Cloud Cover vs Solar UV

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Learning outcomes:

By the end of this activity you should know:

  • the intensity of solar UV radiation incident upon earth’s surface depends, among other factors, on cloud cover
  • in general, cloud cover decreases solar UV radiation; but it can also enhance it in some special cases
  • partly cloudy conditions do not eliminate the sunburn risk.

Clouds are the main cause of variations in the solar UV radiation at a given location for a fixed solar elevation. This is known to many people, but it is more important to know the following facts about the effect of clouds on human exposure to solar UV radiation:

  • Generally, clouds reduce the solar UV radiation that reaches earth’s surface and affects human health. However, clouds cannot block all solar UV radiation and that is why people may get sunburnt even on a cloudy day.
  • Cloud cover causes a greater decrease in the other parts of solar spectrum (particularly visible light and heat) than it does in the UV spectrum. When it is cloudy, it may be cool outside, but definitely not safe from UV radiation.
  • Certain configurations of clouds can increase solar UV radiation at specific sites. This increase could be as much as 30% above the corresponding clear-sky levels.

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CAUTION! Some people stay unprotected for long times outside when there is cloud cover as they don’t feel hot and believe there is no UV radiation.  This may cause overexposure to UV radiation especially during partly cloudy conditions.

Global solar radiation, direct solar UV radiation, diffuse solar UV radiation and total sky images are continuously recorded at the University of Southern Queensland, Toowoomba.

The following presentation shows how clouds affect solar UV radiation measured in units of minimal erythemal dose (MED). The minimal erythemal dose is the exposure that produces a just noticeable erythema on a single individual’s previously unexposed skin.

Questions:

  1. At what time was the highest percentage cloud cover? At that time, what was the percentage reduction in the erythemal exposure compared to the clear day?  Was the percentage reduction in erythemal exposure as high as the percentage cover?
  2. Starting from 10:00 and according to the provided data, what is the required time for an exposure of one MED on the cloudy day and what is the time on the clear day?
  3. What was the percentage reduction in the total erythemal exposure compared to the clear day?
  4. Write the intervals when there was an enhancement in the erythemal exposure because of clouds.