UV Index: Information

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.

Figure 1:



Figure 2:



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