On a typical day, the low temperature will occur late at night or just after sunrise. Also, the greatest warming during the day often occurs between a couple of hours after sunrise and noon. The reasons for this tendency are given below: Often a shallow layer of cool air will be at the surface around sunrise. As the sun shines in the morning, the earth’s surface is warmed and the wind increases to help mix this cool air at the surface with warmer air aloft. The solar warming of the ground also helps warm the air. The ground emits longwave radiation at night while there is no sunlight to warm the ground. This is why temperature typically cools off at night. Heating of the ground is cumulative and the sun angle is getting higher in the morning. This steady supply of increasing solar energy on a clear day tends to warm the air temperature significantly in the early morning hours. In the afternoon, the wind is often a little stronger and mixing of now cooler air aloft mixes with the air at the surface. Also, in the afternoon the sun angle is getting lower. Thus, the decreasing sun angles and mixing of the air reduces the amount of warming that occurs in the afternoon. |