For example, the roof of a low building situated directly south of a much higher building will receive a considerable amount of reflected sunlight, plus some fraction of the long-wave radiation emitted by the wall of the higher building. The annual maximum insolation is nearly the same for all walls facing not more than 90 degrees from south, regardless of latitude. The hour and date of the maximum depend, however, on the wall orientation and latitude. Walls facing east and west receive their greatest irradiation in the middle of the summer about 4 hours before and after noon, respectively, whereas those facing south receive their maximum at noon about the middle of November. Walls facing east and west will have higher surface temperatures than walls facing south, other things being equal, because their maximum irradiation coincides with the maximum ambient air temperature.
Walls always receive some reflected shortwave radiation from their surroundings.
Thus the total radiation incident on a vertical surface is just about the same as that shown in Figure 2 for a flat roof. It is commonly recognized that the surface of a thick masonry wall does not become as hot during a clear summer day as the outer layer of a light curtain wall section, if both walls have the same exposure. The difference lies in the heat storage capacity of the walls. When the radiation incident on a wall or roof surface suddenly changes, as when a cloud moves away from in front of the sun, the S. A. T. increases abruptly, but the temperature of the exposed surface does not reach a new equilibrium value until some time later. The time required depends on the value of the surface conductance and the heat storage capacity of the wall or roof.