Precipitation is any product of the condensation of atmospheric water vapour, which may be solid or liquid water, that falls under gravity.
For precipitation to form:
- the atmosphere must have moisture,
- there must be sufficient nuclei present to aid condensation,
- weather conditions must be good for condensation of water vapour to take place, and
- the products of condensation must reach the earth.
Under proper weather conditions, the water vapour condenses over nuclei to form tiny water droplets of sizes less than 0.1 mm in diameter. The nuclei are usually salt particles or products of combustion and are normally available in plenty. Wind speed facilitates the movement of clouds while its turbulence retains the water droplets in suspension. Water droplets in a cloud are somewhat similar to the particles in a colloidal suspension. Precipitation results when water droplets come together and coalesce to form larger drops that can drop down.
Some of the common forms of Precipitation are:
- Rain
- Snow
- Drizzle
- Glaze
- Sleet
- Hail
Rain:
The Precipitation in the form of water drops of sizes larger than 0.5 mm is known as rainfall. The maximum size of a raindrop is about 6 mm. Any drop larger in size than this tends to break up into drops of smaller sizes during its fall from the clouds. On the basis of its intensity, rainfall is classified as:
SNO.
| Type
| Intensity
|
1
| Light Rain
| Upto 2.5 mm/hr
|
2
| Moderate Rain
| 2.5 mm/hr to 7.5 mm/hr
|
3
| Heavy Rain
| greater than 7.5 mm/hr
|
Snow:
Precipitation in the form of an ice crystals which usually combine to form flakes, with an average density of 0.1 gm/C`m^2` is snow.
Drizzle:
Rain droplets of size less than 0.5 mm and rainfall intensity of less than 1 mm/hr is known as Drizzle. The drops are so small that they appear to float in the air.
Glaze:
When rain or drizzle comes in contact with cold ground at around 0o C, the water drops freeze to form an ice coating called glaze or freezing rain.
Sleet:
Sleet is frozen raindrops of transparent grains which form when rain falls through air at subfreezing temperature.
Hail:
It is a showery precipitation in the form of irregular pellets or lumps of ice of size more than 8 mm. Hails occur in violent thunderstorms in which vertical currents are very strong.
Depending upon the way in which the air is lifted and cooled so as to cause the precipitation, Precipitation are of following types:
- Convective Precipitation
- Orographic Precipitation
- Cyclonic Precipitation
- Frontal Precipitation
Convective Precipitation:
The precipitation caused by the upward movement of warmer air because of its lesser density than its surrounding air is known as Convective Precipitation. The air close to the warm earth gets heated and rises due to its low density, cools adiabatically to form a cauliflower shaped cloud, which finally burst into a thunder storm. Usually the areal extent of such rains is small, being limited to a diameter of about 10 km.
Orographic Precipitation:
The Precipitation in which the moist air masses may get lifted-up to higher altitudes due to the presence of mountain barriers (as it cannot move forward) and consequently undergo cooling,condensation and precipitation is known as Orographic precipitation. Thus in mountain ranges, the windward slopes have heavy precipitation and the leeward slopes light rainfall.
Cyclonic Precipitation:
A cyclone is a large low pressure region with circular wind motion. This type of Precipitation is due to the lifting of moist air due to the pressure differences created by unequal heating of the earth surface. Here, the wind blows spirally inward counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere. There are main two types of Cyclones:
- Tropical Cyclone and
- Extra-tropical Cyclone
Tropical Cyclone:
A tropical cyclone, also called cyclone in India, hurricane in USA and typhoon in South-East Asia, is a wind system with an intensely strong depression with MSL pressures sometimes below 915 mbars The normal areal extent of a cyclone is about 100-200 km in diameter. The isobars are closely spaced and the winds are anticlockwise in the northern hemisphere. The centre of the storm, called the eye, which may extend to about `10-50` km in diameter, will be relatively quiet. However, right outside the eye, very strong winds/reaching to as much as 200 kmph exist. The wind speed gradually decreases towards the outer edge. The pressure also increases outwards. The rainfall will normally be heavy in the entire area occupied by the cyclone.
During summer months, tropical cyclones originate in the open ocean at around `5-10^0` latitude and move at speeds of about `10-30` kmph to higher latitudes in an irregular path. They derive their energy from the latent heat of condensation of ocean water vapour and increase in size as they move on oceans. When they move on land the source of energy is cut off and the cyclone dissipates its energy very fast. Hence, the intensity of the storm decreases rapidly. Tropical cyclones cause heavy damage to life and property on their land path and intense rainfall and heavy floods in streams are its usual consequences. Tropical cyclones give moderate to excessive precipitation over
very large areas, of the order of `10^3\ `km^2` , for several days.
Extratropical Cyclone:
These are cyclones formed in locations outside the tropical zone. Associated with a frontal system, they possess a strong counter-clockwise wind
circulation in the northern hemisphere. The magnitude of precipitation and wind velocities are relatively lower than those of a tropical cyclone. However, the duration of precipitation is usually longer and the areal extent also is larger.
Anticyclones:
These are regions of high pressure, usually of large areal extent. The weather is usually calm at the centre. Anticyclones cause clockwise wind circulations in the northern hemisphere. Winds are of moderate speed, and at the outer edges, cloudy and precipitation conditions exist.
Frontal Precipitation:
When two air masses due to contrasting temperatures and densities clash with each other, condensation and precipitation occurs at the surface of contact, called a front or frontal surface. A front may be:
- Cold front
- Warm front
- Stationary front
If a colder and denser air mass lifts the warm and moist air ahead of it which rises,cools and its moisture condenses to produce clouds and precipitation,it is called Cold front. i.e, if a cold air mass drives out a warm air mass, it is called Cold front.
If a warm and less denser air rises up and over the colder air ahead of it which cools as it rises and its moisture condenses to produce clouds and precipitation,it is called Warm front. i.e, if a warm air masses replaces the retraining cold air mass, it is called Warm front.
If the two air masses are drawn simultaneously towards a low pressure area, the front developed is stationary and it is called stationary front.
Cold front causes intense precipitation on comparatively smaller areas while the precipitation due to warm front is less intense but is spread over a comparatively larger areas.
The phenomenon in which cold fronts move faster than warm front and usually overtake them thus lifting the warm sectoe air above the frontal intersection is known as occlusion and the resulting frontal surface is known as Occluded front.
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