MIKE MOSS SAYS:      Tim,     On a fair night with light winds and no frontal passages, the earth's surface loses heat by way of outgoing long wave infrared radiation, and in turn cools the air in the lower atmosphere. This cooling process continues until incoming short wave radiation from the rising sun begins to exceed the outgoing radiation, and the temperature starts to climb again. The low temperature for a given day can vary somewhat in its timing, depending on variations in cloud cover, wind speed and direction, and whether any frontal boundaries are crossing the area. However, most often the lowest temperature occurs near sunrise or within the hour or so following sunrise.