Picture: UVB intensity at a dangerous 424 on the summit of Paulina Peak in Central Oregon, at solar noon, July 2, 2005.... At 8000 feet elevation, and with loads of open sky all around, Oregon UVB doesn't get much higher than this unless you could add being surrounded by fresh snow on such a high peak (see text below) ....If a reading over 400 appears on your UVB meter, it represents a very severe level of sunburn risk!... 440...Note: I returned to this peak exactly one year later, on July 2, 2006, and measured an even higher 450 on the same meter! --For comparison, on June 6, 2011, I measured the UVB at Evergreen, Colorado at a similar 440. Evergreen is just under 8,000 feet elevation. Since we were a couple weeks before the summer solistice, I'm sure this site can also hit 450.
Summer UVB* intensity in Oregon can reach levels as high as 450**; but it will take special conditions, eg. very high altitude, very clear, dry air... If somehow you were on fresh snow at this time of year, it could go higher yet. Below is a shocking picture of someone I know. I took the picture. This person exposed themselves to severe levels of UVB repeatedly as a teenager and young adult. Now, many decades later, is reaping the benefits-- several basal cell cancers removed, and continuing treatment for pre-cancer condtions, such as illustrated below, where a prescription medication cream has revealed the underlying skin damages!
Normal Willamette Valley summer solistice UVB extremes are in the general range of 280 to 350, depending mainly on the clarity of the atmosphere but at times also affected by the actual density of the ozone layer over that particular area.... See next page for comparison intensity values of Willamette Valley extremes versus extremes found elsewhere on this planet, eg. the tropics, the Arctic, etc.
Important Note: UVB is only part of the Ultraviolet light spectrum. There is considerable research, discussion, and debate in regard to the relative risk of human exposure to UVA vs. UVB vs. UVC.... Here is a very short overview: Violet visible light begins at about 400 nm, the longest visible light (deep red) is at about 700 nm......UVA is closest to the visible light spectrum, with a wavelength of 315-400 nm....UVB is the mid-range, with a wavelength of 280 to 315 nm....UVC is the shortest, with a wavelength of 200 to 280 nm..... What is very clear in the research is that UVC is biologically very dangerous, but it is also the type of UV most highly filtered by Earth's atmosphere.
The Sun's Altitude above the horizon is THE key everyday factor-- "The Higher the Sun, the Bigger the Burn" is a useful guideline that everyone needs to be aware of it in evaluating their day-to-day risk of sunburn--- or their hopes of achieving a suntan.... Sun Altitude, in combination with the overall clarity of the atmosphere, will give surprisingly wide variations from place to place in Oregon, eg. lowland Canby at 6:15pm in late June measured at only 50, while it's still 110 sitting beside Paulina Lake at 6,300 in Eastern Oregon. Eg. 4:00 pm readings of 217 in Canby while at 7,600 feet in Eastern Oregon it is still at 320.... and Southern Oregon, with the sun 3 degrees higher in the sky than Northern Oregon, has given me readings 360-370 in late June in the Rogue River Valley at 1,500 feet elevation.
The absorption of UVB by the Earth's atmosphere is very powerful; the thin Ozone layer*** is Earth's chief filter of the lethal levels of UVB found just above our protective atmosphere---- But there is another powerful factor one must understand. This is the additional protective absorption provided BELOW the ozone layer by the "crud" in the lower atmosphere. This crud is composed of air molecules, clouds, dust, water vapor, and air pollution, and it's what the UVB has to fight through in its final 30,000 feet of descent before it can hit the ground, or your skin... Lots of additional absorption of UVB occurs during this final descent, as you will see as you read below. "Rayleigh Scattering" is a main operative attenuator of UVA..
