Refer to figure.

Convection is the transfer of heat by the movement of a fluid (such as air or water) between areas of different temperature.
Convection results from the tendency of most fluids to expand when heated—i.e., to become less dense and to rise.
When two surfaces are heated unequally, they heat the overlying air unevenly.

The process of convection begins at sunrise and continues as follows:

1. The sun's radiation strikes the ground, heating it.
2. As the ground's temperature warms, it heats the layer of air directly above it through conduction.
3. Because surfaces like sand, rocks, and pavement become warmer faster than ground covered by water or vegetation, air at and near the surface heats unevenly. As a result, some pockets warm faster than others.
4. The faster warming pockets become less dense than the cooler air that surrounds them and they begin to rise. These rising columns of air are called "thermals." As the air rises, heat and moisture are transported upward (vertically) into the atmosphere. The stronger the surface heating, the stronger and higher up into the atmosphere the convection extends. (This is why convection is especially active on hot summer afternoons.)

In the summer the sun activity is greater than in the other seasons.
During noon 12:00 the sun is at its highest point.
At this time solar radiation is highest and a sunburn occurs in the shortest amount of time.
The temperature is at it hottest around afternoon (3pm) because the suns heat has built up since noon and more heat is present at the surface than is leaving it.