Precipitation (meteorology)
:''This article is about the meteorological term. For the chemistry term, see
Precipitation (chemistry) |
Late-summer rainstorm in Denmark |
In
meteorology,
precipitation (also known as hydrometeor) is any form of
water that falls from the sky as part of the
weather to the ground. This includes
snow,
rain,
sleet,
freezing rain,
hail, and
virga. Precipitation is a major component of the
hydrologic cycle, and is responsible for depositing most of the
fresh water on the planet.
|
Condensation and coalescence are important parts of the Water cycle |
Condensation
Precipitation begins forming when relatively warm, moist air rises. As the air cools, water vapor begins to condense on
condensation nuclei, forming clouds. After the water
droplets grow large enough, two processes can occur to form precipitation.
Coalescence
Coalescence occurs when water droplets merge to create larger water droplets, or when water droplets freeze onto an ice crystal. Because of air resistance, the water droplets in a cloud typically remain stationary. When air turbulence occurs, water droplets collide, producing larger droplets. As these larger water droplets descend, coalescence continues, so that drops become heavy enough to overcome air resistance and fall as rain. Coalescence generally happens most often in clouds above freezing.
Bergeron process
The
Bergeron Process occurs when ice crystals acquire
water molecules from nearby
supercooled water droplets. As these ice crystals gain mass, they may begin to fall, acquiring more mass as coalescence occurs between the crystal and neighboring water droplets. This process is temperature dependent, as supercooled water droplets only exist in a cloud that is below freezing. In addition, because of the great temperature differential between cloud and ground level, these ice crystals may melt and become rain as they fall.
Precipitation is any product of the condensation of atmosphereic water vapour that is deposited on the earth's surface. Precipitation that forms aloft is divided into three categories:
Liquid Precipitation;Freezing Precipitation andFrozen Precipitation.
* Types of Liquid Precipitation:
**
Drizzle (DZ)
**
Rain (RA)
* Types of Freezing Precipitation:
**
Freezing Drizzle (FZDZ)
**
Freezing Rain (FZRA)
* Types of Frozen Precipitation:
**
Snow (SN)
**
Snow pellets (SHGS)
**
Snow Grains (SG)
**
Ice Pellets (PL)
**
Hail (SHGR)
**
Ice Crystals (IC)
The code in the brackets is the
METAR Code for the phenomena
The standard way of measuring rainfall or snowfall is the standard rain gage, which can be found in 4-inch/100 mm plastic and 8-inch/200 mm metal varieties.
[http://www.crh.noaa.gov/iwx/program_areas/coop/8inch.php] The inner cylinder is filled by 25 mm/1 inch of rain, with overflow flowing into the outer cylinder. Plastic gages will have markings on the inner cylinder down to 0.25 mm/0.01" resolution, which metal gages will require use of a stick designed with the appropriate 0.25 mm/0.01" markings. After the inner cylinder is filled, the amount inside it is discarded, then filled with the remaining rainfall in the outer cylinder until all the fluid in the outer cylinder is gone, adding to the overall total until the outer cylinder is empty. These gages are winterized by removing the funnel and inner cylinder and allowing the snow/freezing rain to collect inside the outer cylinder. Once the snowfall/ice is finished accumulating, or as you approach 300 mm/12", one can either bring it inside to melt, or use luke warm water to fill the inner cylinder with in order to melt the frozen precipitation in the outer cylinder, keeping track of the warm fluid added, which is subsequently subtracted from the overall total once all the ice/snow is melted.
Other types of gages include the popular wedge gage (the cheapest rain gage and most fragile), the tipping bucket rain gage
[http://www.oardc.ohio-state.edu/williams.2157/Tipping_Bucket_Rain_Gage.htm], and the weighing rain gage
[http://www.weather.gov/glossary/index.php?letter=w]. The wedge and tipping bucket gages will have problems with snow. Attempts to compensate for snow/ice by warming the tipping bucket meet with limited success, since snow may sublimate if the gage is kept much above freezing. Weighing gages with antifreeze should do fine with snow, but again, the funnel needs to be removed before the event begins. For those looking to measure rainfall the most inexpensively, a can that is cylindrical with straight sides will act as a rain gage if left out in the open, but its accuracy will depend on what ruler you use to measure the rain with. Any of the above rain gages can be made at home, with enough know-how.
[http://school.discovery.com/lessonplans/activities/weatherstation/itsrainingitspouring.html]Once someone has a device to measure precipitation, various networks exist across the United States and elsewhere where rainfall measurements can be submitted through the internet, such as CoCoRAHS
[http://cocorahs.org] or GLOBE
[http://www.globe.gov/fsl/welcome/welcomeobject.pl]. If a network is not available in the area where one lives, the nearest local weather office will likely be interested in the measurement.
[http://www.nws.noaa.gov] An important use of precipitation data is for forecasting of river flows and river water quality using
hydrology transport models such as
SWMM, SHE or the
DSSAM Model.
Western
Major elements are prevailing westerly winds and
ocean currents moving toward the
equator. At high
latitudes the current is warmer than land, westerly winds take up moisture and cool when moving over land. When the land is warmer than the ocean, clouds do not produce precipitation, but accrete additional moisture, generating rain eventually at higher elevations. When the land is cooler than the ocean, then westerlies cool as they move inland and rain occurs in the lowlands.
Eastern and central
Polar air masses (above 50° latitude) are distinct from lower latitude air masses. The westerlies are warm air masses that move poleward from 30°N. In the eastern US the westerlies are often laden with moisture from Gulf of Mexico and Atlantic. When polar and westerly air masses meet, precipitation occurs.
Inland continental areas
Areas not proximate to large bodies of water warm more rapidly than
coastal or
lakeside regions. Hot air masses can rise from the center of the continent forming a low pressure area. This low pressure zone draws water laden clouds from the coasts. When this area warms, adiabatic change occurs and ultimately precipitation. The outcome is more summer rain, and less winter precipitation.
*
Monsoon*
Steam*
Umbrella*
Collision/Coalescence; The Bergeron Process*
Report your local rainfall inside the United States at this site (CoCoRaHS)*
Report your local rainfall related to tropical cyclones worldwide at this site
