Evaporation And Condensation
Evaporation is the phenomenon in which an individual substance is transformed into gas. It occurs in almost all cases in nature, but during this process, some latent heat is also released, which causes the transformation of water to occur in a continuous manner.
Latent heat in vaporisation comes in many forms, but it primarily depends on the amount of energy required to break intermolecular bonds of water
An increase in temperature increases water absorption and retention capacity of the given parcel of air.
The movement of air replaces the saturated layer with the unsaturated layer. Hence, the greater the movement of air, the greater is the evaporation.
Condensation- The transformation of water vapour into water. Condensation is caused by the loss of heat.
When moist air is cooled, it may reach a level when its capacity to hold water vapour ceases. Then, the excess water vapour condenses into liquid form.
If it directly condenses into solid form, it is known as sublimation. Condensation also takes place when the moist air comes in contact with some colder object and it may also take place when the temperature is close to the dew point.
When the temperature of the air is reduced to dew point with its volume remaining constant; When both the volume and the temperature are reduced;
When moisture is added to the air through evaporation. However, the most favourable condition for condensation is the decrease in air temperature.
After condensation the water vapour or the moisture in the atmosphere takes one of the following forms — dew, frost, fog and clouds. Condensation takes place when the dew point is lower than the freezing point as well as higher than the freezing point.
Dew: When the moisture is deposited in the form of water droplets on cooler surfaces of solid objects (rather than nuclei in air above the surface) such as stones, grass blades and plant leaves, it is known as dew.
Frost: Frost forms on cold surfaces when condensation takes place below freezing point .i.e. the dew point is at or below the freezing point. The excess moisture is deposited in the form of minute ice crystals instead of water droplets.
Fog and Mist: When the temperature of an air mass containing a large quantity of water vapour falls all of a sudden, condensation takes place within itself on fine dust particles. Fog is a cloud with its base at or very near to the ground. Because of the fog and mist, the visibility becomes poor to zero. In urban and industrial centres smoke provides plenty of nuclei which help the formation of fog and mist. Such a condition when fog is mixed with smoke, is described as smog. The only difference between the mist and fog is that mist contains more moisture than the fog.
Clouds: Cloud is a mass of minute water droplets or tiny crystals of ice formed by the condensation of the water vapour in free air at considerable elevations. As the clouds are formed at some height over the surface of the earth, they take various shapes. According to their height, expanse, density and transparency or opaqueness clouds are grouped under four types : (i) cirrus; (ii) cumulus; (iii) stratus; 4- nimbus.