In meteorology, precipitation (also known as one class 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. It occurs when the atmosphere, a large gaseous solution, becomes saturated with water vapour and the water condenses and falls out of solution (i.e., precipitates). Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapour to the air.
Precipitation that reaches the surface of the earth can occur in many different forms, including rain, freezing rain, drizzle, snow, ice pellets, and hail. Virga is precipitation that begins falling to the earth but evaporates before reaching the surface. 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. 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. Each of these categories can be further subdivided:
The capital letters in the parentheses are the METAR codes for each phenomenon. Mixtures of different types of precipitation, including types in different categories, can fall simultaneously.
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. The air can rise due to convection, large-scale atmospheric motions, or a physical barrier such as a mountain. Conductive cooling occurs when the air comes into contact with a colder surface, usually by being blown from one surface to another, for example from an unfrozen water surface to colder land. Radiational cooling occurs due to the emission of infrared light, either by the air or by the surface underneath. Evaporative cooling occurs when energy from the air is used to evaporate water; this is most effective when dry air blows over a wet surface.
The other way to saturate an air parcel is to add water vapour. An obvious example is when the air parcel is at ground level and water vapour is added by evaporation or evapotranspiration from the underlying water body or land surface. Both evaporation and evapotranspiration also act to warm the air parcel. This is because only the higher-energy water molecules can escape the liquid water's surface tension, and this transfer of energy acts both to cool the evaporating surface and to warm the overlaying air.
The main ways water vapour is added to the air are:
Convection 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. Graupel and hail always indicate convection. In mid-latitudes, convective precipitation is associated with cold fronts (often behind the front), squall lines, and warm fronts in very moist air.
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 (downwind) side. Moisture is removed by orographic lift, leaving drier air (see katabatic wind) on the descending (generally warming), leeward side where a rain shadow is observed.
Orographic precipitation is well known on oceanic islands, such as the Hawaiian Islands, where much of the rainfall received on an island is on the windward side, and the leeward side tends to be quite dry, almost desertlike, by comparison. This phenomenon results in substantial local gradients of average rainfall, with coastal areas receiving on the order of 500 to 750 mm per year (20 to 30 in), and interior uplands receiving over 2.5 m per year (100 in). Leeward coastal areas are especially dry 500 mm per year (20 in)at Waikiki), and the tops of moderately high uplands are especially wet – ~12 m per year (~475 in) at Wai'ale'ale on Kaua'i).
In South America, the Andes mountain range blocks most of the Atlantic moisture that arrives in that continent, resulting in a desertlike climate on the Pacific coast of Peru and northern Chile, since the cold Humboldt Current ensures that the air off the Pacific is dry as well. On the leeward side of the Andes is the Atacama Desert of Chile. It is also blocked from moisture by mountains to its west as well. Not coincidentially, it is the driest place on earth. The Sierra Nevada range creates the same effect in North America forming the Great Basin desert, Mojave Desert and Sonoran Desert.
The Great Sandy Desert has nearly all its rain during from monsoonal thunderstorms or the occasional tropical cyclone rain depression. Thunderstorms occur on an average of 20–30 days annually through most of the area. Although the desert has fairly high precipitation rates, due to the high rates of evaporation, this area remains an arid environment with vast areas of sands.
Other areas of the world which see these rare precipitation events in deserts are northwest Mexico, the southwestern United States, and southwest Asia. In North America, the Sonoran and Chihuahuan Deserts have received some tropical rainfall in the last 10 years. Tropical activity is rare in all deserts, but what rain does arrive there is important to the existence of the delicate ecosystem.
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. 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.
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 or GLOBE. If a network is not available in the area where one lives, the nearest local weather office will likely be interested in the measurement. An important use of precipitation data is for forecasting of river flows and river water quality using hydrological transport models such as SWMM, SHE or the DSSAM Model.
There is no way of telling when the next flood will arrive or how big it will be, but past flooding events can help provide some information as to what to expect.
As with all “probability” events, it is still possible to have multiple “1 in 100 Year Storms” in the same year.
Sensitivities of Orographic Precipitation to Terrain Geometry and Upstream Conditions in Idealized Simulations
May 03, 2012; ABSTRACT This study examines how variations in relatively simple terrain geometries influence orographic precipitation and its...
Sensitivity Studies of the Role of Aerosols in Warm-Phase Orographic Precipitation in Different Dynamical Flow Regimes
Aug 01, 2008; ABSTRACT Aerosols serve as a source of cloud condensation nuclei (CCN) and influence the microphysical properties of clouds. In...
A Numerical Study of the Interaction between the Large-Scale Monsoon Circulation and Orographic Precipitation over South and Southeast Asia
Apr 12, 2012; ABSTRACT A regional climate model is used to simulate the summer monsoon onset in South and Southeast Asia during the year 2000...