The variation throughout the course of the
day (diurnal) of UV radiation is much like that of visible light. Figure 1 overlays a mostly clear day's observations of
the "global" or mostly visible radiation and UV radiation. Notice how UV
radiation is much more attenuated than the global at high solar zenith angles in the early
morning and late afternoon. This is because the two components which make up UV and all
other forms of radiation; the direct and the diffuse, are both
greatly affected at these low sun angles. Direct UV radiation is greatly reduced by the
increased absorption by stratospheric ozone during its increased path length through the
atmosphere (about 6 times more than when the sun is directly overhead). Also, radiation at
the UV wavelengths is scattered much more than visible light. This further decreases the
direct component and increases the diffuse component. As the Sun rises above the horizon,
the amount of absorption in the stratosphere and scattering in the troposphere is reduced.
The result is a drastic increase in UV radiation reaching the surface.
The variation of surface temperature differs significantly from both UV and global
radiation. Where as the diurnal variation of incoming infrared radiation is similar to
that of global radiation, there exists a delay between the time when the earth-atmosphere
system is irradiated to when the temperature begins to increase. This is know as the
thermal response. Depending upon the time of the year the lag in time between the peak
radiation flux reaching the surface (solar noon) to when the surface temperature reaches
its maximum can be as great as 3 to 4 hours. For example, have you noticed that it is
hottest in the mid afternoon not at noon. A typical summer surface temperature plot is
shown in Figure 2 showing how the peak temperature is
reached later in the afternoon. By the time of the temperature maximum, the amount of UV
radiation reaching the surface has decreased almost by half of that at solar noon.