Decrease in temperature due to radiation Can radiation cause the temperature to decrease? Because, in my Physics textbook, it is written that:

ln winter nights, the atmospheric temperature
  goes down. The surfaces of windowpanes, flowers,
  grass, etc., become still colder due to radiation. The
  air near them becomes saturated and condensation
  begins. The droplets condensed on such surfaces are known as dew.

How can radiation cause cooling? Please explain.
 A: Temperature in a gas is defined as :

the average kinetic energy of the molecules composing it. If you read the link, it becomes more complicated but the basic meaning is the same, kinetic energy in any form contributes to the temperature of an object.
At the same time we have black body radiation, an experimental observation which is one of the reasons quantum mechanics had to be adopted as the theory in the microcosm of atoms, but that is another story.
Black body radiation means that all bodies radiate electromagnetic energy to the environment. Energy is leaving the bodies and thus, by the formula above, the temperature falls. If you have many bodies, the temperature falls until it reaches equilibrium in all bodies ( as much radiation energy leaving as entering).
As at night the sun is not radiating energy on the earth, the objects on the earth's surface radiate until they come to an equilibrium with the atmosphere, which radiates away to space continually,until the sun comes up the next day and heats the atmosphere with its radiation. So the dew is due to the fall in temperature.
