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Precipitation (meteorology)

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Long-term mean precipitation by month

In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena) is any product of the condensation of atmospheric water vapour that is deposited on the earth's surface.[1] It occurs when the atmosphere, a large gaseous solution, becomes saturated with water vapour and the water condenses, falling out of solution (i.e., precipitates).[2] Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapour to the air. Virga is precipitation that begins falling to the earth but evaporates before reaching the surface; it is one of the ways air can become saturated. Precipitation forms via collision with other rain drops or ice crystals within a cloud.

Rain drops range in size from oblate, pancake-like shapes for larger drops, to small spheres for smaller drops. Rain drops are not shaped like tear drops, but rather like flattened pancakes. Precipitation that reaches the surface of the earth can occur in many different forms, including rain, freezing rain, drizzle, ice needles, snow, ice pellets or sleet, graupel and hail. While snow and ice pellets require temperatures close to the ground to be near or below freezing, hail can occur during much warmer temperature regimes due to the process of its formation. Precipitation may occur on other celestial bodies, e.g. when it gets cold, Mars has precipitation which most likely takes the form of ice needles, rather than rain or snow.[3]

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Moisture overriding associated with weather fronts is a major method of precipitation production. If enough moisture and upward motion is present, precipitation falls from convective clouds such as cumulonimbus and can organize into narrow rainbands. Precipitation can also form due to forced ascent up the windward side of a mountain or mountain range. On the leeward side of mountains, desert climates can exist due to the dry air caused by compressional heating. The movement of the monsoon trough, or intertropical convergence zone, brings rainy seasons to savannah climes. Precipitation is a major component of the water cycle, and is responsible for depositing most of the fresh water on the planet. Approximately 505,000 km3 (121,000 cu mi) of water falls as precipitation each year; 398,000 km3 (95,000 cu mi) of it over the oceans.[4] Given the Earth's surface area, that means the globally-averaged annual precipitation is about 1 metre (39 in), and the average annual precipitation over oceans is about 1.1 metres (43 in).

Contents

[edit] Types

See also: Precipitation types (meteorology)
A thunderstorm with heavy precipitation

Precipitation is a major component of the water cycle, and is responsible for depositing most of the fresh water on the planet. Approximately 505,000 km3 (121,000 cu mi) of water falls as precipitation each year, 398,000 km3 (95,000 cu mi) of it over the oceans.[4] Given the Earth's surface area, that means the globally-averaged annual precipitation is about 1 m (39 in), and the average annual precipitation over oceans is about 1.1 m (43 in). Precipitation can be divided into three categories, based on whether it falls as liquid water, liquid water that freezes on contact with the surface, or ice. Mixtures of different types of precipitation, including types in different categories, can fall simultaneously.[5]

[edit] How air becomes saturated

[edit] Cooling air to its dew point

Late-summer rainstorm in Denmark

Air contains water vapour, measured in grams of water per kilogram of dry air (g/kg),[6] but most commonly reported as a relative humidity. How much water vapour a parcel of air can contain before it becomes saturated (100% relative humidity) depends on its temperature. Warmer air can contain more water vapour than cooler air before becoming saturated. Therefore, one way to saturate a parcel of air is to cool it. The dew point is the temperature to which a parcel must be cooled in order to become saturated.[7] Water vapor normally begins to condense on condensation nuclei such as dust, ice, and salt in order to form clouds. An elevated portion of a frontal zone forces broad areas of lift, which form clouds decks such as altostratus or cirrostratus. Stratus is a stable cloud deck which tends to form when a cool, stable air mass is trapped underneath a warm air mass. It can also form due to the lifting of advection fog during breezy conditions.[8]

There are four main mechanisms for cooling the air to its dew point: adiabatic cooling, conductive cooling, radiational cooling, and evaporative cooling. Adiabatic cooling occurs when air rises and expands.[9] The air can rise due to convection, large-scale atmospheric motions, or a physical barrier such as a mountain (orographic lift). Conductive cooling occurs when the air comes into contact with a colder surface,[10] usually by being blown from one surface to another, for example from a liquid water surface to colder land. Radiational cooling occurs due to the emission of infrared radiation, either by the air or by the surface underneath.[11] Evaporative cooling occurs when moisture is added to the air through evaporation, which forces the air temperature to cool to its wet bulb temperature, or until it reaches saturation.[12]

Lenticular cloud forming due to mountains over Wyoming.

