During the summer months most of us wants to go on a trip to a hill station to escape from the hotness that we feel right... So, we choose hilly regions as we know that hill station will have cooler temperatures than flat land, but the thing we least know is why?
Let us guess.... is it because of the altitude (height above mean sea level) of the mountain? Or is it due to the air pressure? Or is there some other factor?....Yes, our guess is correct, all of them play role in deciding the cold temperature over a hill station and we will see how?
Let's discuss them briefly...
Our earth is surrounded by a thin layer of air that is atmosphere. This atmosphere exerts pressure on earth that we call as air pressure. If you see Fig. 1. below you can notice that the air molecules at the surface of the earth, but when you move above you can notice less air molecules. This causes high pressure over the surface and low pressure over high altitudes. When there is high pressure the air molecules are so close that they produce heat and inturn, high temperature, while in the low pressure area the air molecules move freely and hence low temperature. In both the figures given below, you can see that over the mountains only less air molecules present above them to exert pressure.
Fig. 1. Air pressure and air density over different heights
Image source: Quizlet.com and scienceabc.com
Another factor is heat source for the atmosphere. Most of us think that the sun is the source for the atmosphere, but that's not the case.... earth is the heat source. You may be wondering how it's possible? Fig.2. Heat source for atmosphere
The atmosphere is mostly transparent to the solar radiation (Fig. 2), as a result earth absorbs most of the radiation and gets heated up. Then the earth heats the air from below. So when moving away from the heat source of the atmosphere (the earth) temperature decreases at high altitudes. This is also a reason for having snow packs at the top of the mountains year around.
Yet another factor for low temperature at high altitude is, when the lower part of the atmosphere is heated the warm air rise, expand and cool (Fig.3.) due to gas law (relation between pressure, volume and temperature)
Fig.3. Warm air rising
Image source: apollo.lsc.vsc.edu
So all the three factors (pressure, heat source and warm air rising) results in low temperature at high altitudes.
Thanks na
Good one..