Is it true even in October, the Hawaiian sun can still burn? (Honolulu: 2013, dangerous)
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I created a program that took a lot of work to make accurate. It tells UV Index and also by skin type, how badly you will burn in 30 minutes, an hour, 2 hours, 3 hours, 4 hours, etc
I factored in Ozone depletion and snow and water reflections and sand and wood and weather cloudcover and altitude
STILL, when I test my program on Honolulu, and compare to other sources, I get that my program underestimates the UV Index for Hawaii. External sources show that Hawaiian sun is more dangerous even in October, than the sun in Boston ever gets.
And this in spite of that the Hawaiian sun angles in October are not higher than Northeastern US
For example, typical July UV index for Northeastern US is around 7 or 8 and sometimes 9
Hawaii July UV index is more than 11
And in October it STILL lingers around 9 most of the month and even some 10s
What is adding to Hawaii's UV that I'm not capturing ? Is it the reflection off the volcanic mountains and the unblocked reflections from the Pacific?
Hawaiian sun is nothing to mess with if your skin is light. But I wonder if the palms and tropical plants also play into UV. I already accounted for the astronomical data. It's beyond the latitude of Hawaii...I already factored that in.
Sun rays in "October" in Hawaii are not as direct as "July" in Massachusetts.
However, Hawaii October sun will burn you quicker than Massachusetts July sun. See where I'm getting?
I've always thought the strength of the UV rays depend on a couple of main things: 1) how far away from the equator (ex. latitude) and 2) altitude, since the higher a place is, the thinner the atmosphere, therefore less blocking of UV rays and higher UV index. I doubt foliage plays a part in the UV index. Hawaii probably has the strongest UV index in the U.S. (except maybe high altitude locations) due to it being much closer to the equator than any other state. Sun rays in October in Hawaii probably are more direct than in Massachusetts in July due to Hawaii being so much closer to the equator.
I've always thought the strength of the UV rays depend on a couple of main things: 1) how far away from the equator (ex. latitude) and 2) altitude, since the higher a place is, the thinner the atmosphere, therefore less blocking of UV rays and higher UV index. I doubt foliage plays a part in the UV index. Hawaii probably has the strongest UV index in the U.S. (except maybe high altitude locations) due to it being much closer to the equator than any other state. Sun rays in October in Hawaii probably are more direct than in Massachusetts in July due to Hawaii being so much closer to the equator.
Yes Hawaii is closer to the equator, but in Mid to late October in Hawaii, the sun's direct rays are on the Southern Hemisphere side from the equator and the sun's direct rays are around 10-11 degrees South of the equator at that time. That's 30 latitude degrees farther south away from where they are in July. In contrast, Honolulu is only 21latitude degrees south from Boston. Hence, the Boston directness of sun in July is NOT more than Hawaii's in October. (obviously there's no mistake Hawaii on any same day as Boston will of course have more direct sun than Boston and I'm not arguing that.)
Honolulu is 21.3 degree north of equator
So by October 20th-ish, Hawaii is some 31 latitude degrees removed from the direct rays of the sun and the highest solar elevation angle is 59 degrees above horizon
Boston July has solar elevation angle of 68 degrees because the sun direct rays are only about 21 degrees removed from Boston
Hawaii October 20th gets a UV index that can reach 9
Most July days for Boston sees UV index level around 8
So factoring in that Hawaii is closer to the equator (in other words, angle of the sun) is not the only driver of the UV index. Something else is pushing up Hawaii's UV index. I am guessing it's the amount of water reflecting UV rays and enhancing the circulating UV. Or something to do with lack of buildings or open unblocked reflecting solar off the ocean in all directions
Or maybe warm temperature does play into UV magnification.
I did an entire math exercise for you. While ozone and clouds factor into UV, angle of the sun compared to Boston is the biggest factor.
The angle of the sun on the first day of summer in Boston is about the same as Honolulu on the first day of fall.
But I'm not looking at the first day of fall
My honeymoon with wife to be we will be arriving in Hawaii on October 20th
That's 30 days into fall
And I'm seeing on October 20th!
UV index in 2013 was a 9.
Season shift is particularly large between first day of Fall and third week October. End of October Hawaii is a less direct sun level than all of the continental USA in July, but the northern US in July doesn't see UV indexes that reach 9
I would think though that the Hawaii sun in October won't be too strong outside the hours of 10 and 2, especially in October when days are less than 12 hours long
For sake of an easier math discussion, lets take late September, when the sun is above the equator or 0 degrees. (Angles below are approximately noon)
The angle of the sun from vertical is 90 degrees.
