The moisture content of the atmosphere in any locality can be measured in a number of ways. The following expressions are used to denote the actual moisture content of the atmosphere: absolute humidity, specific humidity, relative humidity and mixing ratio.
The weight of water vapour per volume of air (usually expressed as grams of water vapour per cubic meter of air) is referred to as the absolute humidity.
The measure of atmospheric humidity is seldom used by meteorologists, because absolute humidity varies with the expansion or contraction of air, even though the amount of water vapour is constant. It shows the absolute humidity for saturated air at different temperatures.
It is the weight of water vapour per weight of a given mass of air including the water vapour (usually expressed as grams of water vapour per kilogram of air). Specific humidity is a more constant property of air, and therefore it is more frequently used in meteorology.
Specific humidity of air changes only as the quantity of water vapour undergoes a change. Since it is measured in units of weight, specific humidity is not affected by changes in pressure or temperature of the air. Specific humidity is directly proportional to the vapour pressure of air and inversely proportional to the air-pressure.
The maximum amount of specific humidity is found in the equatorial zone and the minimum at the poles. Since capacity of the air depends on its temperature, the amount of water vapour present in the air near the equator is much more than what it is in the polar region.
Similarly because of the control of temperature, specific humidity in a particular region is much higher in summer than in winter. Another salient feature of the zonal distribution of specific humidity is that it is higher on the oceans than on the continents.
Relative humidity is defined as the ratio of the amount of water vapour in the air to the amount the air can hold at that temperature (or its capacity). Stated briefly, relative humidity is the ratio of the air’s water vapour content to its water vapour capacity.
This ratio is always expressed as a percentage. Table 29.1 shows that at 15″C the capacity of the air is 10 grams per kilogram.
If the air at 15°C temperature contains 5 grams of water vapour per kilogram, the relative humidity will be expressed as 5/10 or 50 percent. In case air is saturated, the relative humidity is 100 per cent. Under such a condition the rate of evaporation will be very slow.
Relative humidity being dependent on the air’s water vapour content as well as on its capacity, it must change whenever the amount of water vapour in the air changes or whenever the capacity of air changes.
With any addition of moisture by evaporation, the relative humidity will increase. Since vapour pressure is directly proportional to the amount of water vapour present in air, relative humidity may be defined as the ratio of the observed vapour pressure to that required for saturation at the same temperature. Relative humidity can be expressed by the following equation:
where f stands for relative humidity, e for vapour pressure, and es for saturation vapour pressure.
It may be pointed out that multiplication by 100 is simply for expressing relative humidity as a percentage.
It is a graphic illustration of the effect of variation in temperature on the relative humidity of air. It clearly demonstrates how water vapour content of air remains constant, whereas relative humidity decreases from 100 percent to 20 percent as the temperature increases from 5° to 30°C.
It can, therefore, be concluded that with the specific humidity at a constant level, any decrease in air temperature will result in an increase in relative humidity. On the contrary, any increase in air temperature will cause corresponding decrease in relative humidity.
Zonal distribution of relative humidity:
It shows the distribution of relative humidity by latitude zones. The maximum relative humidity is recorded at the equator where the air has been depicted as very moist. From the equator towards the sub-tropical high pressure belt there is a gradual decrease in relative humidity.
In fact, this zone of subsiding and diverging air masses is marked by the lowest value of relative humidity. However, because of decreasing temperatures in higher latitudes from about 30° pole-ward the relative humidity registers a gradual increase.
Since relative humidity is less on land than on water, it is lower in the higher latitudes in the northern hemisphere where from 60° pole-ward it decreases.
Another characteristic feature of the distribution of relative humidity is northward shifting of the belts of highest and lowest relative humidity in July and southward shifting in January following the apparent movement of the sun.
Relative humidity being directly related to the rate of evaporation, it affects man’s comfort. As noted earlier, evaporation is a cooling process. Perspiration from the skin of our body evaporates at a faster rate in dry air or in air with a lower relative humidity.
On the contrary, when relative humidity is high, the rate of evaporation of perspiration is a very slow process. Faster the rate of evaporation from our skin, the more comfortable we feel because of the cooling effects.