The hottest time of day is typically the mid-afternoon. It is commonly believed that noon is the hottest time of day, but due to delays in the heating of the atmosphere and Earth's surface, it is not.
When the sun's radiation reaches the Earth, there is a delay in the time it takes to heat the surface. This latency is called the thermal response. It may take several hours for the temperature to reach its highest point after the sun is at its highest point. This latency varies during each season. It is common for a great portion of UV radiation to have decreased by the time the surface is heated to its highest temperature.