Introduction

• Is the water vapour content of air at any time and place
• But does not describe the droplets of fog, cloud or rain
• May vary in atmosphere time to time
• Is the Moisture holding capacity of air
• Depends on air temperature
• Increases slowly at lower and rapidly at higher temperature

Some terminologies

a) Absolute humidity: It is the weight of water vapor in a given volume of the air. It changes with change in temperature.

b) Specific Humidity: It is the weight of the water vapor per wt. of a given mass of air including the water vapor i.e. dry air+ water vapor.

c) Relative humidity: It is defined as the ratio of the actual amount of the water vapor present in the air to the maximum amount required to saturate at that temperature. Thus, it is the ratio of the water vapor content of air to its water vapor capacity. It can also be defined as ration of observed vapor pressure to that required for saturation at the same temperature.

RH=Ea/Es

Where, RH=Relative humidity; es= saturated vapor pressure, ea= actual vapor pressure

RH α 1/T

It varies from 0 to 100% i.e. if RH is zero, atmosphere is absolutely dry, if 100 it is completely saturated.

d) Mixing ratio: It is the weight of the water vapor per unit weight of the dry air or it is the ratio of density of the water vapor to density of dry air. It is expressed in grams of water vapor per kilogram of dry air.

Numerically it is calculated as;

X = 1+

Where, X= Mixing ratio; e= vapor pressure; es= saturation vapor pressure; P=total air pressure

e) Dew point temperature: Dew point is the temperature to which the air would become saturated (attain 100% RH) if cooled at constant pressure without addition or loss water vapor.

f) Vapor pressure: Part of the total atmospheric pressure attributed by the water vapor is called vapor pressure or partial pressure of water vapor in the atmosphere is called vapor pressure.

g) Saturation Vapor pressure: The vapor pressure of a system, at a given temperature, for which the vapor of a substance is in equilibrium with a plane surface of that substance’s pure liquid or solid phase is called saturation vapor pressure.

h) vapor pressure deficit: Difference between the saturation vapor pressure & its actual vapor pressure is termed as vapor pressure deficit.

i) Boiling point: The temperature at which the vapor pressure is equal to the atmospheric pressure is called the boiling point.

j) Saturation deficit: At any time, the difference the moisture holding capacity of air and its actual humidity is called saturation deficit.