Precipitated moisture falling on the ground takes various forms which depend on the following conditions: (a) the temperature at which condensation takes place, (b) the conditions encountered as the particles pass through the air, (c) the type of clouds and their heights from the ground, and (d) the processes generating precipitation.
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All forms of precipitation regardless of appearance are collectively termed ‘hydrometeors’. Hydrometeors have been classified into 50 specific types, of which only the more common types will be examined here.
The definitions given below have been taken from Volume I of the International Cloud Atlas, 1956. Other details of some more common precipitation forms have also been given here.
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Rain:
It is “precipitation of liquid water particles, either in the form of drops of more than 0.5 mm diameter or in the form of smaller widely scattered drops”. Thus, rain is precipitation of water in liquid state.
The drops of rain are generally larger than those of drizzle. Sometimes rain drops may be of drizzle size, but in that case they are sparse and less in number. Whenever the rain drops fall from high-altitude clouds, some of them evaporate while passing through a layer of dry air.
On occasions, falling rain drops completely evaporate before reaching the ground. Such streaks of rainfall are called virgae, On the contrary, when the precipitation process is very active, the lower air is moist and the clouds are very deep, rainfall is in the form of heavy downpours. In this type of heavy rains, the raindrops are larger and more numerous.
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Drizzle:
It is “fairly uniform precipitation composed exclusively of fine drops of water (diameter less than 5 mm), very close to one another.” When the drops of falling precipitation are very small and of uniform size, and seem to float in the air, it is referred to as drizzle.
The radii of the drops are less than 500 micrometers. It gives out very small amount of water on the ground. The microscopic water particles are affected by the slightest irregularities in air movements.
Besides being small in size, the drops are numerous. They are formed in very low stratus-type clouds with high water content but not subject to much lifting. Relative humidity in the intervening layers of air between the cloud base and the ground is often nearly 100 per cent, so that the small drops never evaporate in their journey.
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Drizzle is often associated with fog and poor visibility. In some places drizzle is often associated with fog and poor visibility. In some places drizzle is called mist. According to Donn, if the droplets in a drizzle completely evaporate before reaching the ground, the condition is referred to as mist.
However, in the International Codes for weather reports, the term ‘mist’ is used when the hydrometeor-mist or fog-reduces the horizontal visibility at the earth’s surface to not less than 1 km.
Snow:
It is “precipitation of white and opaque grains of ice.” In fact, snow is precipitation of solid water, mainly in the form of branched hexagonal crystals or stars. Snow consists of a wide variety of crystal forms of ice.
It may fall from pure ice clouds or from such clouds as are formed of super-cooled water droplets. Snow flakes result from gravitational coalescence. In winter, when temperatures are below freezing in the whole atmosphere, the ice crystals falling from the alto- stratus do not melt and reach the ground as snow.
Heaviest snowfall is reported to occur when the temperature of air from which snow is falling is not much below 0°C, because under such a condition the moisture content is fairly high. The most interesting feature of the shape and size of snow flakes is that they reflect the processes by which they were formed.
Sleet:
In English-speaking countries outside the United States, sleet refers to precipitation in the form of a mixture of rain and snow. But in American terminology, sleet means a form of precipitation consisting of small pellets of transparent or translucent ice, 5 mm or less ill diameter.
Thus, in America, the term ‘sleet’ refers to a frozen rain, which forms when rain, while falling to the earth, passes through a layer or cold air and freezes.
These ice pellets rebound when they strike the hard surfaces. Sleet is commonly associated with showery conditions in unstable air, and is formed when ice particles and super-cooled water droplets are found in close proximity.
Sometimes sleet may grow into hailstorms when violent vertical currents are produced in the atmosphere. According to Petterssen, in the high and mid- latitudes precipitation may sometimes begin as snow at the higher altitudes.
At or below the melting point, the falling precipitation turns into sleet, and finally reaches the ground as rain. In case the rain from warm air aloft passes through a cold layer near the ground, it does not become snow. The falling rain drops will rather freeze into ice-pellets.
Hail:
It is “precipitation of small balls or pieces of ice (hailstones) with a diameter ranging from 5 to 50 mm or sometimes more, falling either separately or agglomerated into irregular lumps.” Hail is the most dreaded and destructive form of precipitation produced in violent thunderstorms or cumulo-nimbus clouds.
Structure of a hailstone resembles that of an onion. The hailstones consist of concentric layers of ice alternating with layers of snow. The very structure of a hailstone is an indication of the complex process of its formation. The strong updrafts of air in cumulo-nimbus clouds carry rain drops to great heights where they get frozen into ice crystals.
These frozen raindrops have to fall down through the layers of cloud to be carried up again. While falling through the super-cooled cloud droplets, the frozen drops get a coating of ice. Since these pellets are lifted up into the layer of cold air and then dropped again and again, hailstones have concentric layers of ice.
They may also grow to sufficient size by a single descent through an updraft. The onion-like structure is due to variations in the rate at which super-cooled droplets accumulate and freeze. Thus, the ultimate size of a hailstone depends upon (a) strength of the updrafts, (b) concentration of super-cooled water, and (c) total length of its path through cloud.
As regards size, hailstones are usually of pea size or even smaller, but in rare cases they attain the size of a baseball. The largest hailstone on record fell on Coffeyville, Kansas, on September 3, 1970. Its diameter and circumference were 14 and 44 centimeters respectively.
It weighed 766 grams.” Such large-sized hailstones indicate many round trips above the freezing level in a large thunderstorm. Hailstones can have various kinds of shapes. They have been found to be conical, ellipsoidal, or amorphous. In each and every hailstone some parts are made up of clear ice, while other parts are milky ice.
The opaqueness is due to the trapped air bubbles. Large hailstones are characterized by alternating layers of clear and opaque ice. Even though hail is closely associated with thunderstorms, the regions where maximum number of hailstorms occurs do not coincide with regions of maximum thunderstorm activity.
For example, thunderstorms occur most frequently in the southern states of the United States, but the greatest number of hailstorms occurs in the Western Great Plains and over the Great Basin area.
Hailstorms seldom occur in the tropics and in the higher latitudes. Oceans are also almost free from them. In both the hemispheres, areas lying between 30° and 60° north and south latitudes have the maximum number of these storms.
Oceans are almost free from strong vertical convective activity; hence the absence of hail storms there. Similarly, in Polar Regions too, there is little convective activity.
In India, the period from March to May offers the ideal conditions for hail storms. In mid-latitude spring and early summer are the periods when maximum number of hailstorms occurs.
The destructive effects of hail storms are well known. Standing crops in the field can be devastated in just a few minutes by severe hail storms which mean tremendous economic loss to the farmers. Besides, hail is a definite hazard to flying because it causes damage to aircrafts.
The most unfortunate aspect of the hailstorm phenomenon is that they are very difficult to forecast, particularly the time and place of their occurrence. All that can be done is to indicate that conditions are favourable for hail in a general area.