Question

how was rainfall caused explain with the help of diagram

Answer 1

Precipitation can be defined in many different ways. One can define it as any aqueous deposit in liquid or solid form that develops in a saturated atmosphere and falls to the ground generally from clouds. A saturated atmosphere is any atmosphere where the relative humidity equals 100%. However this definition includes sleet, hail, dew, fog, rain and snow. Therefore there is a requirement for further investigation into the causes of rainfall and the processes involved in it. There are many key parts to the theory of how precipitation occurs and the processes involved all relate to the hydrological cycle.

Evaporation is first the process where water is heated from the surface of the earth and rises in a gaseous form of water vapour until it reaches the sufficient height and temperature for condensation to occur. Condensation is the change of water from its gaseous form of water vapour into liquid water. Condensation generally occurs in the atmosphere when warm air rises, cools and looses its capacity to hold the water as a gas in the form of vapour. As a result, excess water vapour condenses to form cloud droplets. There are however many different types of condensation just like there is for precipitation.         

Radiation cooling occurs when the ground loses heat very quickly through terrestrial radiation and the air that is in contact with it is cooled by conduction. If the air that is affected by this process is humid then fog or dew will form. This usually happens in the evenings when there is typically a clear and calm sky. Advection cooling is likened to radiation cooling because they involve horizontal rather than vertical movements of air, which means that the amount of condensation created as a result is much, lower. Advection cooling results when warm moist air that moves over cooler land surfaces causing fogs to form as the warm air drifts over the cooler air below.

Other types of condensation come in the form of orographic and frontal uplift. In this process warm, moist air is forced to rise as it crosses a mountain barrier or when it meets a colder and denser mass of air at what is called a front. This is a far more effective method of creating condensation as it has a vertical movement rather than a horizontal movement. Along with the orographic condensation there is also convective condensation that occurs when air gets warmed during the daytime and rises in certain pockets as thermals. When the air expands it uses energy and so loses heat and so the temperature drops. As the air is being cooled by the reduction in pressure with height rather than due to a loss of heat to the adjacent air it is said to be adiabatically cooled.  This process is demonstrated in the diagram below.



The diagram shown above is the process of adiabatic cooling in the condensation process. For condensation to occur the air must be saturated (cooled to dew point). Dew point is the certain temperature to which air must be cooled in order to reach saturation. Also there must be a surface to condense on, for example the condensation nuclei (dust/smoke). Rising air cools adiabatically due to the expansion adiabatic process. This is a temperature change without a heat transfer involved, and so once the condensation level is reached and clouds will form.

Once the process of condensation has started the formation of the actual rain droplets is very important. As the air has now become too unstable to support the water as a vapour and is turning it into its liquid state the water does not just fall from the sky in one great mass. The water is in minute droplet sizes and are so small that they cannot fall straight to the ground because they cannot overcome the updrafts that support clouds simply because their falling velocity is not high enough.

Answer 2

first transpiration and evaporation takes place

then condensation takes place

then percipitation takes place