With temperatures quickly falling through the 20s tonight, let's see how the clear skies above is major factor. 

During the day, the solar radiation from the sun heats up the earth. At night, the earth cools as heat is released and, escapes back into space. On cloudy nights, the clouds acts as a blanket, causing some heat to be sent back to the ground. These nights are warmer. On the other hand, nights with clear skies have nothing blocking the heat from freely escaping back to space. These nights are typically the coolest. 

Tonight is an example of what is called Radiational Cooling at night. Our average low temperatures is 30 degrees for this time of the year, but many places are already in the mid 20s.