions.
Maritime TrDpical air masses are warm and wet witb origins over
oceans in tbe tropics.

Lift can be provided by a number of mecbanisms working alone or in
concert reinfDrcing each Dther. One mechanism is the cold front
wbere a colder, heavier air mass such as a Conti.ental PDlar air
mass cDllides with a warmer, lighter air mass sucb as a Maritime
Tropical air mass. The beavier CDntinental Polar air mass forces
itself under and lifts tbe lighter Maritime Tropical air mass. As
air is lifted in the ataospbere it cools. And if lifted and
cooled enDugh. clDuds and precipitation reault.

Weatber features generally associated witb cold frDnts are upper
air troughs, storm tracka or jet stream maximuma and IDw pressure
centers. Each of these factors add lift to the atmDspbere which
aids in the production of precipitatiDn.

Convection is a lifting mechanism produced when differential
heating of the earth's surface and the air near tbe earth's
surface causes density differences of the air. As air is warmed
it expands and becDaes lighter or less dense. Relatively cooler,
heavier air will sink and replace relatively waraer,lighter air;
thereby forcing the warmer,lighter air to rise. Once convection
is initiated the stability of tbe air will determine if the air
will cDntinue tD rise. If the air remains warmer and lighter than
the ambient air, the ligbter air will continue to rise. This
condition is called unstable as tbe air moves upward and away frDm
its original position. Stable air would be cooler and besvier than
its envirDnment: therefore, sinking back tDward its Driginal
level, Tberefore, as tbe earth is beated , warm air rises.