The water-cycle of the earth and atmosphere
the more common aspects of weather like clouds, rain, snow and fog arise when, because of cooling, the water vapour in the air condenses to liquid, or sometimes solid form. water vapour gets into the air evaporates from the oceans, an to a less extent from lakes rivers. Vegetation in active growth also gives off a large amount of water vapour from its leaves, the supply being drawn from the soil moisture, through the roots. This process is called transpiration.
Most of the water is ultimately returned to earth as rain or snow. The rain that falls on the ground may percolate to supply soil moisture to be later returned to the air through transpiration, or to feed springs. It may also remain on the surface giving rise to small streams which combine to form rivers, with or without lakes; this process is called run-off. There is therefore, a continuous process of exchange of water between the land, sea and atmosphere and it is this that is called the water-cycle.
Rain
Water falling from the sky in any form is called precipitation, whether as fog, sleet, snow, hail or rain.
Rain is precipitation in the form of sizable water drops; it falls out of cloud whereas true cloud droplets mostly 'float' in the air. There are two main processes by which cloud droplets grow to raindrop size. The first is the water-ice transfer process, in which if a cloud is formed in air temperature below freezing many of the droplets do not freeze but remain liquid, in what is called a supercooled state. There is the other rain forming process, this is called coalescence. A few large drops, falling through the slower falling small drops, collide with them and, so to speak, capture them, to form still larger drops.
1) Measurement of rainfall
The amount of rain which falls in a specified time is exposed as the depth of the water it would produce on a large level impermiable surface. Rain fall is measured daily at a very large number of stations. The simple rain gauge consists of a copper or plastic funnel, so mounted that the catch of rain is collected in a bottle or other vessel which is itself protected from the rain.
2) Types of rain
Relief Rain
When a stream of hot air from the ocean meets a line of hills or mountains, it rises to pass over the obstacle and is cooled in the process. The air temperature falls below dew point, clouds form and precipitation may follow on the windward side. The air will go over the crest and down the leeward side, getting warmer as it moves down the other side
Frontal Rain
Warm moist air comes into contact with cooler drier air. The line seperating the two air masses is known as a front. The warmer, moister air rises when it meets the cold, dry air along the front. In doing so the warm air is cooled and clouds form and rain falls.
Cyclonic Rain
It is due to the general rise of air over a large area. When the surface is heated, the surface heats the air directly in contact with it, causing the warm air to rise. The higher the altitude the lower the temperature. This causes the air to cool, condense and form clouds and cause rain.
Other Forms of Precipitation
Snow
When water freezes, its molecules join together to form crystals. If these crystals fall from a cloud, they may attach themselves to other crystals to form snowflakes. A single snowflake may consist of more than 50 individual snowflakes. At temperatures around -40 C (-40 F) crystals will form in clear air, and snow can fall from a clear sky.
Hail
It is a precipitation in the form of pellets or lumps of ice usually occuring in thunderstorms. A large raindrop will be carried upwards in the cloud by a strong up-current reaching low temperatures until it freezes. As the strength of the up-currents are weaker, higher up in the cloud, it begins to fall. When it reaches the lower part of the cloud where the up-currents are stronger it collects another layer of water and is repeated until the hailstone is too heavy for the up-currents in the cloud to support. It will then fall to the earth as hail.
Thunder and Lightning
They arise from electrical charges. Turbulence inside the clouds causes the raindrops to break-up. The smaller drops are carried to the top of the cloud while the larger drops are left in the lower part of the cloud. This separation leads to the separation of electrical charges which gives rise to lightning. On passing through the air the lightning gives rise to heat causing a sudden expansion and contraction of the air. This sets of sound waves which are heard as thunder.
TORNADOES
Tornadoes are the most violent of all wind storms. A tornado occurs over land and is a revolving cyclonic storm. It has a fairly small diameter, but has a high intensity. The pressure at the centre is very low which causes tremendously powerful winds, which have been known to uproot trees, lift cars and trucks, and destroy anything except the most solidly constructed building. As the tornado weakens, it drops the things it has picked up. A tornado appears as a funnel extending out from the cloudbase. A tornado over a sea or lake is known as a water spout. Every year, hundreds of people lose their lives because of the destructive force of tornadoes.
Cloud Classification