[edit] Adding moisture to the air

The main ways water vapour is added to the air are:

  • Wind convergence into areas of upward motion[13]
  • Precipitation or virga falling from above[14]
  • Daytime heating evaporating water from the surface of oceans, water bodies or wet land[15]
  • Transpiration from plants[16]
  • Cool, dry air moving over warmer water[17]
  • Lifting air over mountains[18]

[edit] Formation

Main article: Water cycle
Condensation and coalescence are important parts of the water cycle

[edit] Coalescence and the Bergeron process

Coalescence occurs when water droplets fuse to create larger water droplets, or when water droplets freeze onto an ice crystal, which is known as the Bergeron process. Air resistance typically causes the water droplets in a cloud to 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. In clouds below freezing, when ice crystals gain enough mass they begin to fall. This generally requires more mass than coalescence when occurring 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 difference between cloud and ground level, these ice crystals may melt as they fall and become rain.[19]

[edit] Raindrop characteristics

Raindrops have sizes ranging from 0.1 millimetres (0.0039 in) to 9 millimetres (0.35 in) mean diameter, above which they tend to break up. Smaller drops are called cloud droplets, and their shape is spherical. As a raindrop increases in size, its shape becomes more oblate, with its largest cross-section facing the oncoming airflow. Contrary to the cartoon pictures of raindrops, their shape does not resemble a teardrop.[20] Intensity and duration of rainfall are usually inversely related, i.e., high intensity storms are likely to be of short duration and low intensity storms can have a long duration.[21][22] Rain drops associated with melting hail tend to be larger than other rain drops.[23]

[edit] Causes

[edit] Frontal activity

Main article: Weather fronts

Stratiform or dynamic precipitation occurs as a consequence of slow ascent of air in synoptic systems (on the order of cm/s), such as over surface cold fronts, and over and ahead of warm fronts. Similar ascent is seen around tropical cyclones outside of the eyewall, and in comma head precipitation patterns around mid-latitude cyclones.[24] A wide variety of weather can be found along an occluded front, with thunderstorms possible, but usually their passage is associated with a drying of the air mass. Occluded fronts usually form around mature low-pressure areas.[25] Precipitation may occur on other celestial bodies other than Earth. When it gets cold, Mars has precipitation which most likely takes the form of ice needles, rather than rain or snow.[3]

[edit] Convection

Convective precipitation

Convective rain, or showery precipitation, occurs from convective clouds, e.g., cumulonimbus or cumulus congestus. It falls as showers with rapidly changing intensity. Convective precipitation falls over a certain area for a relatively short time, as convective clouds have limited horizontal extent. Most precipitation in the tropics appears to be convective; however, it has been suggested that stratiform precipitation also occurs.[24][26] Graupel and hail indicate convection.[27] In mid-latitudes, convective precipitation is intermittent and often associated with baroclinic boundaries such as cold fronts, squall lines, and warm fronts.[28]

[edit] Orographic effects

Main articles: Orographic lift, Precipitation types (meteorology), and United States rainfall climatology
Orographic precipitation

Orographic precipitation occurs on the windward side of mountains and is caused by the rising air motion of a large-scale flow of moist air across the mountain ridge, resulting in adiabatic cooling and condensation. In mountainous parts of the world subjected to relatively consistent winds (for example, the trade winds), a more moist climate usually prevails on the windward side of a mountain than on the leeward or downwind side. Moisture is removed by orographic lift, leaving drier air (see katabatic wind) on the descending and generally warming, leeward side where a rain shadow is observed.[18]

