Competitive Exams: Water Vapour and Evaporation

  • The process of transformation of liquid (water) into gaseous from is called evaporation. The process of conversion of vapour into (water) and solid form (ice, snow, frost) is condensation.

Latent Heat

  • Energy in the form of heat is required for the conversion of water into gaseous torm (water vapor). Heat energy is generally measured in the unit of calorie.

  • The potential energy of water is more than ice and that of vapour is more than water. This hidden amount of heating water is the latent heat.

Humidity Capacity

The moisture content (humidity) of the air is measured in grain per cubic foot or in gram per cubic centimetre. Evaporation is the main mechanism through which wa-sher is converted into humidity. Temperature and evaporation are directly positively related and hence humidity and temperature are also directly positively related.

  • The moisture retaining capacity or humidity capacity refers to the capacity of an air of certain temperature to retain maximum amount of moisture content.

  • Humidity capacity is directly positively related with temperature

  • Higher the temperature, higher the humidity capacity and vice-versa.

  • The ratio of increase of humidity capacity also increases with the increasing temperature.

Absolute Humidity

  • The total weight of moisture contained (water vapour) per volume of air at definite temperature is called absolute humidity.

  • Evaporation is the main mechanism through which water is converted into humidity or vapour. Temperature and evaporation are directly and positively related. The absolute humidity decreases from equator towards thepoles and from ocean to the continents.

  • The possibility of preeipitation largely depends on absolute humidity.

  • The air having moisture content equal to its humidity capacity is called ‘saturated aiR'Generally absolute humidity does not change with increase or decrease of temperature. Specific Humidity-It is defined’ as the mass of water vapour in grams contained in a kg Qf air and it represents the actual quantity of moisture present in a definite air

  • It is seldom affected by change in the air temperature measured in the units of weight

  • It is directly proportional to vapour pressure, which is the partial pressure exerted by water vapour in the air and is independent of other gaseS'and is inversely proportional to air pressure.

  • It decreases from equator to pote ward.

  • In real sense, specific humidity is a geographer's yardstick of a basic natural resource water to be applied from equatorial to Polar Regions. It is a measure of quantity of water that can be extracted from the atmosphere as precipitation.

  • In Arctic, it is 0.2 gm/kg. while in equatorial region, it is 18gm/kg.

Relative Humidity (RH)

Relative humidity is defined as a ratio of the amount of water

vapour actually present in the air having definite volume and temperature (i.e.. absolute humidity) to the maximum amount the air can hold (i.e.. humidity capacity).

(Relative Humidity = Absolute Humidity/Humidity Capacity) There is inverse relationship between air temperature and relative humidity, i.e.. relative humidity decreases with increasing temperature while it increases with decreasing tem-perature.

When the humidity capacity and absolute humidity of the air are the same, the air is said to be saturated and relative humidity becomes lOO percent. Relative humidity (RH) can be changed in two ways: First, if the absolute humidity increases due to additional evaporation and secondly, change takes place temperature. Importance of RH: The possibility of precipitation depends on it. High and low relative humidity is indicative of the possibility of wet and dry conditions respectively. Distribution of RH: Equatorial regions are characterized by highest relative humidity.

  • It gradually decreases towards subtropical high pressure belts where it becomes minimum (between 25° -35° latitude) -Latitudes largely control seasonal distribution of relative humidity.

  • Maximum R H is found during summer season between 30° Nand30° S latitudes. Condensation

  • The transformation of gaseous form of water (i.e.. water vapour) into solid form (ice) and liquid form (water) is called condensation. Condensation is opposite to evaporation.

  • The temperature at which an air becomes saturated is called Dew point temperature. Condensation depends on:

  • the percentage of relative humidity of the air

  • the degree of cooling of air.

Cooling of Air and Adiabatic Change

Temperature decreases with increasing height at the rate of 6.5° C per 1000m or 3.6° F per 1000 feet. This rate of decrease of temperature with increasing height is called normal lapse rate.

A definite ascending air with given volume and temperature expands due to decrease in pressure and thus cools. It is apparent that there is a change in temperature of air due to ascent or descent but without addition or subtraction of heat. Such type of change of temperature of air due to contraction or expansion of air is called adiabatic change of temperature.

Adiabatic change of temperature is of two types, viz. Dry adiabatic change and moist adiabatic change.

  • The temperature of unsaturated ascending air decreases with increasing height at the rate of 5.5 F per feet or 10° C per 1000 m This type of change of temperature of unsaturated ascending or descending air is called dry adiabatic rate.

  • The rate of decrease of temperature of an ascending air beyond condensation level is lowered due to addition of latent heat of condensation of the air. This is called moist adiabatic rate

  • In this case temperature of an ascending air beyond condensation level decreases (and hence the air cools) at the rate 3° F per 1000 feet. This is also called retarded adiabatic rate.

Stability and Instability of the Atmosphere

  • Different types of precipitation (dew, rainfall, frost, snowfall, hailstorm, etc.) depend on stability and instability of the atmosphere.

  • The air without vertical movement is called stable air while unstable air undergoes vertical movement.

  • An air mass ascends and becomes unstable when it becomes warmer than the surrounding air mass while descending air mass becomes stable. The stability and instability depends on the relationship between normal lapse rate and adiabatic change in temperature.