Water In The Atmosphere

The creation of acid rain would not be possible without water, another of the major natural constituents of the atmosphere. Lists of the principal gases in the atmosphere—such as Table 2.1—commonly refer to dry air, but the atmosphere is never completely dry. The proportion of water vapour in the atmosphere, in the humid tropics, may be as much as 4 per cent by volume, and even above the world's driest deserts there is water present, if only in fractional amounts. At any one time, the total volume of water in the atmosphere is relatively small, and, if precipitated completely and evenly across the earth's surface, would produce the equivalent of no more than 25 mm of rainfall (Barry and Chorley 1992). In reality, the distribution is very

Figure 2.2 The hydrologie cycle

Figure 2.2 The hydrologie cycle

Figure 2.3 Energy transfer during the change in state of water

uneven, as a result of regional variability in the dynamic processes which produce precipitation. Intense thunderstorms can yield 25 mm of precipitation in a matter of minutes, whereas the same amount may take several months, or even years, to accumulate under more stable atmospheric conditions. Variations such as these account for annual precipitation totals which range from virtually nothing, in some of the world's deserts, to as much as 4,000 mm, in the monsoon lands of the tropics. Precipitation in excess of the quantities normally in the air is made possible by the hydrologic cycle, which circulates water through the earth/atmosphere system, and regularly replenishes the atmospheric reservoir (see Figure 2.2).

Water is unique among the constituents of the atmosphere in that it is capable of existing as solid, liquid or gas, and of changing readily from one state to another. It becomes involved in energy transfer as a result of these changes. For example, energy absorbed during the conversion of liquid water to water vapour is retained by the latter, in the form of latent heat, until the process is reversed. The stored energy is then released (see Figure 2.3). The water vapour may travel over great distances in the atmosphere in the period between the absorption and re-release of the energy, and in this way energy absorbed in one location is transported elsewhere in the system.

Water is also involved in the earth's energy budget through its ability to absorb and reflect radiation. As vapour, it contributes to the greenhouse effect by absorbing terrestrial radiation, whereas in its liquid and solid forms it can be highly reflective. As clouds in the air or snow on the ground, it may reflect as much as 90 per cent of the solar radiation it intercepts. That radiation, reflected back into space, makes no contribution to the energy requirements of the system. In contrast, clouds can also help to retain terrestrial radiation by reflecting it back to the surface.

Water is an integral part of many global environmental issues such as drought, desertification and acid rain. In addition, because of its role in the earth's energy budget, any change in the distribution of water in the earth/ atmosphere system might well augment or diminish the impact of other elements such as the greenhouse effect or ozone depletion which, in whole or in part, make their presence felt through that budget.

The Basic Survival Guide

The Basic Survival Guide

Disasters: Why No ones Really 100 Safe. This is common knowledgethat disaster is everywhere. Its in the streets, its inside your campuses, and it can even be found inside your home. The question is not whether we are safe because no one is really THAT secure anymore but whether we can do something to lessen the odds of ever becoming a victim.

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