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Old 06-11-2014, 07:50 AM
 
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For almost everywhere south of parallel 45 during summer, this is take hours of daylighht minus 6.5 hours. The rule is avoid sun as much as possible when your shadow length is shorter than you which happens when sun is above 45 degrees from horizon

The more north you go in the US, the less dangerous the exposure during the 6.5 hour mid-day period but the sun drops faster in the tropics so by the time sun is at shadow length = your height the tropics is not stronger sun at that time of day anymore.

If you live In the Northern Hemisphere north of Havana but south of Montreal, Minneapolis, or Venice, or Oregon, you get the most hours of safe sun exposure during fall-winter timeframe.

In Minneapolis on parallel 45 your most safe hours of sun = the slightly over 12 hours of daylight you get on Equinox where at the hottest part of day the sun is on Equator 0 and you on 45 makes sun 45 degree angle which is the highest safe level of exposure by the shadow rule. Therefore you can take the 12 hours minus 0.

Let me know your latitude and I can tell you if today you have more safe hours than 12. For all latitudes below 65 degrees north, your number of safe hours of sun is NOT maximized on the summer solstice

On latitude 65 the daylight gain near solstice exceeds the lost hours of safe exposure and this would be unique to graph

However, north of latitude 68 you can have the 24 hours of sun all as "safe" with respect to the shadow rule. This does not account for ozone, altitude, being in snow or in a lake or on a wooden dock. This would apply to flat grass and only UVB sunburn/tanning. Because of UVA which is nearly inescapable with any noticeable daytime sun, you still would do yourself a favor taking sun precautions but you wouldn't have to worry nearly as much.

Note this does not take into account ozone and safe hours also means not totally safe, just that relative to tropical sun midday, you would need at least 9-10 minutes exposure to equal UVB tan/burning of skin-related harm per minute exposure in tropics under same skin type, cloud cover, altitude, and terrain.

Last edited by EricS39; 06-11-2014 at 07:59 AM..
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Old 06-11-2014, 10:21 AM
 
Location: Bremerhaven, NW Germany
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What it is here at 53.5° degrees north in June?
Currently we have about 17 hours of daylight from sunrise to sunset.
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Old 06-11-2014, 10:32 AM
 
Location: United Nations
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At 53.5 °N on June 21 the noon is 50°, at 13:00 is 44°, at 14:00 is 38° and so on. It lasts less than 2 hours.
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Old 06-11-2014, 10:34 AM
 
Location: United Nations
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No I was wrong!
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Old 06-11-2014, 10:35 AM
 
Location: United Nations
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At 53.5 °N on June 21 the noon is 60°, at 13:00 is 54°, at 14:00 is 48°, at 15:00 is 42° and so on. It lasts about 5/6 hours.
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Old 06-11-2014, 10:46 AM
 
Location: Tangerang (6°17 S)
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Don't know but I know there are no days that are completely safe. Minimum sun angle at noon is 60 degrees while maximum is 90 (a.k.a directly overhead).
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Old 06-11-2014, 10:56 AM
 
Location: United Nations
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Quote:
Originally Posted by bronski View Post
Don't know but I know there are no days that are completely safe. Minimum sun angle at noon is 60 degrees while maximum is 90 (a.k.a directly overhead).
Isn't the minimum solar angle at noon 78 for your location?
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Old 06-11-2014, 10:59 AM
 
Location: North West Northern Ireland.
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Quote:
Originally Posted by The East Frisian View Post
What it is here at 53.5° degrees north in June?
Currently we have about 17 hours of daylight from sunrise to sunset.
I have 17 hours and 10 minuets. How can you be having 17 hours?
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Old 06-11-2014, 11:21 AM
 
Location: Tangerang (6°17 S)
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Quote:
Originally Posted by EverBlack View Post
Isn't the minimum solar angle at noon 78 for your location?
No. It's coming soon, only 9 days until the winter solstice.
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Old 06-11-2014, 12:19 PM
 
Location: Rimini, Emilia-Romagna, Italy (44°0 N)
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I live at 44°N.
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