What is meant by derating factor.How it effects the ampacity of the cable.

The longer the cable the less amps it can safely carry. Other things include is the cable in the air, in the ground, in conduit or is it romex, bx, uf. etc. All of these tells you that heat is dissipated by cables carrying electricity. Where does the heat go? This is why a cable hung in the air is rated higher than one that is in a conduit etc