Most of our Precipitation is Associated With Atmospheric Fronts

Research suggests precipitation resulting from atmospheric fronts is an important component of global rainfall, accounting for around 57 percent of total midlatitude (30 to 60 degrees latitude) precipitation. Precipitation associated with warm fronts is the most common type over much of North America. 

There are three main types of precipitation:

1) Convective, where heating of the Earth’s surface causes parcels of air to rise until they have cooled sufficiently for condensation and rainfall;

2) Orographic, where mountains cause passing moist air masses to rise to altitudes where condensation takes place;

3) Frontal or Cyclonic precipitation, driven by interactions between relatively warm and cold air masses.

In frontal precipitation, parcels of denser and generally drier cold air go underneath parcels of warmer air, forcing the warmer air to uplift to the condensation level of the atmosphere and precipitate. This type of precipitation is generally associated with cyclones, where counterclockwise winds blow inward around a low-pressure center (these winds blow clockwise but still inward in the Southern Hemisphere).

Warm fronts, where a warm air mass is moving towards colder air, generally move slower than cold fronts, where a cold air mass is moving towards warmer air. These differences in speed mean differences in precipitation type and intensity.

Cold fronts generally lead to more severe weather, but less overall precipitation: the average precipitation that occurs with a midlatitude cold front is two millimeters versus three millimeters for the average precipitation that occurs with a midlatitude warm front.

In some places, such as the North Pacific and North Atlantic where warm ocean currents create frequent temperature contrasts and frontal activities, 90 percent of total annual precipitation takes place when a front is present. In midlatitude regions on land, the proportions are not that high.

Cold front precipitation accounts for less than 25 percent of annual precipitation over most of North America, while warm front values are between 30 and 58 percent.

For more information on fronts, including diagrams, visit: http://www.srh.noaa.gov/crp/?n=education-airmasses.

(Source: Catto, JL et al. “Relating global precipitation to atmospheric fronts.” Geophysical Research Letters 39 (2012): L10805.)   


 
The material above was generated by Earth Gauge which his supported by the National Environmental Education Foundation based in Washington, DC.  Their web site is: www.earthgauge.net


Return to News Archive

USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.