The formation of dew

At nightfall, the ground, losing heat by radiation, undergoes a continued fall in temperature and the air in contact with the ground also suffers a fall in temperature, heat transfer between the two taking place by convection. Eventually, the temperature of the ground drops below the dew point and condensation forms. Although the rate of heat loss from solid surfaces is roughly constant, depending on the fourth power of the absolute temperature of the surface, all solid objects do not fall in temperature at the same rate. A good deal of heat is stored in the upper layers of the earth, and heat flows outwards to the surface to make good radiation losses therefrom. Thus, good conductors of heat in good thermal contact with the ground will fall in temperature at a rate much the same as that of the main surface mass of the earth nearby. Bad conductors, or insulated objects, however, will not be able to draw heat from the earth to make good their losses by radiation and so their temperature will fall more rapidly and dew will tend to be deposited first on such objects. Examples of these two classes are rocks, which are good conductors and are in intimate contact with the earth and grasses which are poor conductors. Dew tends to form on grass before it does on rocks.

Posted in Air Conditioning Engineering


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