Forms of precipitation in hydrology pdf & Weather system for precipitation

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Forms of precipitation in hydrology pdf & Weather system for precipitation
Forms of precipitation in hydrology pdf & Weather system for precipitation


The term precipitation denotes all forms of water that reach the earth from the atmosphere. The usual forms are rainfall, snowfall, hail, frost and dew. 

Of all these, only the first two contribute significant amounts of water. Rainfall being the predominant form of precipitation causing stream flow, especially the flood flow in a majority of rivers in India, unless otherwise stated, the term rainfall is used in this book synonymously with precipitation. The magnitude of precipitation varies with time and space. Differences in the magnitude of rainfall in various parts of a country at a given time and variations of rainfall at a place in various seasons of the year are obvious and need no elaboration. It is this variation that is responsible for many hydrological problems, such as floods and droughts. The study of precipitation forms a major portion of the subject of hydrometeorology. In this chapter, a brief introduction is given to familiarise the engineer with important aspects of rainfall, and, in particular, with the collection and analysis of rainfall data.

For precipitation to form: 

  1. the atmosphere must have moisture, 
  2. there must be sufficient nuclei present to aid condensation, 
  3. weather conditions must be good for condensation of water vapour to take place, and 
  4. 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. A considerable part of this precipitation gets evaporated back to the atmosphere. The net precipitation at a place and its form depend upon a number of meteorological factors, such as the weather elements like wind, temperature, humidity and pressure in the volume region enclosing the clouds and the ground surface at the given place.

What are the 6 types of precipitation

Some of the common forms of precipitation are rain, snow, drizzle, glaze, sleet and hail.

1. Rain

It is the principal form of precipitation in India. The term rainfall is used to describe precipitations in the form of water drops of sizes larger than 0.5 mm. 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 follows:

 Light rainTrace to 2.5 mm/h
Moderate rain2.5 mm/h to 7.5 mm/h
Heavy rain>7.5 mm/h

2. Snow

Snow is another important form of precipitation. Snow consists of ice crystals which usually combine to form flakes. When fresh, snow has an initial density varying from 0.06 to 0.15 g/cm and it is usual to assume an average density of 0.1 g/cm'. In India, snow occurs only in the Himalayan regions.

3. Drizzle

A fine sprinkle of numerous water droplets of size less than 0.5 mm and intensity less than 1 mm/h is known as drizzle. In this, the drops are so small that they appear to float in the air.

4. Glaze

When rain or drizzle comes in contact with cold ground at around 0°C, the water drops freeze to form an ice coating called glaze or freezing rain.

5. Sleet

It is frozen raindrops of transparent grains which form when rain falls through air at subfreezing temperature. In Britain, sleet denotes precipitation of snow and rain simultaneously.

6. 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.

Weather Systems for Precipitation

For the formation of clouds and subsequent precipitation, it is necessary that the moist air masses cool to form condensation. This is normally accomplished by adiabatic cooling of moist air through a process of being lifted to higher altitudes. Some of the terms and processes connected with weather systems associated with precipitation are given below.

1. Front

A front is the interface between two distinct air masses. Under certain favourable conditions, when a warm air mass and cold air mass meet, the warmer air mass is lifted over the colder one with the formation of a front. The ascending warmer air cools adiabatically with the consequent formation of clouds and precipitation.

2. Cyclone

A cyclone is a large low-pressure region with circular wind motion. Two types of cyclones are recognised: tropical cyclones and extratropical cyclones.

(a) 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.

types of precipitation in hydrology pdf
section of tropical cyclone

During summer months, tropical cyclones originate in the open ocean at around 5-10° 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 km², for several days.

(b) 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.

3. 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.

Also read - Barometric levelling PDF

Also read - Difference Between Shrinkage and Swelling of Soils

Also read - Types of sheet piles pdf

4. Convective Precipitation

In this type of precipitation, a packet of air which is warmer than the surrounding air due to localised heating rises because of its lesser density. Air from cooler surroundings flows to take up its place, thus setting up a convective cell. The warm air continues to rise, undergoes cooling and results in precipitation. Depending upon the moisture, thermal and other conditions, light showers to thunderstorms can be expected in convective precipitation. Usually, the areal extent of such rains is small, being limited to a diameter of about 10 km.

5. Orographic Precipitation

The moist air masses may get lifted up to higher altitudes due to the presence of mountain barriers and consequently undergo cooling, condensation and precipitation. Such a precipitation is known as orographic precipitation. Thus, in mountain ranges, the windward slopes have heavy precipitation and the leeward slopes have light rainfall.

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