Altitudes

Because when pressure decreases temp. also decrease.

Our earth accepts the waves with shorter wavelength from the sun and reflects of longer wavelength. The longer wave length waves are absorbed by the dust particles, air molecules and other impurities in the air etc. and warm the atmosphere.

Reason:

The concentration of these air molecules decreses as we go up, so absorption of heat is less, hence tempertaure decreses as altitude increases.

NOTE: Temperature increases as pressure increaes, it is true only if the volume is kept constant. But in reality, as we go up volume is not always constant.

simple 
with increase in altitude the air pressure decrease ,as temperature is directly proportional to pressure.

Volume of air is constant around our globe or in other words the earth can be considered as isochoric system. As we move higher and higher the pressure decrease and at constant volume the pressure is directly proportional to temp. as a result temp will also decrease