Today, we learnt about the formation of rain, how rain is formed. Basically, there are three types of rain formation, namely
– Orographic (relief) rain
– Frontal rain
– Convectional rain
So firstly, how does rain come from the clouds? Clouds are actually made up of mass of water droplets. These water droplets are very small, so small that they can remain in the air. So, these water droplets would coalesce (blend/come together) together to form larger water droplets and once these larger water droplets are large enough, they would then be heavy enough to precipitate down as rain. Basically, mass of water droplets would make up cloud droplets and many of these cloud droplets would make up your cloud!
Then why do clouds look dark when it is going to rain? When it is going to rain, there would be many of these cloud droplets. So, as it is too dense, it will block and scatter the light from the sun, thus it will appear dark! Water droplets actually will bounce off each other. Therefore, a dust particle is needed to stick the water droplets together, preventing them from bouncing off.
What is adiabatic cooling? Adiabatic cooling is basically the cooling of air when it rises. We know that warm air rises and when it rises, it becomes cooler and this drop in temperature is due to the decrease in atmospheric pressure at higher altitudes. Thus, if the pressure of the rising air decreases, it will expand. Therefore, when it expands, it will lose heat to the surrounding and cool down and thus the water vapour would condense into water droplets. Similarily, adiabatic heating would refer to when the air is being forced to descend, it would be compressed and thus its temperature would rise. However, in order for condensation, the air must contain moisture, if not there won’t be water vapour! IMPORTANT!
So now lets move on to the different types of rain.
Orographic rain is basically when warm maritime air is forced upwards due to a mountain range, it will undergo adiabatic cooling and thus the water vapour would condense into water droplets and form clouds. And when these clouds get too heavy to be suspended, heavy rain falls. However, rain would only take place around the top of the mountain range as when air descends on the other side of the mountain range, it will undergo adiabatic heating, compressed and warmed and thus will produce little rain as condensation would cease, creating a rain shadow.
Frontal rain is basically when two air masses of different temperature, density and humidity meet such as the meeting of a tropical maritime air and a polar air mass. A zone called a front separates them. At the warm front, the lighter warm air would rise over the cold air which remains close to the ground. The warm air would then undergo adiabatic cooling and thus rain would fall. The rain would fall steadily ranging from a few hours to several days.
Convectional rain basically occurs when the continuous heating of the ocean from the sun would cause the moist air above to heat up. The warm air would undergo adiabatic cooling and thus rain will fall as well. The cycle is like convection as its name suggests. However, this only occurs commonly in warmed or heated areas such as equatorial/tropical regions and normally happens in the afternoon, evening period. The rain would be intense however short-lasting.
Okay, im done. I know its alot and sorry for putting up so late at night. But anyway, good luck to everyone for the test tmr. Happy mugging!