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As a result of evaporation, visible water converts into vapour. The process of conversion of water into vapour due to solar radiation is called evaporation. Through this process oceanic water reaches into the atmosphere. Evaporation of water from oceans takes place more from continents. Its main reason is sufficient availability of water. People knew about the process of evaporation about 2,500 years back as Greek scholars have given description about loss of water from water sources due to evaporation, but systematic information about the process of has become available only during the last few decades.
It is now clear at present that through vaporization conversion of surface water takes place in gaseous form. Latent heat of vaporization converts liquid into vapour which is about 539 calories per gram on 100°C. Thermal energy of latent heat while increasing at fast speed converts granules into dynamic energy. Whenever any liquid is evaporated, thermal energy of liquid reduces after evaporation. Its example is water falling in the form of showers and during bathing, due to evaporation of water, temperature of body reduces.
After converting of water particles into gaseous form, they behave differently in comparison to liquids. In gases, every particle is isolated and is away from the particles in liquid form. Vapour particles located above a water lake on a summer day will be 40 times farther from the water level at the time of relative humid air.
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During evaporation average mutual distance between particles increases. Water expands on a large scale. At normal temperature, one gram liquid water is one millilitre whereas, at 25°C temperature one gram saturated water vapour is approximately 42,000 millilitre. This expansion is about 42,000 times. In this state, vapour particles are at quite a distance.
This fact is also important that up to about 60 per cent, water vapour is so heavy like other atmospheric gases that it inclines air to rise above water level. Besides such presence of high thermal energy in vapour, it also possesses some normal gaseous qualities. It can be compressed and can also expand. Its particles can become pressure free, which it is called ‘partial pressure’. An English chemical scientist, John Dalton had established this rule of partial pressure in 1801 after different experiments.
At certain temperature and pressure, quantity of water vapour existing in air of a certain size is called humidity. Relative humidity of saturated air is 100 per cent. Less than 100 per cent air shows that it has still some capacity. For example, at a temperature of 44° Celsius, if temperature of 100 per cent relative humidity air is made 15.5°C , then its relative humidity will become 50 per cent because with increase in temperature, the capacity of absorbing humidity increases. Thus, it is clear that fog existing in the morning disappears as soon as sun rises because the heat in the air vaporises the fog away.
Factors Affecting Evaporation:
In nature, evaporation continues till water gets heat. Rate of evaporation of water continues changing according to environment. Thermal energy is essential for evaporation and it can be obtained from the radiation of the sun. Relative humidity of air and movement of air above water surface also affect the rate of vaporization.
Near the equator where sea and air both are hot, water is taken away in large quantity through evaporation. Tropical climate is typically humid. There exist such conditions in the eastern continental group, Amazon basin and Central Africa. Cool air prevailing near the Poles cannot absorb humidity because of minimum temperature. Hence, Arctic climate is typically dry.
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Effect of Water Quality:
Quality of water is the main reason affecting the rate of evaporation of water. Increase of one per cent salinity in water reduces one per cent rate of evaporation of water. Due to this reason, as compared to saline sea water of oceans there is more evaporation from clean water portions.
Effect of Water Depth:
Water has the capacity to absorb thermal heat. Hence, bigger water reservoirs can store more thermal energy. One deep lake will absorb more thermal energy as compared to a shallow lake which will create difference in rate of evaporation. It also happens with change of seasons. In cold winter climate, temperature of a shallow lake would be near 4°C (39°F) and when its surface starts freezing its temperature would reach 0°C (32°F).
Measuring Evaporation:
Theoretically speaking, evaporation can be measured on the basis of rate of vaporization over any water level. This is possible only in laboratories. This function is very difficult in open water area because nature cannot be controlled so minutely. John Dalton discovered the ‘Rule of Partial Pressure’ in 1801. He tried to measure evaporation indirectly by measuring air on water level and pressure of vapour by using an equation which was later on revised by many researchers. His method is called ‘Mass Transfer Method’ of measuring evaporation.
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As per one estimate, 4, 55,000 cubic kms water from the sea reach’s in the atmosphere every year through evaporation. Similarly, 62,000 cubic kms reaches the atmosphere through evaporation from water sources situated on the earth. As against it, when 98,000 cubic kms water through evaporation enters into the atmosphere, it is absorbed in it.
Water vapour can be absorbed in atmosphere in a definite quantity. As soon as the relative humidity becomes 100 per cent, air is called saturated. It has been seen that hardly one per cent of the total water available on earth is used by the hydrological cycle. It normally believed that nowhere is there equality between evaporation and precipitation.
As estimated by M. I. Budyoko (1971), evaporation from oceans is more than rain. According to him, loss of 46,000 cubic kms water takes place from oceans every year. On the other side, 62,000 cubic kms water evaporates every year from the land, but It receives 1, 08,000 cubic kms water through rain. Hence, the land receives 46,000 cubic kms additional water every year. Such additional water again reaches oceans through surface flow. Thus, more rain is received by continents as compared to evaporation.