Precipitation refers to all the moisture that comes to the earth from atmosphere. This may be in the form of rain, snow, sleet, fog, dew or hail. Precipitation can occur only when the air containing moisture is cooled sufficiently to condense a part of the moisture which is present in the atmosphere. The atmospheric air always contains moisture. Evaporation from the oceans is the major source (about 90%) of the atmospheric moisture for precipitation. Continental evaporation contributes only about 10% of the atmospheric moisture for precipitation. The atmosphere contains the moisture even on days of bright sun shine.
For the occurrence of precipitation, some mechanism is required to cool the atmospheric air sufficiently to bring it to (or near) saturation. This mechanism is provided by either convective systems (due to unequal radiative heating or cooling of the earth’s surface and atmosphere) or by orographic barriers (such as mountains due to which air gets lifted up and consequently undergoes cooling, condensation and precipitation) and results into, respectively, convective and orographic precipitations. Alternatively, the air lifted into the atmosphere may converge into a low pressure area (or cyclone) causing cyclonic precipitation. Artificially induced precipitation requires delivery of dry ice or silver iodide or some other cloud seeding agent into the clouds by aircraft or balloons.
Forms of Precipitation
1) Drizzle
A light steady rain in fine drops (0.5 mm) and intensity <1 mm/hr.
2) Rain
The condensed water vapour of the atmosphere falling in drops (>0.5 mm, maximum size - 6 mm) from the clouds.
3) Glaze
Freezing of drizzle or rain when they come in contact with cold objects.
4) Sleet
Frozen rain drops while falling through air at subfreezing temperature.
5) Snow
Ice crystals resulting from sublimation (i.e., water vapour condenses to ice).
6) Snow flakes
Ice crystals fused together.
7) Hail
Small lumps of ice (>5 mm in diameter) formed by alternate freezing and melting, when they are carried up and down in highly turbulent air currents.
8) Dew
Moisture condensed from the atmosphere in small drops upon cool surfaces.
9) Frost
A feathery deposit of ice formed on the ground or on the surface of exposed objects by dew or water vapour that has frozen.
10) Fog
A thin cloud of varying size formed at the surface of the earth by condensation of atmospheric vapour (interfering with visibility).
11) Mist
A very thin fog.
Classification of Rainfall Based on Rate
|
Sl. No. |
Type of Rainfall |
Rate (mm/hr) |
|
1 |
Light |
upto 2.5 |
|
2 |
Moderate |
2.5
- 7.5 |
|
3 |
Heavy |
>7.5 |
Types of Precipitation
Adiabatic cooling resulting from the vertical transport of air mass is the primary cause of condensation and hence of precipitation. Depending on the conditions responsible for the vertical motion of the air mass, precipitation can be classified into the following four types.
1) Convective Precipitation
Convective precipitation results from the heating of the earth's surface. The warm ground heats the air over it. As the air warms, the air molecules begin to move further apart. With increased distance between molecules, the molecules are less densely packed. Thus, the air becomes “lighter” and rises rapidly into the atmosphere. As the air rises, it cools. Water vapour in the air condenses into clouds and precipitation.
2) Orographic Precipitation
Orographic precipitation results when warm moist air moving across the ocean is forced to rise by large mountains. As the air rises, it cools because a higher elevation results in cooler temperatures. Cold air cannot hold as much moisture as warm air. As air cools, the water vapour in the air condenses and water droplets form. Clouds forms and precipitation (rain or snow) occurs on the windward side of the mountain.
This air is now dry and rises over top the mountain. As the air moves back down the mountain, it collects moisture from the ground via evaporation. This side of the mountain is called the leeward side. It receives very little precipitation.
3) Cyclonic Precipitation or Frontal Precipitation
Cyclonic or frontal precipitation results when the leading edge of a warm, moist air mass (warm front) meets a cool and dry air mass (cold front). The molecules in the cold air are more tightly packed together (i.e., more dense) and thus, the cold air is heavier than the warm air. The warmer air mass is forced up over the cool air.
As it rises, the warm air cools, the water vapour in the air condenses, clouds are formed and it result in precipitation. This type of system is called Frontal Precipitation because the moisture tends to occur along the front of the air mass. In another words, unequal heating of the earth’s surface creates low and high pressure regions. Movement of air masses from the high pressure regions to low pressure regions displaces the low pressure air upward to cool and cause precipitation. Cyclonic precipitation can be of two types.
a) Frontal Precipitation
A frontal is called as the hot moist air mass boundary. This precipitation is caused by the expansion of air near the frontal surface.
b) Non-Frontal Precipitation
This is a cold moist air mass boundary that moves and results in precipitation.
There are two main types of cyclones namely Tropical Cyclone (also called hurricane or typhoon) of comparatively small diameter of 300-1500 km causing high wind velocity and heavy precipitation and the Extra-Tropical Cyclone of large diameter up to 3000 km causing wide spread frontal type precipitation.
When two air masses due to contrasting temperatures and densities clash with each other, condensation and precipitation occur at the surface of contact. This surface of contact is called a ‘front’ or ‘frontal surface’. If a cold air mass drives out a warm air mass’ it is called a ‘cold front’ and if a warm air mass replaces the retreating cold air mass, it is called a ‘warm front’. On the other hand, if the two air masses are drawn simultaneously towards a low pressure area, the front developed is stationary and is called a ‘stationary front’. Cold front causes intense precipitation on comparatively small areas, while the precipitation due to warm front is less intense but is spread over a comparatively larger area. Cold fronts move faster than warm fronts and usually overtake them, the frontal surfaces of cold and warm air sliding against each other. This phenomenon is called ‘occlusion’ and the resulting frontal surface is called an ‘occluded front’.
Characteristics of Precipitation in India
India receives more than 75% of its annual precipitation during the monsoon season (June to September). The monsoon (i.e., south-west monsoon) originates in the Indian Ocean and appears in the southern part of Kerala by the end of May or the beginning of June. Monsoon winds, then, advance and cover the entire country by mid-July. The monsoon season is not a period of continuous rainfall. The temporal and spatial variability of the magnitude of rainfall results into regions of droughts and floods. Assam and the north-eastern region are the heavy rainfall regions (with average annual rainfall ranging from 2000-4000 mm) and Uttar Pradesh, Haryana, Punjab, Rajasthan and Gujarat constitute low rainfall regions (with average annual rainfall less than about 1000 mm). Western Ghats receive about 2000-3000 mm of annual rainfall.
Around mid-December, the western disturbances cause moderate to heavy rain and snowfall (about 250 mm) in the Himalayas and Jammu and Kashmir and other northern regions of the country. Low pressure areas formed in the Bay of Bengal during this period cause some rainfall in the south-eastern parts of the country.
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