Changes in Insolation

The amount of insolation that an area receives can vary over the course of a day or over a year. The highest point of the Sun's path in the sky is the time when the maximum amount (intensity) of insolation for the day reaches a location. The warmest part of the day is usually a few hours later. This is because the land absorbs the sunlight and reradiates it out to the atmosphere, warming it up. We measure the air temperature when giving temperature readings. The coldest part of the day is just before sunrise, when the Earth's surface has reradiated much of the energy it absorbed during the hours of sunlight.

Over the course of a year, the Sun reaches its highest point on June 21 for anyone living north of the Tropic of Cancer. The maximum air temperature for this area is delayed until July. The reason is similar to the daily changes. The ground needs time to absorb the energy and to reradiate it to the atmosphere. On the other end of the year, the Sun reaches its lowest noontime point on December 21. For the same reasons as above, the coolest month for the region is January.