Precipitation (meteorology)

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).

Types of precipitation

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:

Liquid precipitation:

Drizzle (DZ)
Rain (RA)

Freezing precipitation:

Freezing drizzle (FZDZ)
Freezing rain (FZRA)

Frozen precipitation:

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.

How air becomes saturated

Air contains water vapour, measured in grams of water per kilogram of dry air (g/kg), 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. Because of this property, 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.

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:

precipitation falling from above (e.g. stratus forming in the rain under a higher cloud)
daytime heating evaporating water from the surface of oceans, water bodies or wet land
transpiration from plants
drier air moving over open water (e.g. Lake effect snow off the Great Lakes in winter).

How precipitation forms

Condensation

Precipitation begins forming when warm, moist air rises. As the air cools, water vapour 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 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 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.

Ways of precipitation

Frontal activity

Stratiform or dynamic 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.

Convection

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 effects

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.

Tropical activity

Tropical activity in general consists of 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 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.

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.

Rainfall characteristics

Size and shape

Raindrops have sizes ranging from 0.1 to up to approximately 9 mm (0.004 to 0.4 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.

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 gauge, which can be found in 100-mm (4-in) plastic and 200-mm (8-in) metal varieties. 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. 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.

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.

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% 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 multiple “1 in 100 Year Storms” in the same year.