Question

why are hill stations cooler in summers than the plains ?

Answer 1

Your question about why temperatures on hill stations aren't "warmer", is a good one. But in reality, the air is not warmed directly by the sun's rays, but by infrared, or long-wave radiation from the earth. The sun's rays strike
the earth, and are absorbed by the earth, which raises the temperature of the earth. The earth re-radiates the energy at a wavelength dependent on the temperature of the earth. The air is able to absorb the earth radiation, and becomes warmer. At high altitudes, as on mountain tops, strong winds keep the air mixed, and prevent much rise in temperatures near the surface of the mountain. Also as you go up the heating factor, the land becomes progressively lesser. Vegetation also reduces heat as most of the suns energy is used to evaporate water.

As you go up in altitude from the surface of the earth, the air cools at a fixed rate. This is called the "lapse rate," and for unsaturated air, that rate is about 5 deg. C. for each thousand feet of altitude. For saturated air, the rate is about 3.5 deg.C. for each thousand feet of altitude. Wind mixes this colder air on mountain tops and hill stations...

Answer 2

The drop in temperature at Hill Stations and Mountains is because of the change in ATMOSPHERIC PRESSURE. There is a relation between PRESSURE and TEMPERATURE. Both Pressure and Temperature vary proportionally. This is what THERMODYNAMICS says. But I hate equations and formulas. Let's have a practical explanation and good understanding of this FACT The sun does not heat the air directly. It actually warms the surface of the earth. When the surface is warmed up, it warms up the air layer above it. ... Hence the temperature would be reduced by the time they come to higher parts of the atmospher