Sun Avoidance Hours Versus Calendar Date
by Tom Chester
Remember the constant advice we get in the summer to stay out of the sun between 9 am and 3 pm? Over the Christmas vacation of 1995, I got interested in quantifying that advice versus date after reading a Sky & Telescope article that gave the dependence of uv on sun angle, especially to find out what the corresponding times were in winter.
The conventional advice implies that one should stay out of the sun anytime the intensity is greater than 0.4 of maximum. That would imply staying out of the sun between:
- 9:04 and 2:56 on Jun 21,
- 9:08 and 2:52 a month earlier or later,
- 9:36 and 2:24 in the middle of April and August,
- 10:24 and 1:36 in the middle of March and September,
and one is free to be out anytime during most of winter.
For those interested, it turns out the uv drops off much more steeply than just the path length through the atmosphere, presumably because the atmosphere is optically thick to uv. Since the received uv radiation dies out quickly at earlier and later times, there is not much benefit to picking a number smaller than 0.4 of maximum uv. In other words, the total received uv radiation is relatively small up to the time when the intensity hits 0.3 or 0.4 of maximum uv. However, one will receive quite a bit more total uv radiation by staying out even a bit later than those times, since the uv quickly increases to the maximum.
Here's the quick answer for a latitude of 33.3° versus calendar date:
(Click on graph or link for bigger and better image.)
Fig. 4. Time of day in the morning (local time) at which ultraviolet light from sun equals 0.4 times the maximum at noon on the summer solstice, ~21 June.
Remember that the afternoon time is (noon) + (noon - morning time).
Here's the long answer, including how to compute the same curve for any latitude.
Finally, note that I don't recommend any amount of exposure to the sun at all. If you have been exposed to a fair amount of sun, typical for those over 40 years old, take a look at the skin on your face and hands and compare it to places that have received very little sun, like perhaps your stomach or buttocks. The difference in the skin's appearance is solely due to sun damage. There were absolutely striking facial pictures in Science News around 1990 of an ~80 year-old monk who had virtually never been exposed to sun and that of an ~40 year-old Vietnamese farmer who had been constantly in the sun. From the appearance of the faces, you would have guessed that the monk was in his 30s, and the farmer was in his 90s.
If anyone wants a Quattro Pro or Excel spreadsheet that does the above calculation, let me know. It can easily be customized to give the results for any latitude, although the spreadsheet is not highly automated, requiring the user to construct the table above by varying parameters.
Source:
http://la.znet.com/~schester/calculations/sun_avoidance_times.html
Comments and feedback: Tom Chester
Tom Chester is the Two Micron All Sky Survey (2MASS) science team supervisor of Intrared Processing and Analysis Center (IPAC) at Caltech.
Astronomical Archive Produced by 2Mass Telescope Project
Home
Last modified: 08/15/03
 |
