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Factors
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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.
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.
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.
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.
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 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)
The following description of the seasonal cycle of precipitation based on gridded precipitation fields has been drawn from the data section of the Arctic Climatology Project Arctic Meteorology and Climate Atlas.
The gridded fields illustrate the seasonal cycle of Arctic precipitation. During winter, precipitation is highest over the Atlantic sector. This represents the effect of frequent cyclone activity associated with the Icelandic Low. High precipitation totals are also found south of Alaska corresponding to the Aleutian Low. The lowest amounts are found over the central Arctic Ocean and land areas, where cyclone activity is uncommon and anticyclonic conditions are more the rule. From spring into summer, the pattern changes. The precipitation maxima over the Atlantic side and south of Alaska weaken, attended by increases in precipitation over the Arctic Ocean and land. The increases over the Arctic Ocean correspond to the seasonal increase in cyclone activity in this area. This is also true for the increase in precipitation over land. Perhaps surprisingly given the high latitude, the summer maximum in terrestrial precipitation is also contributed to by convective activity (that is, by thunderstorms). Autumn shows the transition back to the winter regime. The maxima associated with the Icelandic and Aleutian lows is reestablished, and precipitation decreases over land.
The following description of the seasonal cycle of snow based on gridded snow depth fields has been drawn from the data section of the Arctic Climatology Project Arctic Meteorology and Climate Atlas.
The winter fields indicate greater snow depths over the North American side of the Arctic Ocean. This is because temperatures are lower in this region, so that snow falling during the autumn months tends to more readily accumulate compared to other areas. However, March shows a tendency for deeper snow cover towards the Atlantic side of the Arctic Ocean. This is because winter snowfall is comparatively high in this area due to the northward penetration of storms associated with the Icelandic Low. Moving through spring and summer, the pattern of greater snow depths on the North American side is reestablished. Again, this is because this region tends to be colder, but here the effect is to result in slower melt. Most of the snow is melted by August. The season's first snowfall tends to occur in September. Because snow densities are broadly similar across the Arctic Ocean, the seasonal cycle of snow water equivalent is similar to that of snow depth.
Land areas with high winter snow depths correspond in part to regions with fairly high elevations (for example, Alaska and parts of Siberia). This is understood in that higher-elevation areas tend to be somewhat cooler, so snow can more easily accumulate in autumn. Snowfall may also be enhanced by orographic uplift of air masses. For other areas, such as Northwestern Eurasia, large snow depths appear to be more directly a reflection of synoptic activity. Note the low winter snow depths over east-central Siberia, where the strong Siberian High suppresses precipitation. The seasonal melt of the snow cover occurs earlier over the North American side. By July, snow cover has essentially disappeared.
