What is radiation?
Radiation is the transfer of enery through a vacuum in the form of waves.
What is heat?
Heat is a form of energy that can move from the hotter object to the cooler one.Tha temperature of the cooler object rises while that of the hotter one falls until both are at the same temperature.
Warm air is less dense than cold air.
Why?
As temperature rises molecules begin to move faster, bouncing off each other causing there to be less molecules per given area, therefore decreasing the density. (also why cold causes volume to decrease and warm temperature causes volume to increase)