Let us now understand the thermal structure of our atmosphere. If you think that sun heats up our atmosphere, you are absolutely wrong. Solar radiation from the sun reaches earth directly.
Thereafter, atmosphere is heated by re-radiation of heat by the earth. It is easily visible because you can see that as we go to mountains or hill tops the temperature reduces.
Sensible heat is transferred from one place to another by three means. One of them is Conduction the other is Convection and the third one is Radiation.
In conduction, heat transfer occurs between objects by direct contact. In the case of convection heat is transferred when molecules of the fluid travel from one place to another taking the heat along with them.
In the case of radiation, heat transfer occurs without involving particles of the medium. This type of heat transfer takes place due to the difference in temperature. Radiation is possible even in the absence of any medium like vacuum.
Latent heat is defined as the heat or energy that is absorbed or released during a phase change of a substance. It could either be from a gas to a liquid or liquid to a solid or the other way around.
Latent heat is transferred in the atmosphere by evaporation, condensation and sublimation. Evaporation is the process by which water changes from a liquid to a gas or vapour.
Condensation is the process in which water vapour in the air changes into liquid water. Sublimation is the process in which solid converts to gas without changing through the liquid phase.
We say that an air parcel is saturated, when it is holding the maximum possible amount of water vapour at a given temperature and pressure. At higher temperatures, we need more water vapour to saturate air as compared to lower temperatures.
Therefore in the tropics, where the temperature is high, 4% of water vapour is required for saturation. Conversely, nearer to the poles even negligible amount of water vapour is good enough for saturating the air. In meteorology, when we say saturated air, we mean to say that the air has got 4% of water vapour.
Relative Humidity (RH) Is a ratio, of the amount of atmospheric moisture present, relative to the amount that would be present if the air were saturated. Relative humidity is expressed in terms of percentage. In saturated air, the relative humidity would be equal to 100%.
We had said that whenever the quantity of water vapour is less than 4% we say that the air is unsaturated. In terms of relative humidity, in an unsaturated air the relative humidity would be less than 100%.