Introduction to Water Cycle and Related Terms

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Condensation, Sublimation and Latent Energy

From the gaseous state, molecules may pass into the liquid state by condensation. If the temperature is below freezing point, they can pass directly into the solid state to form ice crystals or snow. When ice changes into water vapour it is called sublimation. Water needs energy to change into water vapour. The water molecules absorb the heat energy from the surroundings; this energy is called latent energy and is sealed in water vapour.

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Conventional Rainfall

In the equatorial areas the sun heats the ground and the air near it also gets heated. As the air gets heated, it spreads and becomes lighter and moves upwards. When it moves upward it cools and water vapour condenses to form clouds and heavy rain occur. This type of rain is known as conventional rainfall.

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Evaporation

The process of changing of water from the liquid state to the gaseous state is called evaporation. For example: Wet clothes dry in the sun because the water present in them changes into water vapour and evaporates.

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Effects of precipitation

Precipitation has diverse effect:
  • Heavy rainfall may lead to flood.
  • Scarcity of rainfall may lead to drought.
  • A good amount of rainfall at the right time can increase crop production while severe rain can cause damage to the crops.
  • It also affects the means of transportation.

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Humidity and measurement of relative humidity

Humidity: The amount of water vapour in the air is its humidity. More water vapour in the air indicates higher humidity.
Relative humidity: It is more important from the point of view of weather studies. It is expressed as the ratio between the actual amount of water vapour present in the air at a given temperature and the amount of water vapour air can hold at the temperature. The relative humidity is always expressed in percentage.

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Fog, Dew and Frost

Fog- When the temperature of the air near the surface of the earth reduces, water vapour condenses. In this process vapour turns into microscopic water particles and float in the air. When the density of these particles increases, fog occurs.
Dew- When moisture-laden air near the earth's surface comes into contact with very cold objects, condensation of vapour takes place. They turn into very small water droplets. This is called Dew.
Frost- If the temperature of the air is less than zero degree, the water droplets stuck to the surface of the cold object freeze. This frozen water droplet is called frost.

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Forms of condensation

Various forms of condensation include:
(a) Mist; (b) Fog; (c) Frost; (d) Dew; (e) Hail; and (f) Clouds.

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Multiple uses of water

Humans use huge amounts of water not only for drinking and washing but also in the process of production. Water is being used for irrigation, hydro-electric power generation, industry, domestic use, transportation, fisheries and also for recreational purposes. 

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Cyclonic rainfall

Cyclonic rainfall occur due to the influence of a cyclone. In a cyclonic rainfall the air from the surrounding region comes toward the center of the cyclone and start moving upwards. As it rises, the temperature of the air reduces, condensation occur and rainfall takes place.

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Example of Precipitation

Precipitation: Condensed water vapour falls back to the Earth in various forms. This process is called precipitation. The different forms of precipitation are rain, drizzle, snow, sleet and hail.

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Orographic rainfall

When the moisture-laden winds get obstructed by high mountain ranges, they start going upward along the slopes. With altitude the temperature of these winds drop and condensation occur and rain takes place. The windward side of the mountains gets more rain while the leeward side gets lesser rain.

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Types of rainfall

There are three different types of rainfall:
(a) Convectional rainfall
(b) Orographic or relief rainfall
(c) Cyclonic or frontal rainfall

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Measurement and calculation of rainfall

Rain Gauge is the instrument used to measure rainfall.
For calculating rainfall for weather charts, readings are taken daily. With these recordings a monthly chart is prepared and these charts are used for calculating the total annual rainfall.
1. Total monthly rainfall
2. Total annual rainfall

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Moisture

Water is the only substance which can be naturally exist in three states - solid, liquid and gas. Very cold places on the Earth's surface have water in the solid state. Rainfall generally comes in the form of liquid and the water vapour present in the atmosphere is in the form of gas. The processes of evaporation, condensation and precipitation complete the hydrological cycle which is simply termed as "moisture".

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Evapotranspiration

Evapotranspiration is the process by which water is transferred from the land to the atmosphere by evaporation from the soil and other surfaces and by transpiration from plants.

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Hail

It is precipitation of small balls or pieces of ice. Hail is the most destructive form of precipitation formed in thunderstorm. The strong up-drift of air in the clouds carry rain drops to great height where they get frozen into ice crystals.

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Snow as precipitation

It is precipitation of white and opaque grains of ice. Snow is the precipitation of solid water. In winter when temperature are below freezing point in the whole atmosphere, the ice crystals falling from the clouds do not melt and reach the ground as snow.