![]() ![]() The atmospheric moisture or humidity is nothing but water vapour which has escaped from oceans, rivers, lakes, ponds, plants, animals and humans into the atmosphere. Evaporation and Condensation:Įvaporation Evaporation is the process by which matter changes from liquid to a gaseous or vapour state. The relative humidity is the lowest during early morning, especially after long, calm, clear nights due to low capacity of the air to hold moisture at a low temperature. Looking at the diurnal variation, the absolute humidity is high during the afternoon and comes down as the temperature comes down. With altitude, the capacity of air to hold moisture decreases because the temperature also decreases. The marine air may be saturated to the extent of 80%, while the continental air may be only saturated up to 20%. Latitudinally, the atmospheric moisture decreases from the equator towards the poles in an irregular manner with the latitudinal temperature gradient. Since it is measured in units of weight (usually grams per kilogram), the specific humidity is not affected by changes in pressure or temperature. It is expressed as the weight of water vapour per unit weight of air, or the proportion of the mass of water vapour to the total mass of air. The temperature at which saturation occurs in a given sample of air or water vapour begins to change into water is known as the dew point. If it has half the amount of moisture that it can carry, the air is unsaturated and its relative humidity is only 50%.Ī given sample of air becomes saturated without any actual change in its moisture content, provided its temperature falls or it cools to the required extent. Thus, relative humidity of the saturated air is 100%. At this temperature, the air cannot hold any additional amount of moisture. Air containing moisture to its full capacity at a given temperature is said to be ‘saturated’. The relative humidity determines the amount and rate of evaporation and hence it is an important climatic factor. (ii) A decrease in temperature (hence, decrease in moisture-holding capacity) will cause an increase in relative humidity. ![]() (i) If moisture is added by evaporation, the relative humidity will increase.
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