Thus - for first day of fall
Honolulu = 21.3 degrees - 0 degrees (sun above equator) = 21.3 degrees or 68.7 degree sun angle. (90 is directly overhead)
Boston = 42.35 - 0 degrees = 42.35 degrees or 47.65 sun angle, far lower than Honolulu.
In summer, (approximate, as the sun actually goes a little north of Hawaii, but for discussion purpose)
Honolulu = 21.3 degrees - 21.3 degrees (Sun about above Tropic of Cancer) = 0 degrees, or 90 degree sun angle (directly overhead)
Boston = 42.35 - 21.3 = 21.05 or 68.95 degree sun angle, or approximately the same Honolulu has the first day of fall in late September.
Heck, lets take first day of winter....
Honolulu = 21.3 - (-21.3) (Sun about over Tropic of Capricorn), = 42.6 or 47.4 degree sun angle.
Boston = 42.35 - (-21.3) = 63.65, or a very low in the sky 26.35 degree sun angle.
Although it should be noted, UV index is far more than angle of the sun. Altitude, cloud cover, ozone are also factors.
Altitude for Honolulu is sea level so not a factor. Though other parts of interior Oahu altitude would become a factor (maybe more for Maui). Ocean reflection is probably the most likely factor. Hawaii is not likely an ozone thin region of the world because the ozone is thicker in the tropics and thinner in the arctic and Antarctic, with with exception of parts of the Southern Hemisphere such as Australia
My honeymoon with wife to be we will be arriving in Hawaii on October 20th
That's 30 days into fall
And I'm seeing on October 20th!
UV index in 2013 was a 9.
Season shift is particularly large between first day of Fall and third week October. End of October Hawaii is a less direct sun level than all of the continental USA in July, but the northern US in July doesn't see UV indexes that reach 9
I would think though that the Hawaii sun in October won't be too strong outside the hours of 10 and 2, especially in October when days are less than 12 hours long
No. No. No. And No.
You need to go back to the math I originally did for you.
The first day of fall in Hawaii has a higher sun angle than the first day of summer in Boston.
Furthermore, Hawaii has a higher sun angle than the entire continent US THE ENTIRE YEAR.
You will burn in January in Hawaii. You will really burn in October.
So on October 20, 2013 Honolulu Hawaii the solar noon was 12:16pm, sun peaked at 58.2 degrees above horizon
Sun direct rays over 10.6 degrees south of the equator...added to the 21.3 degrees Honolulu is north of the equator
That's 32 degrees to subtract from 90 and there's the 58 degrees above horizon
And the UV Index was 9 that day
October 20, 2012, UV index was also 9
October 20, 2011 UV Index was also 9
Source:Moderator cut: link removed, linking to competitor sites is not allowed
In contrast on a summer July day when sun is directly on around 21 degrees north of equator and Boston is 42 degrees North, solar noon around 12:50pm that's the 69 degrees Above horizon, more than Hawaii 10/20/2013's 58 degree solar height...
And UV index is not above Hawaii's. It's between 8 and 9
Add to this that if it is even too close to call, the summer Boston daytime lasts 15+ Hours, while the Honolulu October 20th day length is under 12 hours
You need to go back to the math I originally did for you.
The first day of fall in Hawaii has a higher sun angle than the first day of summer in Boston.
Furthermore, Hawaii has a higher sun angle than the entire continent US THE ENTIRE YEAR.
You will burn in January in Hawaii. You will really burn in October.
No in January Hawaii's sun angle at noon is not high compared to summer even for places in Canada.
In fact , the solar elevation angle on the Arctic Circle marker on the summer solstice, if you do the math, is higher than the solar elevation angle at the Tropic of Cancer on the Winter Solstice. Not to add if that isn't enough, the former is 24 hours of daylight and the latter is less than 12 hours of daylight.
Arctic circle solar elevation angle peak summer solstice is 47 degrees above horizon 90 - (66.5-23.5)
Tropic of Cancer solar elevation angle peak winter solstice is 43 degrees above horizon 90 - (23.5 + 23.5)
So I think there's other factors here
Ocean reflection, fewer overcast days, the terrain of Hawaii
Last edited by EricS39; 07-21-2014 at 11:20 PM..
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