![]() Cooling of a larger layer of air near to the ground may produce mist of fog, which freezes if the dew point is below 0✬. If air in contact with the ground is cooled to its dew point, dew or frost will form, dew if the point is above 0✬, or frost if it is below 0✬. Both air temperature and absolute humidity will determine what type of condensation will occur when the air is cooled. The dew point of humid air will be higher than the dew point of dry air. The temperature of the dew point will depend upon the absolute content of water vapour, that is the absolute humidity, measured in g/m 3. Further cooling below the dew point will induce condensation of the excess water vapour. When air is cooled, relative humidity increases, until at a particular temperature, called the dew point, the air becomes saturated. The lower the air temperature, the smaller the maximum possible capacity for vapour. Condensation of water vapour occurs when the temperature of air is lowered to its dew point.Īll air contains water vapour of varying quantities. ![]() Condensation is not a matter of one particular temperature but of a difference between two. In the atmosphere, condensation may appear as clouds, fog, mist, dew or frost, depending upon the physical conditions of the atmosphere. If, however, a steady flow of air exists to remove the newly formed vapour, the air surrounding the water source will remain dry, "thirsty" for future water.Ĭondensation is the process whereby water vapour in the atmosphere is returned to its original liquid state. ![]() As the moisture content of the air increases, evaporation will diminish. On still days, water evaporating to the air remains close to its source, increasing the local humidity. It follows, therefore, that the presence of wind will also increase evaporation. Drier air has a greater "thirst" for water vapour than humid, moist air. ![]() The humidity of the surrounding air will also influence evaporation. Rates increase when temperatures are higher an increase of 10✬ will approximately double the rate of evaporation. The rate of evaporation will depend upon a number of factors. At temperatures between 100✬ and 0✬, only some of the molecules in the water have enough energy to escape to the atmosphere and the rate at which water is converted to vapour is much slower. At 100✬, the boiling point, all water will rapidly be turned to vapour, for the energy supplied to the water is enough to break all the molecular bonds in water apart. The water cycleĮvaporation of water from the Earth's surface forms one part of the water cycle. This cycle of evaporation, condensation and precipitation is called the water cycle of the Earth and atmosphere (see Figure 4.4). The water vapour is returned to the surface in rain, hail, sleet or snow, where it is returned to the soil for uptake by vegetation or to surface streams, rivers and lakes and ultimately by the sea. Water also enters the atmosphere by evapotranspiration from plants and trees. These include puddles, ponds, streams, rivers, lakes and oceans. ![]() Water vapour enters the atmosphere by evaporation from surface bodies of water. This condensation is the source of all clouds and rain. Lesson 3 demonstrated that when air rises and cools, it becomes saturated and releases water vapour as condensation. There is always water vapour present in the atmosphere. RESOURCES INDEX | HOMEPAGE | CONTENTS | PDF | HELP | BACK | NEXT ISBN 0-8493-0487-3.Atmosphere, Climate
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