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PRECIPITATION PROCESSES

A. INTRODUCTION

Adiabatic Processes

processes that produce a change in temperature within a gas due to expansion or compression





expansion: air cools





compression: air warms





changes in pressure may cause expansion or compression

Lapse Rates

ELR: environmental lapse rate varies





DALR: dry adiabatic lapse rate = 1^oC/100m





SALR: saturated adiabatic lapse rate = 0.6^oC/100m

B. OROGRAPHIC UPLIFT  

Precipitation on windward side of mountains

Rainshadow desert on leeward side of mountains

Photo: Orographic precipitation in the Andes. Chile is wet and green while Argentina is brown and dry.

Image credit: Courtesy MODIS, NASA. Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images

C. CONVECTIONAL UPLIFT  

Cause:

differential heating of atmosphere





warm air less dense than cold; warm air rises

Example: the cloudy intertropical convergence zone

Image source: NASA Earth Observatory http://earthobservatory.nasa.gov/Newsroom/NewImages/images_topic.php3?topic=atmosphere&img_id=4028

Atmospheric Stability

tendency for air to either rise or fall through the atmosphere





stable conditions inhibit air from rising





unstable conditions cause air to rise spontaneously

Determining Stability

compare ELR temperature to temperature of rising air at the same altitude





if rising air is warmer than the ELR, it' s unstable and will continue rise





unstable air rises and cools until it reaches an altitude where its temperature equals that of the ELR





condensation level: altitude of the base of a convectional cloud





stability level: altitude of the top of a convectional cloud

Examples

Air Mass Thunderstorms

C. CYCLONIC (convergent) UPLIFT

Convergence in cyclones causes air to rise

Midlatitude Cyclones  

Tropical Cyclones (hurricanes)    

D. FRONTAL UPLIFT

Air Mass

a uniform body of air that moves as a unit

Temperature	Moisture
A: Arctic (really cold)	m: maritime (moist)
P: polar (cold)	c: continental (dry)
T: tropical (warm)

Air masses of North America

Fronts

boundary between contrasting air masses





stationary front: boundary is stationary on the landscape





cold front: cold air moves into a region occupied by warmer air





warm front: warm air moves into a region occupied by colder air





occluded front: a cold front "catches up" with a warm front forcing all warm air off the ground

Midlatitude Cyclones

Image credit: ©Bruce Molnia, Terra Photographics. Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images

Image credit: Ralph F. Kresge, NOAA National Weather Service Historic Album http://www.photolib.noaa.gov/historic/nws/clouds3.html

Image credit: Ralph F. Kresge, NOAA National Weather Service, Historic NWS Collection http://www.photolib.noaa.gov/historic/nws/wea02047.htm

Severe Thunderstorms and Tornados

Image credit: C. Clark NOAA Photo Library, NOAA Central Library; OAR/ERL/National Severe Storms Laboratory (NSSL) http://www.photolib.noaa.gov/nssl/nsind1.htm

Image credit: NOAA Photo Library, NOAA Central Library; OAR/ERL/National Severe Storms Laboratory (NSSL) http://www.photolib.noaa.gov/nssl/nsind3.htm

E. SUMMARY

Lapse Rates

ELR, DALR and SALR

Uplift Mechanisms

orographic uplift





convectional uplift; atmospheric stability





cyclonic (convergent) uplift





frontal uplift: air masses; stationary, warm, cold, and occluded fronts

Global Precipitation Pattern

Equatorial low (ITCZ): convectional uplift





Subpolar low and Polar Front: cyclonic and frontal uplift (midlatitude cyclones)





mountains of western North America, Andes Mountains, Himalayas: orographic uplift





seasonally variable precipitation between subtropical high pressure cells and Polar front