Question

why does only 51%of the insolation reach the earth surface?

Answer 1

This is called insolation. Only 51% of this insolation reaches and is absorbed by the earth's surface. The rest is absorbed by water vapour, dust and clouds, or is reflected by the earth's surface and scattered by particles in the air (the albedo effect).

Answer 2

The remaining 51 %  reaches the earth which in turn warm the layer of air above it by conduction,convection,and radiation