In Hawaii, Mount Waiʻaleʻale, on the island of Kauai, is notable for its extreme rainfall, as it has the second highest average annual rainfall on Earth, with 460 inches (12,000 mm).[29] Storm systems affect the state with heavy rains between October and March. Local climates vary considerably on each island due to their topography, divisible into windward (Koʻolau) and leeward (Kona) regions based upon location relative to the higher mountains. Windward sides face the east to northeast trade winds and receive much more rainfall; leeward sides are drier and sunnier, with less rain and less cloud cover.[30]

In South America, the Andes mountain range blocks Pacific moisture that arrives in that continent, resulting in a desertlike climate just downwind across western Argentina.[31] The Sierra Nevada range creates the same effect in North America forming the Great Basin and Mojave Deserts.[32][33]

[edit] Within the tropics

Rainfall distribution by month in Cairns showing the extent of the wet season at that location
See also: Monsoon, Tropical cyclone, and Wet season

The wet, or rainy, season is the time of year, covering one or more months, when most of the average annual rainfall in a region falls.[34] The term green season is also sometimes used as a euphemism by tourist authorities.[35] Areas with wet seasons are dispersed across portions of the tropics and subtropics.[36] Savanna climates and areas with monsoon regimes have wet summers and dry winters. Tropical rainforests technically do not have dry or wet seasons, since their rainfall is equally distributed through the year.[37] Some areas with pronounced rainy seasons will see a break in rainfall mid-season when the intertropical convergence zone or monsoon trough move poleward of their location during the middle of the warm season.[21] When the wet season occurs during the warm season, or summer, rain falls mainly during the late afternoon and early evening hours. The wet season is a time when air quality improves,[38] freshwater quality improves,[39][40] and vegetation grows significantly. Soil nutrients diminish and erosion increases.[21] Animals have adaptation and survival strategies for the wetter regime. Unfortunately, the previous dry season leads to food shortages into the wet season, as the crops have yet to mature. Developing countries have noted that their populations show seasonal weight fluctuations due to food shortages seen before the first harvest, which occurs late in the wet season.[41]

Tropical cyclones, a source of very heavy rainfall, consist of large air masses several hundred miles across with low pressure at the centre and with winds blowing inward towards the centre in either a clockwise direction (southern hemisphere) or counterclockwise (northern hemisphere).[42] Although cyclones can take an enormous toll in lives and personal property, they may be important factors in the precipitation regimes of places they impact, as they may bring much-needed precipitation to otherwise dry regions.[43] Areas in their path can receive a year's worth of rainfall from a tropical cyclone passage.[44]

[edit] Measurement of precipitation

Standard Rain Gauge
See also: Rain gauge, Disdrometer, and Snow gauge

The standard way of measuring rainfall or snowfall is the standard rain gauge, which can be found in 100-mm (4-in) plastic and 200-mm (8-in) metal varieties.[45] The inner cylinder is filled by 25 mm (1 in) 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 in) resolution, which metal gages will require use of a stick designed with the appropriate 0.25 mm (0.01 in) 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. Some add anti-freeze to their gage so they do not have to melt the snow or ice that falls into the gage.[46] Once the snowfall/ice is finished accumulating, or as you approach 300 mm (12 in), 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.[47]

Other types of gages include the popular wedge gage (the cheapest rain gage and most fragile), the tipping bucket rain gage, and the weighing rain gage.[48] 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.[49]

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[50] or GLOBE.[51] If a network is not available in the area where one lives, the nearest local weather office will likely be interested in the measurement.[52]

