Why does temperature decrease as we increase elevation?

Two reasons...the 1st being more important than the 2nd ... why the temperature generally decreases with altitude in the troposphere.

1) Sun warms the ground.

The ground ... in turn ... warms the lower troposphere by conduction ... the same way heat is transmitted from a kitchen stove to a pot or skillet.

The farther away from the heat source ... the less heating; therefore ... the air temperature decreases with increasing altitude.

2) Air density is lower at higher altitudes.

Temperature is a measure of kinetic energy (how fast things are moving around).

At lower altitudes...there are more air molecules than at higher altitudes.

The more air molecules there are to bump into each other...the higher the temperature; therefore ... at higher altitudes ... where there are fewer air molecules ... the air temperature is lower.

It's called adiabatic lapse rate, air is expanded at higher elevation, so the heat it contains is diluted, heat being quantity times temperature.

Because the atmosphere becomes more diffused and dispersed.