Simple Geometry tells us that the lower the sun is, the more miles of air it must pass through to reach your skin. Therefore, the effects of high altitude are seen to be more pronounced in the early morning and later afternoon hours. This leads to a generalization that suntan/sunburn risk typically extends over more hours of the day at high altitude sites. Keep this in mind next time you have a tailgate picnic at Timberline Lodge at 6000 feet (see next page for an actual mid-September reading).... 5,000 feet altitude in the mountains of Eastern Oregon in mid-August has given me readings of 391.
Another corollary of atmospheric absorption is that absorption of UVB speeds up very rapidly once the sun altitude sinks below approximately 30 degrees above the horizon.... During the summertime in the mid-latitudes, this means that UVB intensity is still relatively high at 4 p.m, one hour before people get off work, but by 5 p.m. will be diminishing very rapidly, eg. 226 at 4 p.m., down to 93 at 6 p.m. and a mere 30 by 7 p.m (readings taken July 20, 2003, lowland Willamette Valley). In terms of sun altitude, we get UVB at 226 with a sun 48 degrees high in the sky, but only 93 with the sun at 27 degrees high at 6 p.m., and 30 with the sun at 17 degrees altitude at 7 p.m. ....Therefore, a worthwhile question for the person going out for an evening in the sun after work is this one: "how worthwhile/cost-effective is it to apply lots of sun cream if your exposure won't begin until 5:30 or 6 p.m.?"
Clouds-- Myth: "You can get burned right through clouds. Always use sun protection and sun creams." Since Western Oregon is so frequently cloudy, if this was correct, you would be wearing sun tan cream in the rain much of the year. What is the real truth? The data revealed by the UVB meter, and common experience, says there is some truth to the myth, but the warning is quite oversimplified and at times quite erroneous. The best rule of thumb I can create is this one:
If the sky were clear and the time of year/time of day is such that suntan/sunburn might occur, AND the sun is obscured by clouds, THEN if the sun is still shining brightly enough through the clouds that it makes your eyes hurt, or severely squint, then about 50% of the UVB is coming through. If it is cloudy enough that you can't really tell where the sun even is, then UVB strength is likely at less than 20%. If the clouds are dense enough that it seems kinda dark and gloomy, or if the clouds are actually raining a real rain (not just a mist), then UVB risk is probably negligible (unless your skin is extraordinarily sensitive to burning). The above guidelines are admittedly just guidelines, and anyone concerned about serious skin issues should consult a medical professional.
Clouds, part two-- during Oregon's recent spate of giant forest fires, I was able to measure the effects of forest fire smoke, and found the effects on UVB to be considerable. Even on days of what appeared to be light forest fire smoke, the UVB at noon was often reduced 50 points eg. from 300 to 250--- and on days of heavy smoke when the sun's orb became dim and reddish, UVB would drop steeply, to readings of only 50-100, even at zenith sun position! My theory is that forest fire smoke causes a stronger UVB blocking effect than normal water vapor couds. I believe this is due to high concentrations of particulates.
Clouds, part three-- clouds are observed to cause what I call Omnidirectional effects, especially the overcast layers commonly known as "early morning clouds" in Northwestern Oregon. Such clouds can cause a type of UVB scattering, where UVB can be at significant levels in a direction of the sky quite far away from where the sun really is! Keep in mind that the UVB level is still only 50% or less of what it would be if the sun was shining from a clear sky. The classic scenario among mountain climbers and skiers for serious sunburn and snowblindness is related to this scattering effect--- here one combines several big danger factors: 1.late spring or early summer, giving a high sun altitude, 2. high altitude above sea level, 3. UV-reflective new snow, and adds in (4.) scattering-- the mountainside being shrouded in ground fog/blowing low clouds that are actually quite thin and transmitting up to 50% of the clear-sky UVB--- so in the equation the climber ends up receiving the equivalent of 100% or more of sea level UVB shining very omnidirectionally into the climbers' eyes, nostrils, the roofs of their mouths (as they gasp for air), and the underside of their chins--- creating severe sun burns in tender places very difficult to protect with clothing or sun creams.
Link back to my pages about Oregon's largest climbing club--- Mazamas page ....