[edit] Return period

See also: 100-year flood

The likelihood or probability of an event with a specified intensity and duration, is called the return period or frequency.[53] The intensity of a storm can be predicted for any return period and storm duration, from charts based on historic data for the location.[54] The term 1 in 10 year storm describes a rainfall event which is rare and is only likely to occur once every 10 years, so it has a 10% likelihood any given year. The rain fall will be greater and the flooding will be worse than the worst storm one would expect in any single year. The term 1 in 100 year storm describes a rainfall event which is extremely rare and which will occur with a likelihood of only once in a century, so has a 1% likelihood in any given year. The rain fall will be extreme and flooding to be worse than a 1 in 10 year event. As with all probability events, it is possible to have multiple "1 in 100 Year Storms" in a single year.[55]

[edit] Forecasting

Main article: Quantitative precipitation forecast
Example of a five day rainfall forecast from the Hydrometeorological Prediction Center

The Quantitative Precipitation Forecast (abbreviated QPF) is the expected amount of liquid precipitation accumulated over a specified time period over a specified area.[56] A QPF will be specified when a measurable precipitation type reaching a minimum threshold is forecast for any hour during a QPF valid period. Precipitation forecasts tend to be bound by synoptic hours such as 0000, 0600, 1200 and 1800 GMT. Terrain is considered in QPFs by use of topography or based upon climatological precipitation patterns from observations with fine detail.[57] Starting in the mid to late 1990's, QPFs were used within hydrologic forecast models to simulate impact to rivers throughout the United States.[58] Forecast models show significant sensitivity to humidity levels within the planetary boundary layer, or in the lowest levels of the atmosphere, which decreases with height.[59] QPF can be generated on a quantitative, forecasting amounts, or a qualitative, forecasting the probability of a specific amount, basis.[60] Radar imagery forecasting techniques show higher skill than model forecasts within 6 to 7 hours of the time of the radar image. The forecasts can be verified through use of rain gage measurements, weather radar estimates, or a combination of both. Various skill scores can be determined to measure the value of the rainfall forecast.[61]

