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Precipitation, its causes, Classification, and Types

 Precipitation is known as all states of water that reach the earth from the atmosphere. The usual precipitation is rainfall, hail snowfall frost, and dew. Among these, all types of precipitation, rainfall and snowfall contribute a significant amount of water. Rainfall is the predominant form of precipitation. It is used synonymously with precipitation. It is usually expressed as depth either mm or cm. The magnitude of precipitation varies with the location, season, and time.

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Causes of Precipitation
Fig1. Causes and process of Precipitation

In the process of precipitation, there is a sequence process of the lifting of an air mass in the atmosphere where it cools and some of its moisture condenses forms precipitation.

The three main mechanisms of atmosphere lifting are frontal lifting where warm air is lifted over cooler air by the frontal passage; orographic lifting. In which air mass rises to pass over the mountain ranges and convective lifting where the air is drawn upward by convective action. Such as in the center of the thunderstorm cell, convective cells are initiated by surface heating.

Which causes a vertical instability of moist air and is sustained by the heat of transformation of vaporization given up as water vapor rises and condenses. The condensation process increase in the size of the droplet becomes heavy (= 0.1 mm dia.) some of them (droplets) increase in size by impact and aggregation and larger drops (= 0.1-3 mm) form and then precipitation starts to fall.

Forms of Precipitation are:

Some of the common forms of precipitation are rainfall, snowfall, drizzle, sleet, and hail.

  • Rain : A form of Precipitation in the form of water drops of size more than 0.5 mm, Maximum size of raindrop is approximately 6 mm.
Light rainTrace to 2.5 mm/hr
Moderate rain2.5 mm/hr to 7.5 mm/hr
Heavy rain>7.5mm/hr
  1. Snow : Consist of ice crystal which usually combiner to form flakes. , Initial density varying form 0.06 to 0.15 gm/cm3.
  2. Drizle : Fine sprinkle of numerous water droplets of size <0.5 mm. , Intensity less than 1mm/hr, Water drop appears to float in the air due to small size.
  3. Glaze : When rain or drizzle cons in contact with cold ground 0  0C. the water drop freezes to form an ice coating called glaze or freezing rain.
  4. Sleet : Frozen raindrops of transparent grains.
  5. Hail : Showery precipitation having the irregular form of pellets or lumps of ice of size ranging from 5mm to 50mm or above. The density of the hail is normally 0.8 gm/cm

Causes of precipitation :

  1. Cooling of air to the dew point temperature to produce saturation condition.
  2. Being a condensation of moist air masses.
  3. Growth of droplet.
  4. Accumulation of moisture of sufficient intensity to account for the observed rates of rainfall.

OR, When the water is evaporated and diffused into the atmosphere. Moist air masses get cool to form condensation. After condensation tiny water drops formed and precipitation occurs.

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Types and Classification of Precipitation :

According to the factor responsible for the lifting of the air mass, precipitation is often classified. In nature, the effects of the various types of cooling causing precipitation are often inter-related and the resulting precipitation cannot always be identified as being of any one type.

Precipitation is classified by the type of mechanism that produces it. Each type of precipitation has characteristics that have important hydrological consequences. Three types of precipitation are distinguished.

1. Convective Precipitation:

  • It is the precipitation resulting from the upward movement of air that is warmer from the surrounding air. The air close to the warm earth gets heated and rises because of its lesser density. The effect may be caused by the difference between land and taken hot bare field and cool woods. Streets and roof of city warmer.
  • Cooler air replaces the warmer surrounding convective cell.
  • Warmer air continuously rises up undergoes cooling and results in precipitation.
  • Aerial extent is small limited to diameter (ø) about 10km.

2) Cyclonic Precipitation :

Precipitation associated with cyclones (Low-pressure region surrounded by a larger pressure region) is termed as cyclonic precipitation. It results from the lifting of air converging into the low-pressure of the cyclone. Cyclonic move at the rate of about 30 to 50 km/hr and give medium to high-intensity rainfall over a larger area. It may result in heavy precipitation and damages enough life and properties.

(i) Frontal  Precipitation :

If the line of demarcation between cold and warm air is sharp. It is called a front in case of temperature contrast heavy precipitation is usually located near fronts. The precipitation along and associated with the front is known as frontal precipitation.

(ii) Non frontal Precipitation :

In the case of the non-frontal precipitation. The moist warm air mass is stationary and the moving cold air mass meets it. Thus due to the lightness of warm air, there is the passive ascent of warm over cold air owing to the active undercooling. When the warm air cools down at a higher altitudes Precipitation occurs.

Precipitation due to Orographic features
Fig.2. Precipitation due to Orographic features

3) Orographic Precipitation:

The moist air masses may get lifted-up to higher altitudes due to the presence of topographic barriers as mountains and consequently undergo cooling condensation and precipitation. Such a type of precipitation is known as orographic precipitation.

Precipitation is heavier on windward slops and lighter on leeward slopes.

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