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Current Category » Agricultural Meteorology

Precipitation and Forms of Precipitation

It can be defined as earthward falling of water drops or ice particles that have formed by rapid condensation in the atmosphere.

Forms Of Precipitation

A. Liquid form

B. Solid form

C. Mixed form

1. Rain

1. Snow

1. Sleet

2. Drizzle

2. Hail

2  Hail




A. Liquid Form

1. Rain:
Rain is defined as precipitation of drops of liquid water. The clouds consists of minutes of minutes droplets of water of about 0.02 mm diameter.  When these minute water droplets in clouds combine and form large drops that become so large that they can not remain suspended in the air and they fall down as rain.  The droplets are formed by repaid condensation.  The rain drops have diameters ranging from 0.05 to 0.06 cm (0.5 to 0,6 mm) The line joining the places of equal rainfall called Isohyets.

Types of Rain:

I) Convectional rains:
Due to heating, the air near the ground becomes hot and light and starts upward movement (This is known as convection.) as air moves  upward it cools at the DALR (9.80C/km) and becomes saturated(having RH 100%) and dew point is reached where the condensation.           begins . This level or height is known as condensation level.  Above condensation level air cools at SALR (5 0C/km) clouds are formed.  Then further condensation results into precipitation.  These rains are known convectional rains.

II) Ographic or relief rains:
When the moist air coming from sea encounters mountain or relief barrier, it can not move horizontally and has to overcome mountain. When this air rises upward, coolsdown, cloud is formed and condensation starts and giving precipitation. These rains are known as or orographic rains thus high rains are possible on the windward side of the mountain.  After crossing the mountain divide, when air descends downward, the air is compressed and it warmed up at DALR. This warm air does not give any precipitation on the leeward region.  This is known as rain shadow region.

III) Cyckibuc/Frontal and Convergent rains:
Frontal precipitation is produced when two opposing air currents with different temperature meet, vertical lifting takes place which gives rise to condensation and precipitation.  When the humus and warm air mass meets the cold air mass, the colder air being denser tends to push below the warmer air and replace it.  The boundary zones along which two air masses meet are called as fronts. When the mixing of warm and moist air with cold air mass takes place, the temperature of the warm and air falls down, saturation occurs and may give precipitation and it also responsible for cyclone formation and rains received from cyclones are called cyclonic reins.

Thunder Storms:
It is the atmospheric disturbance which is always accompanied by thunder and lightening and sometimes by hail.  It is a local storm covering comparatively small area and often causing damage.  Its chief
Characteristics are an immense cumulo-nimbus cloud accompanied by copious precipitation, a marked drop in temperature and a more or less destructive out rushing squall wind which precedes the rainfall.  Thunder storms occur in every part of the world and their frequency decreases with increase in latitude. 

Storms are of two types:

1) Frontal or general thunderstorm:

This occurs over wide areas in connection with passing of a cyclonic disturbance.

2) Local thunder storm:
This forms as a result of strong local convection.

3) Drizzle:
It is more or less uniform precipitation of very small and numerous raindrops which are carried away even by light wind.  The drizzle drop is less than 0.5 mm in size, and precipitate at the rate usually less than 1 mm per hour.

4) Shower:
Precipitation lasting for a short time with relatively clear intervals is called shower. This occurs from the passing clouds.

B) Solid Form:
1.  Snow:
Snow is defined as precipitation of water in the solid form of small Or large ice crystals. It occurs only when the condensing medium has a temperature below freezing temperature, snow is generally in the form of individual crystals or in flakes that are aggregates of many crystals.  Snow flakes are formed in high clouds. Snow is measured with snow gauge.

2. Hail:
Hail is a precipitation of solid ice. On a warm sunny day, a strong Connective column may cause the formation of pellets having spherical Shape and concentric layers of ice. Such a formation is known as hail.

C. Mixed Form:
1. Sleet
Simultaneous precipitation of the mixture of rain and snow is called as sleet.

2. Hailstorm:
Rainfall associated with hail stones is called hailstorm.
Mechanism or process of Rain formation or Process of Precipitation:

There are two methods by which rain drop is formed.

1. Bergeron mechanism:
There are two methods by which rain drop is formed.

1. Bergeron mechanism:
The cloud having cold temperature is cold temperature is cold cloud. In these clouds Ice particles are formed due to very low temperature (-150C to -250C). These ice particles are grow rapidly by deposition of water vapors (sublimation) developing in to hexagonal shaped ice crystals. These ice crystals on collision form snow pellets and melt into water droplets when falling on ground through warm atmosphere. This mechanism is suggested by Swedish Meteorologist Bergeron in 1933. Artificial rain making is based on these mechanisms.

2. Collission and coalescence mechanism:
The cloud having slightly higher temperature is not cloud. In these Clouds fine water droplets exist instead of ice particles. This fine water Droplets colloid and coalesce (combine) and grow into the larger size and fall on earth as rain drop.

Current Category » Agricultural Meteorology