On a hot summer day the air above the land is shimmering? GIVE REASON

The surface is hotter than the air. This means the air heats up & consequently rises (air - the same as all gases - expands as it is heated, causing the pressure to drop to a level lower than the air above it & that makes it rise). The rising air plays havoc with lightwaves & the shimmering you see is due to lightwaves being distorted.

Even a very short distance above the surface the air is cooler. Hotter air rising into it causes heat transference and resultant turbulence and this can also add to the effect.