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 familiarize the engineer with important aspects of rainfall and, in particular, with the collection and analysis of rainfall data.
For precipitation to form:
The atmosphere must have moisture
There must be sufficient nucleii present to aid condensation
Weather conditions must be good for condensation of water vapour to take place
The products of condensation must reach the earth.
Under proper weather conditions, the water vapour condensed over nucleii to form tiny water droplets of sizes less than 0.1 mm in diameter. The nucleii 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.
FORMS OF PRECIPITATION
Some of the common forms of precipitation are:
Rain, Snow, Drizzle, Glaze, Sleet, Hail
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
Snow is another important form of precipitation. Snow consists of ice crystals which. usually combine to form flakes. When new, 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 Jndia, snow occurs only in the Himalayan regions.
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.
When rain or drizzle comes in contact with cold ground at around 00 C,the water drops freeze to form an ice coating called glaze or freezing rain.
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.
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.
TYPES OF PRECIPITATION
These are regions of high pressure, usually of large area 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.
In this type of precipitation a packet of air which is warmer than the surrounding air due to localized 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 area extent of such rains is small. being limited to a diameter of about 10 km.
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 light rainfall.