Here's the actual weather from a mesonet station in Norman, Oklahoma on Sept. 10, 1996, near the autumnal equinox (when day and night are of equal length). The graph runs approximately from sunset on the 9th to sunset on the 10th. Time is plotted in GMT: local midnight (standard time) is 06Z and local noon is 18Z. The evolution of temperature is similar to what is observed on most sunny days. You should click on this image so that you get a copy for future reference as you read the rest of this module.

The characteristics of this temperature plot provide important hints for what's causing the changes in temperature.

Notice that the diurnal temperature cycle is not symmetrical. The temperature rises a lot faster than it falls.

Notice that the temperature starts rising rapidly, and then more gradually. The most rapid rise is during the morning, and the warmest temperature is reached around 3:00 PM (21Z).

Notice also that the temperature starts falling rapidly, and then more gradually. The most rapid fall is around sunset (6:00 PM or 00Z), but the coldest temperature is not reached until around dawn (12Z).