[edit] See also

[edit] References

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  2. ^ The Weather World 2010 Project (1999). Precipitation: hail, rain, freezing rain, sleet and snow. University of Illinois. Retrieved on 2009-01-02.
  3. ^ a b Dr. Jim Lochner (1998). Ask an Astrophysicist. NASA Goddard Space Flight Center. Retrieved on 2009-01-16.
  4. ^ a b Dr. Chowdhury's Guide to Planet Earth (2005). The Water Cycle. WestEd. Retrieved on 2006-10-24.
  5. ^ Jan Jackson (2008). All About Mixed Winter Precipitation. National Weather Service. Retrieved on 2009-02-07.
  6. ^ Steve Kempler (2009). Parameter information page. NASA Goddard Space Flight Center. Retrieved on 2008-12-27.
  7. ^ Naval Meteorology and Oceanography Command (2007). Atmospheric Moisture. United States Navy. Retrieved on 2008-12-27.
  8. ^ FMI (2007). Fog And Stratus - Meteorological Physical Background. Zentralanstalt für Meteorologie und Geodynamik. Retrieved on 2009-02-07.
  9. ^ Glossary of Meteorology (2009). Adiabatic Process. American Meteorological Society. Retrieved on 2008-12-27.
  10. ^ TE Technology, Inc (2009). Peltier Cold Plate. Retrieved on 2008-12-27.
  11. ^ Glossary of Meteorology (2009). Radiational cooling. American Meteorological Society. Retrieved on 2008-12-27.
  12. ^ Robert Fovell (2004). Approaches to saturation. UCLA. Retrieved on 2009-02-07.
  13. ^ Robert Penrose Pearce (2002). Meteorology at the Millennium. Academic Press, pp. 66. ISBN 978-0-12-548035-2. Retrieved on 2009-01-02.
  14. ^ National Weather Service Office, Spokane, Washington (2009). Virga AND Dry Thunderstorms. Retrieved on 2009-01-02.
  15. ^ Bart van den Hurk and Eleanor Blyth (2008). Global maps of Local Land-Atmosphere coupling. KNMI. Retrieved on 2009-01-02.
  16. ^ Krishna Ramanujan and Brad Bohlander (2002). Landcover changes may rival greenhouse gases as cause of climate change. NASA Goddard Space Flight Center. Retrieved on 2009-01-02.
  17. ^ National Weather Service JetStream (2008). Air Masses. Retrieved on 2009-01-02.
  18. ^ a b Dr. Michael Pidwirny (2008). CHAPTER 8: Introduction to the Hydrosphere (e). Cloud Formation Processes. Physical Geography. Retrieved on 2009-01-01.
  19. ^ Paul Sirvatka (2003). CLOUD PHYSICS: Collision/Coalescence; The Bergeron Process. College of DuPage. Retrieved on 2009-01-01.
  20. ^ United States Geological Survey (2009). Are raindrops tear shaped? United States Department of the Interior. Retrieved on 2008-12-27.
  21. ^ a b c J . S. 0guntoyinbo and F. 0. Akintola (1983). Rainstorm characteristics affecting water availability for agriculture. IAHS Publication Number 140. Retrieved on 2008-12-27.
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  23. ^ Norman W. Junker (2008). An ingredients based methodology for forecasting precipitation associated with MCS’s. Hydrometeorological Prediction Center. Retrieved on 2009-02-07.
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  25. ^ David Roth (2006). "Unified Surface Analysis Manual". Hydrometeorological Prediction Center. http://www.hpc.ncep.noaa.gov/sfc/UASfcManualVersion1.pdf. Retrieved on 2006-10-22. 
  26. ^ Houze, Robert (October 1997). "Stratiform Precipitation in Regions of Convection: A Meteorological Paradox?". Bulletin of the American Meteorological Society 78 (10): p. 2179. doi:10.1175/1520-0477(1997)078<2179:SPIROC>2.0.CO;2. http://ams.allenpress.com/archive/1520-0477/78/10/pdf/i1520-0477-78-10-2179.pdf. Retrieved on 2007-11-27. 
  27. ^ Glossary of Meteorology (2009). Graupel. American Meteorological Society. Retrieved on 2009-01-02.
  28. ^ Toby N. Carlson (1991). Mid-latitude Weather Systems. Routledge, pp. 216. ISBN 978-0-04-551115-0. Retrieved on 2009-02-07.
  29. ^ Diana Leone (2002). Rain supreme. Honolulu Star-Bulletin. Retrieved on 2008-03-19.
  30. ^ Western Regional Climate Center (2002). Climate of Hawaii. Retrieved on 2008-03-19.
  31. ^ Paul E. Lydolph (1985). The Climate of the Earth. Rowman & Littlefield, pp. 333. ISBN 978-0-86598-119-5. Retrieved on 2009-01-02.
  32. ^ Michael A. Mares (1999). Encyclopedia of Deserts. University of Oklahoma Press, pp. 252. ISBN 978-0-8061-3146-7. Retrieved on 2009-01-02.
  33. ^ Adam Ganson (2003). Geology of Death Valley. Indiana University. Retrieved on 2009-02-07.
  34. ^ Glossary of Meteorology (2009). Rainy season. American Meteorological Society. Retrieved on 2008-12-27.
  35. ^ Costa Rica Guide (2005). When to Travel to Costa Rica. ToucanGuides. Retrieved on 2008-12-27.
  36. ^ Michael Pidwirny (2008). CHAPTER 9: Introduction to the Biosphere. PhysicalGeography.net. Retrieved on 2008-12-27.
  37. ^ Elisabeth M. Benders-Hyde (2003). World Climates. Blue Planet Biomes. Retrieved on 2008-12-27.
  38. ^ Mei Zheng (2000). The sources and characteristics of atmospheric particulates during the wet and dry seasons in Hong Kong. University of Rhode Island. Retrieved on 2008-12-27.
  39. ^ S. I. Efe, F. E. Ogban, M. J. Horsfall, E. E. Akporhonor (2005). Seasonal Variations of Physico-chemical Characteristics in Water Resources Quality in Western Niger Delta Region, Nigeria. Journal of Applied Scientific Environmental Management. Retrieved on 2008-12-27.
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  41. ^ Marti J. Van Liere, Eric-Alain D. Ategbo, Jan Hoorweg, Adel P. Den Hartog, and Joseph G. A. J. Hautvast. The significance of socio-economic characteristics for adult seasonal body-weight fluctuations: a study in north-western Benin. British Journal of Nutrition: Cambridge University Press, 1994.
  42. ^ Chris Landsea (2007). Subject: D3) Why do tropical cyclones' winds rotate counter-clockwise (clockwise) in the Northern (Southern) Hemisphere? National Hurricane Center. Retrieved on 2009-01-02.
  43. ^ Climate Prediction Center (2005). 2005 Tropical Eastern North Pacific Hurricane Outlook. National Oceanic and Atmospheric Administration. Retrieved on 2006-05-02.
  44. ^ Jack Williams (2005). Background: California's tropical storms. USA Today. Retrieved on 2009-02-07.
  45. ^ National Weather Service Office, Northern Indiana (2009). 8 Inch Non-Recording Standard Rain Gage. Retrieved on 2009-01-02.
  46. ^ Chris Lehmann (2009). 10/00. Central Analytical Laboratory. Retrieved on 2009-01-02.
  47. ^ National Weather Service Office Binghampton, New York (2009). Rainguage Information. Retrieved on 2009-01-02.
  48. ^ National Weather Service (2009). Glossary: W. Retrieved on 2009-01-01.
  49. ^ Discovery School (2009). Build Your Own Weather Station. Discovery Education. Retrieved on 2009-01-02.
  50. ^ Community Collaborative Rain, Hail & Snow Network (2009). Community Collaborative Rain, Hail & Snow Network Main Page. Retrieved on 2009-01-02.
  51. ^ Global Learning and Observations to Benefit the Environment Program (2009). GLOBE Home Page. GLOBE. Retrieved on 2009-01-02.
  52. ^ National Weather Service (2009). NOAA's National Weather Service Main Page. Retrieved on 2009-01-01.
  53. ^ Glossary of Meteorology (2009). Return period. American Meteorological Society. Retrieved on 2009-01-02.
  54. ^ Glossary of Meteorology (2009). Rainfall intensity return period. American Meteorological Society. Retrieved on 2009-01-02.
  55. ^ Boulder Area Sustainability Information Network (2005). What is a 100 year flood? Boulder Community Network. Retrieved on 2009-01-02.
  56. ^ Jack S. Bushong (1999). Quantitative Precipitation Forecast: Its Generation and Verification at the Southeast River Forecast Center. University of Georgia. Retrieved on 2008-12-31.
  57. ^ Daniel Weygand (2008). Optimizing Output From QPF Helper. National Weather Service Western Region. Retrieved on 2008-12-31.
  58. ^ Noreen O. Schwein (2009). Optimization of quantitative precipitation forecast time horizons used in river forecasts. American Meteorological Society. Retrieved on 2008-12-31.
  59. ^ Christian Keil, Andreas Röpnack, George C. Craig, and Ulrich Schumann (2008). Sensitivity of quantitative precipitation forecast to height dependent changes in humidity. Geophysical Research Letters. Retrieved on 2008-12-31.
  60. ^ P. Reggiani and A. H. Weerts (2007). Probabilistic Quantitative Precipitation Forecast for Flood Prediction: An Application. Journal of Hydrometeorology, February 2008, pp. 76-95. Retrieved on 2008-12-31.
  61. ^ Charles Lin (2005). Quantitative Precipitation Forecast (QPF) from Weather Prediction Models and Radar Nowcasts, and Atmospheric Hydrological Modelling for Flood Simulation. ACTIF project. Retrieved on 2009-01-01.

[edit] External links

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