Why are nights with a clear sky sometimes colder than nights that are cloudy?

Clouds act like a blanket and actually prevent the heat gained from the previous day from escaping out to space. Humidity levels throughout the atmosphere play a role, also. The lower the humidity, the more heat will radiate out into space. Other factors such as wind speed affect the temperature. On windy nights, the cold air (which is heavier than warm air) cannot collect at the surface. The wind keeps the air mixed preventing the heat from radiating into space. Now, if an arctic air mass is moving into your area, the advection of the cold air can be just as effective in bringing down temperatures to frigid levels. However, once the winds subside as the high pressure area moves in, the following night will usually be colder. Snow cover plays a role in surface temperatures as it reflects the sun’s light and heat during the day and produces ideal conditions for rapid heat loss at night.