The following table gives the results for the latitude of Fallbrook, CA, 33.3°.
The first column is the declination of the sun, and the right hand column gives the approximate dates at which the sun reaches that declination. The times are the local (~standard) times of the time in the morning at which the uv from the sun reaches the given percentage of maximum on 21 June. The afternoon time that corresponds to the same level of uv is 24 - morning time, using 24 hour time, or 12 - morning time in p.m. time.
| UV Intensity (fraction of maximum) | dates | ||||||
|---|---|---|---|---|---|---|---|
| sun declination | sun angle at noon | 0.5 | 0.4 | 0.3 | 0.2 | 0.1 | |
| 23 | 10.3 | 9.37 | 9.03 | 8.67 | 8.3 | 7.8 | 21 Jun |
| 20 | 13.3 | 9.5 | 9.13 | 8.77 | 8.4 | 7.87 | 21 May, 21 Jul |
| 10 | 23.3 | 10 | 9.6 | 9.23 | 8.8 | 8.3 | 19 April, 24 Aug |
| 0 | 33.3 | 11.07 | 10.4 | 9.87 | 9.4 | 8.8 | 21 March, 21 Sep |
| -10 | 43.3 | 11.2 | 10 | 9.53 | 24 Feb, 18 Oct | ||
| -20 | 53.3 | 10.87 | 21 Nov, 21 Jan | ||||
| -23 | 56.3 | 12 | 21 Dec | ||||
A quicker way to get the results for most other latitudes is to use the column giving the sun angle at noon. The sun angle at noon is your latitude minus the solar declination for latitudes above 23°. (Take the absolute value of the difference for lower latitudes.) If your latitude is 50°, for example, at the summer solstice on 21 June, the sun angle at noon would be 50 - 23 = 27°. This is a bit less than halfway between the sun angles of 23.3 and 33.3 in the table. Hence the "9 am to 3 pm" advice translates to "10 am to 2 pm" for you.
Find your latitude:
US Gazetter
Chris Michels, Northern Arizona University.
Although the table is incomplete for smaller sun angles, at a sun angle of 0° there is only a 4% increase in the amount of uv radiation at noon. Hence the times are little changed. San Diego County is already nearly the worst case for maximum uv, having a sun angle of 33° - 23° = 10° at noon on June 21. The absolute maximum would of course be at a latitude of +23°. The equator receives less uv radiation on June 21, having a sun angle at noon of 0° - 23° = -23°, equivalent to Fallbrook's radiation in April and August! (The received radiation is the same for positive or negative sun angles.)
Source:
http://la.znet.com/~schester/calculations/sun_avoidance_times.html
Comments and feedback: Tom Chester
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Last modified: 08/15/03