Precipitation

Precipitation, usually falling as snow, is the factor of arctic climate most important to the hydrological cycle. The principal forms of precipitation are rain, drizzle, freezing rain, snow, ice pellets (sleet), and hail. Precipitation form depends on the source cloud and the temperature of the air below the cloud. Moisture may be deposited not only by precipitation but as dew or hoar frost on a horizontal surface or on a vertical surface as rime crystals. Rain falling on a surface that is below 0 degrees Celsius forms glaze.

How Is Precipitation Formed?

Cloud development is not always an indication of coming rain or snow. Although all precipitation occurs because of condensation, cloud physicists have determined that condensation alone cannot cause cloud droplets to grow into raindrops. This is because updrafts within clouds are usually strong enough to prevent cloud particles from leaving the base of a cloud and falling to the earth's surface. Even if droplets or ice crystals descend from a cloud, their downward drift is so slow that they might travel only a short distance before evaporating in the unsaturated air beneath the cloud. Cloud particles must grow large enough to counter updrafts and survive a descent to the earth's surface as raindrops or snowflakes without completely evaporating.

Two processes that allow raindrop formation have been identified: the collision-coalescence process in "warm" clouds, and the Bergeron process in "cold" clouds.

The Collision-Coalescence Process

For precipitation to form in clouds, relatively large droplets (larger than 40 micrometers in diameter), must be present. As a large droplet falls through a cloud, it collides and coalesces with smaller water droplets in its path. Collision and coalescence are repeated over and over again until the droplet is so large that it is heavy enough to fall out of the cloud as a raindrop (less than one millimeter in diameter).

The Bergeron Process

Most precipitation that falls to earth's surface originates through the Bergeron process. The process requires the coexistence of water vapor, ice crystals and supercooled liquid water droplets (water that has been cooled below the freezing temperature, but is still in liquid form). In "cold" clouds (typically -10 degrees Celsius to -20 degrees Celsius), water vapor deposits on the ice crystals. Hence, the ice crystals grow larger while the supercooled water droplets lose mass, that is, there is a migration of mass from the supercooled droplets to the ice crystals. As the ice crystals grow larger and heavier, they start to collide and coalesce with water droplets and ice crystals in their path, thereby growing still larger. Eventually, the ice crystals become so heavy that they fall out of the cloud. If the air below the cloud is above freezing, snowflakes melt and fall as raindrops.

Once a raindrop or a snowflake leaves a cloud, it enters unsaturated air where either evaporation or sublimation can take place. In general, the longer the journey to the ground and the lower the relative humidity of the air beneath the clouds, the greater the quantity of rain or snow that returns to the atmosphere as vapor through evaporation or sublimation.

Eventually an ice crystal or water droplet becomes big enough (heavy enough) to start to fall. Often the particles will catch updrafts as they fall and circulate in the cloud a few times to pick up more water or ice. Many particles that start out as ice crystals never reach the ground. For instance, if the air is very dry, they simply evaporate, while relatively warm air will change ice into rain, and strong winds can break ice crystals into smaller fragments.

How is precipitation measured?

Precipitation is collected and measured using a precipitation gauge (often referred to as a rain gauge) and is usually reported in millimeters or inches. There are numerous designs for rain gauges: some measure precipitation with a ruler, some in which precipitation is measured by pouring it into a graduated cylinder, some self-recording, some meant to be emptied every day, and some meant to collect unattended for a season. The basic rain gauge used in the United States has a cone-shaped funnel eight inches in diameter at the top that directs rainwater into a long, narrow cylinder. Rainwater that accumulates in the narrow cylinder is measured by a graduated scale at some fixed time once every 24 hours, and the gauge is then emptied.

Rain and snow are usually measured in the same gauge (snow is melted before measuring), but the measurement of precipitation that falls as snow is subject to greater error due to wind effects. Wind produces turbulent eddies around gauges, and prevents a large number of raindrops or snowflakes from entering gauge orifices. The magnitude of this error depends on wind speed, gauge type, and the type of precipitation. For an unshielded gauge, the "undercatch" ranges from about 60 percent of the true catch at a wind speed of 20 mph for rain, and 40 percent of the true catch for snow. Various methods of shielding the gauge have been devised to mitigate this problem and ensure a more representative catch (for example the Tretyakov shield, Nipher shield, and the Wyoming gauge, which consists of a normal gauge surrounded by a circular snow fence). Corrections to precipitation measurements for wind and for other error sources must be made before precipitation measurements can be used in climatologies. Errors can be especially high in the Arctic because the overall amount of precipitation is small, and most of it falls as snow. (For more information on corrections to precipitation measurements, see the Atlas technical documentation for the gridded precipitation fields.)

In practice, the only reliable observations of the amount of solid precipitation are made by measuring the thickness and density of the snow deposited on the ground. Owing to the irregular topography of a natural snow cover, many such measurements (usually made at fixed distances along a "snow course") are required to obtain an accurate measure of the amount of solid precipitation.

Precipitation Amounts in the Arctic

The amount of precipitation in a given region depends on the amount of available atmospheric water vapor (precipitable water), as well as on the processes that cause condensation, in particular the uplift of air associated with cyclones and fronts, as well as convection. If all the water vapor in the atmosphere were condensed, the earth's surface would be covered, on average, with a 25 mm layer of water. However, since the amount of water vapor the atmosphere can hold decreases with decreasing temperature, the amount of water that can be condensed from the air generally decreases with latitude. In general, the amount of precipitable water in the humid tropics is more than 40 mm, while near the pole, it is often less than 5 mm.

In some parts of the Arctic, warm ocean currents bring heat and moisture to the air and frontal activity results in increased precipitation. For instance, southern Iceland, southern Alaska and parts of the Norwegian coast receive in excess of 3000 mm of precipitation per year. In contrast, inland areas of the Arctic with continental climate and lower temperatures receive less than 150 mm of precipitation per year.

The following figures show the annual cycle of precipitation phase in the central Arctic Ocean compared with the northern Atlantic. The phase of precipitation is either solid (snow) or liquid (rain). Note that in the central Arctic, even in July the majority of the precipitation falls as snow.

Precipitation in the Arctic

Precipitation phase as a percentage of total precipitation from the central Arctic Ocean (left) and from the northern Atlantic Ocean (right). Gray shading is solid, black is liquid, and white is mixed precipitation. (Clark et al. 1996)


A description of the seasonal cycle of precipitation based on gridded precipitation fields has been drawn from the data section of the Atlas.

A description of the seasonal cycle of snow based on gridded snow depth fields has been drawn from the data section of the Atlas.