Since so many frozen, sun-starved folks in modern times use air travel to take mid-winter trips to warm, sunny places, it seems important to focus on the subject of sunburn risks in the tropics.
Remember on the previous page how I stated the cardinal rule of UVB?
The tropics burn, and they burn bad, and they burn year-round, and they burn for relatively more hours per day than places in the high latitudes like the USA's northern States or Northern Europe.
"Altitude is All, and there is a twist, too"
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. In the tropics, even in mid-winter, the sun rises to high altitudes year-round. As an example, I chose Quito, Equador, which is essentially right on the equator (zero degrees latitude)..... Even at winter solistice (Dec. 21) the sun rises at noon to 67 degrees high in the sky, which is higher than it gets even on the summer solistice in places like Seattle, Washington or London, England. Thus, the UVB is as strong on Dec. 21 on the equator as it is on June 21st. at such northern cities!
2. Worse yet, due to the celestial mechanics of the Earth/Sun in space, the way the sun transits the sky in the tropics leads to addtional UVB danger. In Quito, on Dec. 21, the sun rises very rapidly in the morning to high enough altitude to be dangerous, reaching 30 degrees high by 8:30 am, and staying higher than that all day, all the way until about 3:45 p.m.
3. 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 my example of Quito, height above sea level
is an additional risk factor, as Quito lies nearly 10,000 feet
high in the Andes Mountains! Thus, even in midj-winter, the UVB
strength in Quito is similar to the beaches of Florida in mid-summer!
I estimate my Solarmeter in Quito on Dec. 21 would read over 400,
a very high intensity indicating great sunburn risk.
Additionally, the thin atmosphere of high altitudes leads to relatively stronger UVB even when the sun is at lower heights in the sky, thus compounding the influence of #2 above. In Quito, this means that the amount of time per day when the UVB is very strong is sensational-- the rule of thumb for Quito even in mid-winter would be: "High sunburn risk from about 9am to 3pm any day the sun is shining."
In Oregon (latitude 45) how do 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, Oregon 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.... the two things to understand about the tropics are that UVB there varies considerably less by season than in the high latitudes, and that UVB in the tropics is ALWAYS high, no matter what month it is.
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. THE TROPICS IN MID-WINTER--- the resort paradise of the Seychelles Islands (south of India and several degrees south of the equator) turned in a Dec. 21 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. THUS, I somewhat arbitrarily say that anytime the sun is above about 30 degrees in the sky, there is significant sunburn risk.
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).
Concerns?
