Take an ice cube out of the freezer. Drop it in a glass of water. Take the temperature of the water immediately after you drop the ice in. Take it again after 5 minutes. Which measurement is going to be colder?

Drop a red-hot nickel ball into a glass of water. Measure the temperature of the water immediately, and then again after 5 minutes. Which measurement is going to be hotter?

Basically, the surface of the earth holds heat, which causes the atmospheric temperature to lag behind the heat input from solar insolation. The cold surface is still warming long after the summer solstice; the hot surface is still cooling long after the winter solstice. The atmospheric temperature extremes are closer to the equinoxes than the solstices.