Snow reflections: old dirty summer snow can be pretty variable, at times adding as low as 10%, but if a little cleaner can reflect about 30%. And fresh, new snow can reflect a lot of UVB, much more than water reflections. Some data is offered from three sessions: one on Mt. Rainier at 7,700 feet; the other two on Mt. Hood, the first set taken at 7,600 ft, and the most recent set at 6,700 ft.-- see next page)
Water UVB reflections: generally measure at less than 20%, but effect should not be totally ignored. Eg. 5:00 pm July 20 reading of sky UVB was at 173, but although the sun reflection off the calm water felt hot and strong, its UVB content measured only 26.
Open Sky: I have measured a general effect of being under lots of open sky versus being only in a patch of sunlight among trees. So far measurements have shown that total UVB increases some 10 to 20% if one locates oneself out in middle of a large, open expanse, eg. on top of a mountain, in the middle of a big athletic field, or out on a boat dock. These are all the places where I have recorded the very highest UVB readings.
Latitude: Don't forget that Oregon spreads over some 4 degrees of latitude, thus significantly affecting solar altitude. Therefore, remember this: "Southern Oregon Burns Better." The "Best Sunburns" to be had in Oregon would be found while Spring Skiing in late May on fresh snow at 7,000 feet on Southern Oregon's Mt. Ashland ski area (UVB estimate: approximately 525). In the graph below, what's most important to note is the high correlation between two factors: your latitude and the time of (solar) day. The latitude values are 20, 40 and 60 degrees. The most obvious observation is that at high latitudes, the intensity is rapidly declining, even at solar noon.
Your Town's Tanning/Burning Season: a comparison of Portland, Oregon, Seattle, Washington, Anchorage, Alaska, San Francisco, California, and Los Angeles in terms of their suntan/sunburn seasons....."how much of each year do you have to worry about sunburn/suntan in Oregon, and how does this compare to other locations?"
Here is a brief example of daily readings at a low altitude site in the Willamette Valley near summer solistice (Latitude = 45 degrees, 20 mins.) (the Portland area)
10:00 am--- UVB measured at 207--- already strong enough to achieve sunburn/suntan.
11:00 am--- 279--- (compare December 21, with UVB reading at same location only 40 on a clear day!)
12:30 pm--- 331--- sun approaching solar zenith at 1:12 pm.
1:12 pm--- 326--- sun at true solar zenith for the day (compare December 21, reading only 55 at solar noon).
1:12 pm--- 001--- measurement taken on dash of my car through the untinted windshield. Myth: "you can get a tan through your car's windows." Truth: windshields of cars pass vitually zero UVB.... Side windows, if untinted, reportedly can pass up to about 25% of the UVB, but I personally have never measured even an untinted side window that passed more than 3%.
1:45 pm--- 344--- one of the highest values I have observed while in the lowland Willamette Valley. (for comparison, on Sept. 2, 2011, I recorded 374 at 5000 feet in Eastern Oregon, more than two months after Summer Solistice!)
4:30 pm--- 218--- sky is crystal clear and it is 94 degrees; most people are still at work in their offices.
5:00 pm--- 168--- sun is only half as strong in UVB rays as it was around solar noon.
6:00 pm--- 101--- UVB less than one third as strong as a few hours ago. Most weary workers reach the park or lake and begin slavering on expensive sun creams to prevent sunburn! There are still nearly 3 hours of heat and sunshine left until sunset, but UVB risk is diminishing very rapidly as the sun sinks lower. Meanwhile, solar infrared radiation levels remain high, and so one's skin continues to perceive strong heat sensations.
6:30 pm--- 67--- sun UVB a scant one-fifth as strong as it was in early afternoon. Meanwhile, it is 90 degrees and there are still well over two hours of sunshine left in the day. At this hour, the sun is at the same strength/altitude that it has during the Winter solistice in late December (remember how difficult it is to get a suntan in Oregon in December?)
