WeatherWhys for December 16,2015 from accuweather.com

Clouds can regulate temperatures both during the day and night. During the day, a thick vail of clouds will prevent a large percentage of solar radiation from reaching the ground, which keeps the air cool. At night, clouds act as a blanket and return a portion of the heat radiated by the earth back to the ground. Since less heat is escaping, the temperature does not fall as much.