Precipitation (meteorology)

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This article is about the meteorological term. For the chemistry term, see Precipitation (chemistry)

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 vapor 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, snow, sleet, and hail. Virga is precipitation that begins falling to the earth but dries up before it reaches 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]

Contents 1 Types of Precipitation 2 How Air becomes Saturated 3 How precipitation forms 3.1 Condensation 3.2 Coalescence 3.3 Bergeron process 4 Ways of precipitation 4.1 Frontal Activity 4.2 Convection 4.3 Orographic effects 4.4 Tropical Activity 5 Rainfall Characteristics 5.1 Size and Shape 5.2 Intensity and Duration 5.3 Intensity and Area 5.4 Intensity and Drop Size 6 Measurement of precipitation 7 Return period 8 Frequency of Flooding 8.1 1 in 10 Year Storm 8.2 1 in 100 Year Storm 9 See also 10 References 11 External links

[edit] Types of Precipitation

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]

[edit] 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 (ie., RH50%). How much moisture a parcel of air can hold before it becomes saturated (RH100%) 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.

• Cooling mechanisms
  • Lift (Daytime Heating, Convective, Orographic)
  • 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 changes phase to vapour)

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)

[edit] How precipitation forms

[edit] Condensation

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.

[edit] Coalescence

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.

[edit] 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.

[edit] Ways of precipitation

[edit] 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.

[edit] Convection

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.

[edit] Orographic effects

Main article: Orographic lift

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 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 Pacific moisture that arrives in that continent. On the leeward side of the Andes is the Atacama Desert of Chile. It is also blocked from moisture by moutains to its west as well. Not coincidentially, it is the driest place on earth. The Rocky mountain range creates the same affect in North America forming the Great Basin desert, Mojave Desert and Sonoran desert.

[edit] Tropical Activity

Tropical rainfall is large air masses several hundred miles across with low pressure at the centre and with winds blowing around the centre in either a clockwise direction (Southern hemisphere) or anti-clockwise (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.

[edit] Rainfall Characteristics

[edit] Size and Shape

Rainfall occurs when moisture in the atmosphere condenses into drops. Raindrops occur in any shape up to approximately 9 mm mean diameter after which they tend to break up. However, they do tend, if turbulence does not interfere, toward an aerodynamically stable shape (tear-drop) because this affords the least surface resistance to movement.

[edit] 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.

[edit] Intensity and Area

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

[edit] Intensity and Drop Size

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

[edit] Measurement of precipitation

See also: Rain gauge, Disdrometer, and Snow gauge

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 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,^[5] and the weighing rain gage.^[6] 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.^[7]

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^[8] or GLOBE.^[9] If a network is not available in the area where one lives, the nearest local weather office will likely be interested in the measurement.^[10] 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.

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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% 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.

[edit] See also

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

[edit] References

[edit] External links

• 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
• Global Precipitation Climatology Center GPCC
• Falling Rain Genomics
• Types of rainfall (BBC)

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