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In meteorology, precipitation (also known as hydrometeor) is any product of the condensation of atmospheric water vapor that is deposited on the earth's surface. It occurs when the atmosphere (being a large gaseous solution) becomes saturated with water vapour and the water condenses and falls out of solution (i.e., precipitates).[1] Air becomes saturated via two processes, cooling and adding moisture.

Precipitation that reaches the surface of the earth can occur in many different forms, including rain, freezing rain, drizzle, snow, sleet, and hail. Virga is precipitation that begins falling to the earth but evaporates before reaching the surface. Precipitation is a major component of the hydrologic cycle, and is responsible for depositing most of the fresh water on the planet. Approximately 505,000 km³ of water falls as precipitation each year, 398,000 km³ of it over the oceans.[2] Given the Earth's surface area, that means the globally-averaged annual precipitation is about 1 m, and the average annual precipitation over oceans is about 1.1 m.


Types of precipitation

A distant storm producing rain

Precipitation that forms aloft is divided into three categories:

  • Liquid precipitation;
  • Freezing precipitation; and
  • Frozen 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)
    • Graupel (GS)
    • Ice crystals (IC)

The code in the brackets is the METAR Code for the phenomena[3]

How air becomes saturated

Air contains moisture, measured in grams per kilogram of dry air (g/kg), but most commonly reported as Relative Humidity in percent (e.g., RH50 percent). How much moisture a parcel of air can hold before it becomes saturated (RH100 percent) depends on its temperature. Warmer air has a higher capacity for holding moisture than cooler air. Because of this property of the air, one way to saturate a parcel of air is to cool it. The dew point is the temperature that a parcel needs to be cooled to for saturation to occur.

Late-summer rainstorm in Denmark
  • Cooling mechanisms
    • Lift (Convective, mechanical, positive vorticity advection)
    • Conductive cooling (warm air moves over a cool surface)
    • Radiational cooling (heat radiates off into space at night)
    • Evaporative cooling (air temperature falls as liquid water uses the energy to change phase to vapor)

The other way to saturate an air parcel is to add moisture to it.

  • Adding moisture
    • Precipitation falling from above (stratus forming in the rain under a higher cloud)
    • Daytime heating evaporates water from the surface of oceans/lakes
    • Drier air moves over open water (snow streamers off the Great Lakes in winter)

How precipitation forms

Condensation and coalescence are important parts of the Water cycle


Precipitation begins forming when 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 occurs when water droplets fuse to create larger water droplets, or when water droplets freeze onto an ice crystal. 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.

Bergeron process

The Bergeron process occurs when ice crystals acquire water molecules from nearby supercooled water droplets. As these ice crystals gain required mass, they begin to fall. This generally acquires 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 differential between cloud and ground level, these ice crystals may melt and become rain as they fall.

Ways of precipitation

Frontal activity

Stratiform, or large-scale, precipitation occurs as a consequence of slow (cm/s) ascent of air in synoptic systems, such as along cold fronts, and in advance of warm fronts. Similar ascent is seen around Tropical cyclones outside of the eyewall, and in comma head precipitation patterns around mid-latitude cyclones.


Convective precipitation

Convection rain, or showery, precipitation occurs from convective clouds, e.g., cumulonimbus or cumulus congestus. It falls as showers, with rapidly changing intensity and at one moment only over patches, as convective clouds have limited horizontal extent. Convective precipitation is most important in the tropics. Graupel and hail always indicate convection. In midlatitudes, convective precipitation is associated with cold fronts (often behind the front), squall lines, and warm fronts with significant available moisture.

Orographic effects

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 (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 desert-like, 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 inches), and interior uplands receiving over 2.5 m per year (100 inches). Leeward coastal areas are especially dry 500 mm per year (20 inches)at Waikiki), and the tops of moderately high uplands are especially wet ~12 m per year (~475 inches) 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 desert like 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 coincidentally, 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.

Tropical activity

Tropical rainfall is large air masses several hundred miles across with low pressure at the center and with winds blowing around the center in either a clockwise direction (Southern hemisphere) or counterclockwise (Northern hemisphere). Precipitation arises when a warm front is formed by an advancing mass of warm air, which moves up an inclined surface of retreating cold air and is chilled in the process of being lifted up resulting in rainfall. (Manning 1997)

The Great Sandy Desert has nearly all its rain during from monsoonal thunderstorms or the occasional tropical cyclone rain depression. Thunderstorm days average 20-30 annually through most of the area (Burbidge 1983) 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 drylands, are Northwest Mexico, South West America, and South West Asia. In North America in the Sonoran and Chihuahuan desert have received some tropical rainfall in the last 10 years. Tropical activity is rare in all deserts but what rain does arrive here is important to the delicate ecosystem existing.

Rainfall characteristics

Size and shape

Raindrops have sizes ranging from 0.1 to up to approximately 9 mm 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.

Intensity and duration

These 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.

Intensity and area

We can expect a less intense rainfall over a large area than we can over a small area.

Intensity and drop size

High intensity storms have a larger drop size than low intensity storms.

Measurement of precipitation

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.[4] 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 lukewarm 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, and the weighing rain gage.[5] 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.[6]

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[7] or GLOBE.[8] If a network is not available in the area where one lives, the nearest local weather office will likely be interested in the measurement.[9] 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.

Return period

The likelihood or probability of an event with a specified intensity and duration, is called the return period or frequency. The intensity of a storm can be predicted for any return period and storm duration, from charts based on historic data for the location.

Frequency of flooding

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.

1 in 10 year storm

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 amount of rain which will fall will be greater and cause the flooding to be worse than for a 1 in 1 year event.

1 in 100 year storm

The term “1 in 100 year storm” (see also 100-year flood) 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 percent likelihood in any given year. The amount of rain which will fall will be extreme and cause far greater flooding to be worse than for a 1 in 1 year event.

As with all “probability” events, it is still possible to have two “1 in 100 Year Storms” in the same year.

See also

  • Monsoon
  • Steam
  • Umbrella
  • Rainfall patterns in the United States
  • List of meteorology topics


  1. Precipitation: hail, rain, freezing rain, sleet and snow - University of Illinois. Retrieved November 15, 2007.
  2. The Water Cycle - Dr. Art's Guide to Planet Earth. Retrieved November 15, 2007.
  3. Canada. Atmospheric Environment Service. MANOBS : manual of surface weather observation. [Ottawa : Govt.], 1977-
  4. NOAA's National Weather Service Weather Forecast Office - Retrieved November 15, 2007.
  5. National Weather Service - Retrieved November 15, 2007.
  6. Build Your Own Weather Station - Discovery Education. Retrieved November 15, 2007.
  7. CoCoRaHS - Retrieved November 15, 2007.
  8. The GLOBE Program - Retrieved November 15, 2007.
  9. New Interactive Drought Information Website - Retrieved November 15, 2007.

External links

All links retrieved November 15, 2007.

Meteorological data and variables

Atmospheric pressure | CAPE | CIN Dew point | Heat index | Humidex | Humidity | Pot T | Sea surface temperature | Temperature | Theta-e | Visibility | Vorticity | Wind chill


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