The key thesis undergriding the information on this page is a startling one!
"Altitude is All"
In calculating your risk of sunburn/suntan on a clear, sunny day, the season of the year, your latitude, and most other factors can be essentially forgotten---- except your two types of altitude:
1. The sun's altitude in the sky above the horizon line..... If the sun is below a certain level, you aren't at risk even in Hawaii or Los Angeles. If it is above a certain level, you are at risk even at unlikely places and times quite far north; eg. Vancouver, B.C. in early April (see "200 UVB Season" below).
2. Your height above sea level.
On clear days the very transparent, thinner atmosphere of high
altitudes definitely leads to increased UVB intensity, especially
at times of day normally thought to be safer from sunburn. 
Although the sun's altitude in the sky is
definitely a much larger determinant of UVB intensity, earthly
altitudes 6,000 or more feet above sea level can add substantially
to sunburn risk.... In the picture at the top of the page, I am
at 14,000 feet on snow in late May at 41 degrees latitude, and
thus I estimate my Solarmeter would have read over 550, an astounding
intensity indicating grave sunburn risk.
Similarly, the situation pictured on the left, from Mt. Hood's Hogsback at 10,500 ft, in late June, would have produced a UVB situation nearly as dangerous..... Even in mid-September on Mt. Hood, I measured noon UVB at 353 in the Timberline Lodge parking lot at 6000 feet! Higher on the mountain at 7600 ft, at 3:15 pm in the afternoon, with scattered fresh snow on the ground, I measured UVB at a strong 261, with the snow-reflected UVB ranging from 100 to 160 additional units-- a combination that could sunburn the unwary climber quite easily-- yes, that was 3:15 in the afternoon in mid-September at 45 degrees latitude! (see "200 UVB Season" section below)...... Finally, a mid-winter data set on January 23 revealed how severely the low sun angles of winter affect UVB: we were at 6,700 ft. on Mt. Hood with brillant new snow all around. My meter could only reach 122. What was rather interesting was how strong the UVB reflected off the snow was-- in the direction of the sun, glazed new snow powerfully reflected at 96, while pointing the meter directly away from the sun but at the brillant mountain gave a reading of 60. These January 23 levels, although certainly not the intense UVB found during the summers even in the lowland, would be capable of giving a person that "Skier's Tan" if one spent the whole day out in it..
.... My most recent UVB studies about snow reflections occurred Sept. 9, 2007 on Mt. Rainier, WA... At an elevation of 7,700 feet, standing on a vast but somewhat dirty snowfield, the sky UVB ws 335 at 2:00 pm, while UVB reflections off the snow were at about 102. (illustrating the effects of altitude, one day later in Olympia, WA at sea-level on a very clear day the 2:00 pm. UVB was 90 points less (245)....
How do Willamette Valley UVB levels compare to places in the tropics and places in the Arctic? Using data supplied by the USDA UV-B Monitoring and Research Network (use link), I can provide a general context to answer this question:
Zero to Three--- The approximate total range of Ultraviolet intensity on the Planet, in locations where ordinary people might be able to drive to in a car, appears to be on a scale of 0-3... By this I mean that zenith maximums at Fairbanks, Alaska (the Arctic) are about 0.13*, while the 10,000 foot high Mauna Loa observatory in the tropics in Hawaii (19.5 degrees) returns an intensity fully three times higher (0.39)... The highest values I found in the Continental US were 0.29 in Southern Florida (25 degrees latitude), 0.29 in Arizona at 7,000 feet at 36 deg. lat., and 0.28 in Colorado at 10,500 feet and 40 deg latitude. An unimpressive 0.195 was the intensity meaured in Eastern Washington, which was the closest comparable measuring station to the Willamette Valley. .... Thus, Willamette Valley intensities are about one-half the intensities of the tropics, and exactly halfway between the maximum and minimum values to be found during normal worldwide travelling.
8/1/2005, 10,000 feet high in Hawaii-- recently I discovered a United Kingdom website with a picture of a Solarmeter 6.2 meter in action at 10,000 ft. in Hawaii, returning a value of 545. This was atop Haleakala Crater on Maui, with the sun at an altitude of almost 90 degrees (straight overhead, in other words). This value probably cannot be bested in any other place on Earth that an ordinary car can drive you to--- except, I theorize, if you could catch a flight to La Paz, Boliva at summer solistice--- the city lies at an incredible 12,000 feet elevation, and typically has the extreme low humidity that seems to assist UVB in penetrating to the earth's surface. Yes, take me there for the experiment! Click here to see the UK website mentioned above. This group recently posted UVB measurement results about worldwide levels on winter solistice 2005, with Solarmeter users worldwise turning in results. A site in the resort paradise of the Seychelles Islands (south of India) turned in a result nearly as strong as Haleakala Crater, but the Seychelles are at sea level, and are a place much more commonly visited by "average" tourists (the UVB reading was 530).
* the USDA network uses much more sophisticated and expensive equipment than my Solarmeter 6.2, and its measurements of UV-B should not be construed as exactly comparable, eg. the USDA network uses Erythemal doses, which Solarmeter 6.2 does not measure.
Portland, Oregon vs. Seattle, Washington vs. Vancouver, B.C, vs. Anchorage, Alaska, vs. San Francisco, California, vs. Los Angeles, California, vs. El Paso, Texas.
First, let's establish somewhat arbitrarily the UVB levels of 200 and 150 as our reference points.
In terms of sun altitude capable of producing these UVB intensities at sea level, my research points to these figures:
Sun 40 degrees high = 200 UVB on a day rated as a "high UVB day" (Note, however, that more than 50% of clear days will measure less than this when the sun is at this altitude, due to haze, smoke, and other factors).
Sun 33 degrees high = 150 UVB, on high UVB day.
In terms of a dose (erythemal dose, MEDs, and other technical topics) let's talk about the sun being able to apply this UVB level for about one hour. Our goal is to give persons a sense of what the UVB risk/suntan "Season" actually is in various locales, referencing Portland, Oregon as our reference point. (45 1/2 degrees latitude, sea level).
In Portland, the 200 UVB level season begins when the sun first rises to 40 degrees high at solar noon and stays that high for about an hour; this gives March 12th as the beginning of the season, and September 30th as the end of sunburn/suntan season...(but on nearby Mt. Hood, at high altitude, the season is much longer).
In Seattle, the 200 UVB season shrinks to March 17th to September 25th.
In Vancouver, B.C., the 200 UVB season is small, from only March 21st. to September 23rd. (latitude 49 16')
In Anchorage, Alaska, the 200 UVB season is brief, from April 21st. to August 23rd. (latitude 61 10')
In San Francisco, the 200 UVB season has a long run, from about February 21st to October 19th.
In Los Angeles, the 200 UVB season occupies most of the year, beginning February 8th, and lasting until November 1st.
In Portland, the 150 UVB season begins about February 25th, and ends about October 18th.... Note: at Portland the UVB intensity at Winter Solistice (Dec. 22) sinks to a feeble 48 (and this was on the most dry, brillant day I was able to measure)!
In Vancouver, B.C., the 150 UVB season runs from about March 4th to October 11th.
In Anchorage, even the 150 UVB season is short, only from April 1st, to September 11th.
In San Francisco, the 150 UVB season begins about February 1st, and ends about November 11th.
In Los Angeles, the 150 UVB season is only forty days shy of running all year, being from January 10th to December 1st.
In El Paso, Texas (latitude 31 46') the 150 UVB season becomes year-round, especially considering the higher altitude and typically dry, transparent desert air in El Paso, (elevation 3800 feet).
brucej@Oregonphotos.com