7:10 pm--- UVB at 25--- it is still 87 degrees, and the sun still FEELS HOT on the skin as infra-red rays continue to come through the atmosphere strongly---- but there is no danger of sunburn anymore, unless you are extraordinarily senstive, eg. an albino.
9:04 pm--- the sun finally goes down, but its UVB has long since declined to negligible levels.
The highest UVB reading I could manage to urge out of my meter in Portland (160 feet elevation) at Winter Solistic was a feeble 48 (during a series of very dry, brillant clear days)...... Even a month after Solistice time (January 23) and at 6,700 feet elevation on Mt. Hood, with lots of new snow all around, I could only get up to a rather puny 122.
* All UVB measurements on the SolarMeter are provided in uW per square centimeter. (Note: this is similar to, but not the same as, the more worldwide measure called the "UVI," the "clear-sky UV Index," measured where 1 unit equals 25 mW/square meter)
** All UVB measurements shown on this page are taken with the SolarMeter Model 6.2 UVB meter, which has a spectral response of 280 to 320 nm, with a peak sensitivity of 300 nm; most studies find maximum biological damage (eg. sunburn) to occur at between 305 and 310 nm, so this range should be close to the heart of the sunburn/suntan range for human skin. OregonPhotos.com thanks SolarMeter for their gracious support in my efforts to provide hard data to the public of Oregon about their State. A link to SolarMeter is provided below.
*** The Ozone Layer exists 6 to 30 miles high in Earth's stratosphere -- Earth's highest mountain is just under 6 miles high. The lower atmosphere also contains much smaller amounts of ozone. Scientists have estimated that the thickness of the entire Ozone layer, if compressed to sea level pressures, would be well under one inch thick! Yet its filtering ability for UVC radiation is nearly total, and is very high for both UVB and UVA. The much-publicized damage to the Ozone Layer combines with natural fluctuations in the density of the Ozone Layer ("ozone column") above any region to somewhat randomly increase or decrease the UVB intensities that will be measured by a meter such as the one used in my study. The "ozone column" is a scientific entity, and the reader should note that the lowest readings of this value under the Antarctic "Ozone Hole" are about 100, about 1/3 what would be considered normal values for mid-latitudes. This kind of major reduction in ozone definitely does increase the UV intensity; for example, Ushuaia, Argentina, latitude 54.8 degrees south, is usually far enough away from the notorious Antarctic "Ozone Hole" that it is unaffected by the Hole... However, on November 15, 2006, the Hole briefly expanded and included Ushuaia, sending UV Index readings up to a level of 10, which was fully twice the normal level. For comparison value, a UV Index of 10 would be a typical summertime reading for a city in the US's "Sunbelt," such as Phoenix or Los Angeles. Thus the reader can appreciate that the Media scares over the Ozone Hole do have some basis in fact--- in the unlikely event that ozone depletion levels in the mid-latitudes ever reached the severe levels of the seasonal Antarctic Ozone Hole environmental impacts would be major. If mid-latitude UV levels doubled for extended periods of time, there really would be biological stress on humans, animals, plants, the environment in general. Humans really would have to heed their skin doctors' advice and wear protective clothing, use good sunglasses and apply lots of SPF40 during the 6 months of higher sun each year-- it would be akin to protecting oneself from the extreme UV that already exists when one is doing high altitude mountain climbing on snow.
***** Vitamin D, the skin, and UVB--- Reader Andrew L. states that the skin's vitamin D production occurs between 290 and 305 nm, with peak production at 297 nm- these are reputedly a bit shorter wavelengths than those that produce maximum biological damage. Andrew L. stated that the best way to get ones skin to produce a good dose of Vitamin D was to expose as much skin area as possible around solar noon, or whenever the UVB was the highest that day, then cover up for the rest of the day to avoid burning or other skin damage. My personal opinion if one were to follow Andrew L.'s route would be to use caution if you are under high UVB conditions (eg. 300+ UVB) and/or if you are a person with skin that easily burns.