There is an old, though erroneous, saying that it is "darkest before the dawn." Actually, the darkest time is at mid-night (not midnight), the halfway point between sunset and sunrise. A similar adage expresses the belief that it is coldest just before dawn and begins to warm with the appearance of dawn's first light, but observations and physical theory show us that the coldest hour of the day is not before dawn but often an hour or so after sunrise.
Before looking into this adage, we must first set some parameters for the discussion. I am talking here of a clear night with no air mass changes during the period to confuse the discussion. (Of course, a cold front passing hours after dawn can produce a temperature less than the morning low at a later hour.) We start the clear night period at sunset which totally cuts off all direct solar heating. Unless we have an air mass change during the night (which can bring warmer or colder air), the surface layer of the earth radiates some of its energy away, and the air in the lowest atmosphere cools.
All objects, including you and me, radiate heat away at a rate proportional to their temperature (actually the fourth power) and receive energy back from every object radiating in their view. When more radiant energy is gained than lost, the object generally warms. When more energy is lost than gained, the object cools. When there is a balance between gain and loss, the object can maintain a constant temperature.
During most of the day, the incoming solar energy is stronger than the outgoing heat radiation (called terrestrial radiation when referring to the Earth system) and the planet's surface warms. (See "Laying Some Groundwork: Balancing Radiation" for more details on the earth's radiation balance.) Between sunset and rise on a clear night, however, the Earth's surface receives no solar heat but continues to lose its heat and thus cools. Clouds and high humidity can retard that heat loss by absorbing the outgoing radiation and redirecting some back to the surface. (That is why I specified a clear night.)
The surface radiation budget is the meteorologist's way of keeping track of how much radiant heat is gained or lost at the surface which helps determine the air temperatures including the maximum and minimum. The full surface radiation budget equation can become very complicated, but the added details expand on a very simple equation that looks very much like a bank or personal financial balance:
