RELATIVE HUMIDITY

Relative humidity is the measure of the amount of water (moisture) in air as compared to the maximum amount of water the air can absorb at the same temperature, expressed as a percentage. When air cannot absorb any more moisture (means fully saturated), is relative humidity is 100%. 

In case of condition, where air in the room has the capacity to absorb 100 ml of moisture but it does not have any moisture or 0 ml moisture, thus air present in the room will be a dry air. The relative humidity of this dry air room will be 0%.

In case of condition, where air in the room has the capacity to absorb 100 ml of moisture but it has only 25 ml of moisture, thus the relative humidity of this room will be 25%.

Similarly, the relative humidity of room C and D will be 75% and 100% respectively.

The relative humidity of air is dependent not only on the amount of moisture in the air, but also on its temperature as the amount of water required to saturate air is itself dependent on the temperature. It is seen that in convective drying of wet solid where warm air is blowing over the surface of a wet solid, the relative humidity may rise during the drying process due to two reasons.

(a)Uptake of evaporated water vapour from the wet solids and

(b)Cooling of warm air as it transfers heat to the wet solids.

If the cooling of air is excessive, the temperature of the air falls to a value known as the dew point, when liquid water will condense and be deposited.

DRY BULB AND WET BULB TEMPERATURE

• Dry bull temperature is also known as ambient or room temperature because the bulb of the thermometer is in contact with the air of the room. If the thermometer shows 30°C temperature, means dry bulb temperature is 30°C.
• Wet bulb temperature is determined by wrapping moist cotton on the bulb of thermometer.

In this case air at high temperature comes in contact with moisture of the cotton and the heat is absorbed form the air and hence the air is cooling down. The temperature recorded at this condition sis called wet bulb temperature. This will always be less than the dry bulb temperature. If the thermometer shows 25°C, means wet bulb temperature is 25°C.

SIGNIFICANCE OF WET BULB TEMPERATURE

1.By the wet bull temperature, we can get the idea of relative humidity. The air has the capacity to absorb the moisture. The humidity is the amount of moisture in the air. if the relative humidity is 100% then air does not absorb any moisture from wet cotton. This will not even cool down the air thus the wet bull thermometer wrapped with moist cotton is of no use because air has not taken any moisture from this wet cotton and the temperature will be equivalent to dry bulb temperature. It implies that when relative humidity is 100%, dry bulb temperature is equal to wet bull temperature.

2.The difference between dry bulb temperature and the wet bulb temperature is called the wet bull depression. When the relative humidity is low, wet bulb temperature will also be low and the difference between dry bulb temperature and wet bulb temperature i.e., wet bulb depression will be high. Reversely, if relative humidity is high, wet bulb temperature will also be high and the difference between dry bulb temperature and wet bulb temperature i.e., wet bulb depression will be low.