On a night when the sky is clear, the temperature may dip considerably
On a night when the sky is clear, the temperature may dip considerably; on a cloudy night the temperature will usually dip much less. The reason for this different is that
(a) the clear sky allows radiant energy to escape readily from the earth
(b) on a cloudy night the clouds are warm and therefore retard the cooling.
(c) the clear sky allows the cold of outer space to reach the earth
(d) currents of air carry the heat away from the earth on a clear night