Climatology/Humidity

Humidity
Water vapour present in the air is known as Humidity.At any specific temperature amount of water vapour that can be held by air has a definite limit known as saturation point.Air at saturation point is known as saturated air.Temperature at which saturation occurs is known as Dew point. Capacity of air to absorb water vapour increases with increase in temperature. There are three type of humidity:-
 * Absolute Humidity
 * Relative Humidity
 * Specific Humidity

Absolute Humidity
The actual amount of the water vapour present in the atmosphere is known as the absolute humidity.It is the weight of water vapour per unit volume of air and is expressed in terms of grams per cubic metre.The absolute humidity differs from place to place on the surface of the earth.

Relative Humidity
The percentage of moisture present in the atmosphere as compared to its full capacity at a given temperature is known as the relative humidity.

Relative Humidity = [Actual amount of water vapor in air (absolute humidity)/humidity at saturation point (the maximum water vapor air can hold at a given temperature)] X 100

The change of air temperature, the capacity to retain moisture increases or decreases, the relative humidity is affected. Relative humidity is greater over the oceans and least over the continents (absolute humidity is greater over oceans because of greater availability of water for evaporation).The relative humidity determines the amount and rate of evaporation.Air containing moisture to its full capacity at a given temperature is said to be ‘saturated’. The temperature, air cannot hold any additional amount of moisture.relative humidity of the saturated air is 100%.If the air has half the amount of moisture that it can carry,it is unsaturated and its relative humidity is only 50%.

Specific Humidity
It is expressed as the weight of water vapour per unit weight of air.it is measured in units of weight (usually grams per kilogram), the specific humidity is not affected by changes in pressure or temperature.Absolute Humidity and Relative Humidity are Variable whereas Specific Humidity is a constant.

Condensation
Transformation of water vapour into water, caused by loss of heat when moist air is cooled.Cooling may reach a level when air’s capacity to hold water vapour, then excess of water vapour condenses into liquid form.If water vapour directly condensed into solid form, it is known as sublimation.In free air, condensation results from cooling around very small particles termed as condensation Particle of dust, smoke & salt from oceans are particularly good nuclei as they absorb water (Hygroscopic nuclei).

Dew
The temperature at which saturation occurs in a given sample of air is known as dew point.Dew forms when moisture is deposited in form of water droplets on cooler surfaces of solid objects such as stone, glass, blades, plant leaves etc.It forms when temperature of air falls below dew point but above freezing point.Dew point occurs when Relative Humidity = 100%.

Frost
Frost forms on solid surfaces when condensation takes place below freezing point i.e. 0 °C means dew point is below freezing point.

Evaporation
Evaporation is the process by which matter changes from liquid to a gaseous or vapour state. The atmospheric moisture or humidity is nothing but water vapour which has escaped from oceans, rivers, lakes, ponds, and humans into the atmosphere.Heat energy is required for evaporation to take place.In case of atmospheric moisture, the energy is provided by solar radiation. The water molecules, supplied with this energy, get the required motion to escape and conserve this energy as latent heat of vaporisation. when the vapours get condensed into water drops, this energy is released in the form of latent heat of condensation.evaporation is associated with a cooling effect.

Factors Affecting Rate of Evaporation
Several factors affecting the rate of evaporationare following:-
 * 1) Amount of water available:-Rate of evaporation is greater over the oceans than over the continents.
 * 2) Temperature:-high temperature implies greater availability of energy for evaporation.the rate of evaporation is directly proportional to temperature of the evaporating surface.
 * 3) Relative humidity:- the moisture-holding capacity of air at a given temperature is limited, dry air,evaporates more water than moist air.evaporation is greater in summer.
 * 4)  Area of evaporating surface:-larger surface area exposed to heat implies enhanced evaporation.
 * 5)  Air Pressure:-It exerted on the evaporating surface.Lower pressure over open surface of the liquid results in a higher rate of evaporation.
 * 6) Composition of water:-Evaporation is inversely proportional to salinity of water. Rate of evaporation is always greater over fresh water compare to salt water.