• Precipitation has been defined as water in liquid or solid forms falling to the earth. Rain, snow, hail and sleet are the common forms of precipitation. Fog dew, frost are, however, been excluded from precipitation.
• Precipitation involves the process of evaporation, condensation, saturation and precipitation. The process of condensation involves a change from water vapuor to liquid, while the process of precipitation the falling out of water as rain, hail or sleet.
• Droplets produced by the condensation process are very small in size, averaging less than 10 micrometers in diameter (compare with the human hair which is about 75 micrometers in diameter).
Forms of Precipitation
All forms of precipitation are collectively termed hydrometeors. The major types of precipitation are rain, drizzle, snow, sleet, and hail. A brief account of each one of them has been given as under.
– Rain is precipitation of water in liquid state.
– The liquid water particle, either in the form of drops or more than 0.5 mm diameter or in the form of smaller widely scattered drops. Whenever the rain drops fall from high altitude clouds, some of them evaporate while passing through a layer of dry air
– When the drops of falling precipitation are very small and of uniform size, and seem to float in the air, It is called as drizzle.
– Drizzle is fairly uniform precipitation composed exclusively of uniform water drops. They are formed in very low stratus type clouds with a high water content. The relative humidity in the inter 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.
– It is precipitation of white and opaque grains of snow. In other words, snow is precipitation of solid water. Generally, in the winter season, when temperatures are below freezing in the whole atmosphere, the ice crystals falling from the alto stratus clouds do not melt and reach the ground as snow.
– Sleet is a type of precipitation in the form of mixture of rain and snow. It is a frozen raIn, which forms when rain, while falling to the earth, passes through a layer of cold air and freezes. Sometimes, sleet may grow into hailstorms when violent vertical currents are produced in the atmosphere.
– A type of precipitation which falls in the form of small pellets of ice (hailstones) with a diameter between 5 to 50 mm and sometimes more. Hailstones are generally of pea size or even smaller, but in rare cases they attain the size of a baseball. Hail is the most destructive form of precipitation produced in violent thunder storms or cumulonimbus clouds. The structure of a hail resembles to that of an onion.
– Hailstorms seldom occur in the tropics and in the higher latitudes. Oceans are also almost free from them. In both the hemispheres, area lying between 30° to 60° north and south latitudes, have the maximum number of these storms.
Types of Precipitation
On the basis of characteristics precipitation may be classified under the following categories:
1. Convectional Precipitation: The convectional precipitation occurs in the areas of intense heat and abundant supply of moisture. Solar radiation is the main sources of heat to produce convection currents in the air. Convection rainfall generally results from the cumulus clouds. Thunder, lighting and occasional hails are the characteristics of this type of precipitation. The belt of doldrums generally receives this type of rainfall. Convectional rainfall is less effective for crops as much of its water is drained off in the form of surface drainage.
2. Orographic Precipitation: The type of precipitation resulting from a vertical uplift of an air stream by the topographic barrier (mountains etc,) In fact, for heavy rainfall to occur it is necessary for cyclonic or convective process to be operative because the Orographic component is normally weak and acts merely as a triggering mechanism. This type of precipitation is generally found on the wind ward sides of mountain ranges, while the leeward side receives insignificant rainfall.
3. Cyclonic or Frontal Precipitation: Cyclonic precipitation occurs when deep and extensive air masses converge and move upward which lead to their adiabatic cooling. The frontal rainfall is a characteristic of the temperate latitudes. These latitudes are the zone of convergence of the warm and cold air masses. The rainfall in these latitudes is generally in the form of drizzle. The frontal rainfall is widespread and continues for longer periods. In North West Europe and North America the rainfall is mainly of cyclonic origin. In the north-western parts of India also the winter rainfall is of frontal origin.
Distribution of Precipitation
• The world distribution of precipitation is highly uneven.
• The distribution of rainfall is closely influenced by the latitudes, temperature, relative humidity, atmospheric disturbance, and mountain barriers, movement of air masses and differential heating and cooling of the land and water surfaces.
• The average annual precipitation over the whole earth is about 80 cm (30 inches).
• The heaviest rainfall is recorded in the equatorial and monsoon regions. Heavy rainfall is also recorded in the temperate regions on the western margins of the continents.
• In the tropical latitudes, the average annual precipitation decreases from east to west while in the temperate latitudes there is a decrease in rainfall from west to east.
• The dry regions of the subtropical high pressure belt and the polar areas receive little precipitation.
• The equatorial belt is generally a region of abundant precipitation. The Amazon basin, Congo basin and several places in the islands of South East Asia receive more than 200 cm of precipitation annually. Heavy rainfall is also recorded in the Monsoon region (Mausinram and Cherapunji in Meghalaya-India) where the average annual rainfall is around. 1000 cm.
• In the temperate latitudes the precipitation occurs largely because of the temperate cyclones. Places like London, Bonn, Copenhagen, and Paris receive over 100 cm of annual precipitation.
• In the Polar Regions the low precipitation is due partly to the subsidence of air in the high pressure belts and partly due to the low temperatures of air which holds little moisture.
• The oceans record more precipitation than the land areas, and the Southern hemisphere receives more rainfall than the Northern.
Condensation is the transformation of gaseous form of water into solid form i.e. ice and liquid form i.e. water.
Mechanism of Condensation
The mechanism of condensation depends upon the amount of relative humidity present in the air. When the air achieves 100 per cent relative humidity, it is called as saturated air (no more water vapour can added to it).
The temperature at which air becomes saturated is known as dew point and condensation starts only at this point. If the temperature at dew point is above freezing point condensation occurs in the form of fog, rainfall etc. On the other hand if the dew point is below freezing point condensation occurs in the form of snow, frost etc.
How saturation of air is achieved
Method 1: When the absolute humidity at a given temperature is raised equal to the humidity retaining capacity of the air.
Method 2: When the temperature of the air is reduced to such an extent that the humidity capacity becomes equal to its absolute humidity.
Concept of Adiabatic Change of Temperature
If the change in temperature of air takes place by the ascent and descent of air and no addition or subtraction of heat occurs then this process is known as adiabatic change of temperature.
When the air is warmer than the surrounding air-mass it ascends. Due to upward movement of air volume increases and the temperature decreases. As the dew point is achieved, condensation process starts and leads to formation of clouds, fog and ultimately leads to precipitation. Thus instability of air causes different weather phenomenon. In the process explained above there is no addition or subtraction of heat and the process is solely due adiabatic change of temperature.
On the other hand if the dew point is not achieved, the air becomes colder than surrounding air. Thus the air descends and becomes cooler. This leads to stability of air and weather phenomenon get hampered.
Cloud is a mass of minute water droplets or tiny crystals of ice formed by the condensation of the water vapour in free air at considerable elevations. As the clouds are formed at some height over the surface of the earth, they take various shapes. According to their height, expanse, density and transparency or opaqueness clouds are grouped under
Clouds are very significant because:
• They cause all forms of precipitation.
• They play a major role in the heat budget of the earth.
• They reflect, absorb some part of incoming solar radiation as well as some part of long-wave terrestrial radiation re-radiated by the earth.