Sun Protection

UVA rays

If you're sitting in the shade, you'll still get some UVA, but a reduced amount. You know how you can still see things in the daytime when you're not in direct sunlight? That's due to scattering, which is light bouncing off surfaces and reaching you indirectly. UVA works pretty much the same way (though it's scattered a bit more). It's hard to say exactly how much UVA will reach you without using a meter, but you can roughly estimate it based on how bright it is. (source)

Windows

tl;dr: UV-B doesn't go through windows, but UV-A does, pretty much all of it.

Standard window glass, according to the International Ultraviolet Association, will allow UV-A to pass through while almost 100% of the UV-B and UV-C light is blocked. Therefore, some UV light will enter your home and potentially affect your skin. Some of these effects could include increased freckles and increased sensitivity to sunlight that could result in rashes such as photodermatitis. (source)
The Skin Cancer Foundation states that windshield glass is specifically treated to remove the UV-A; however, the side windows and rear window will allow the UV-A to penetrate the interior. Therefore, care should be taken in vehicles, as well. Window tints can be applied to automobiles, but be aware that states have regulations on tinting. (source)
Typical window glass such as that found in homes will block most of the UV light from entering your house with the exception of UV-A light. (source)
"Ordinary window glass passes about 90% of the light above 350 nm, but blocks over 90% of the light below 300 nm." UVAI ranges from 340 to 400nm, visible light ranges from 390 to 700nm (source)
Obviously, every bit of UVA that reaches you can damage your skin and it's up to you to make that judgement call, but in my opinion indirect indoor levels are low enough that I'm not worried. The nice side of the truck driver's face was getting tons of indirect sunlight after all... (source)
By law, front windshields are treated to filter out most UVA, but side and rear windows generally aren’t. (source)

Clouds

tl;dr: No clear answer on how much UVA makes it through clouds. Seems like most, or close to all, do.

Clouds filter out some UV rays from reaching the ground. However, it depends on the type of cloud. Dark, water-filled clouds may block out more UV rays than high, thin clouds. (source)
Clouds can block up to 70-90% of these UV-B rays during times of extreme overcast. (source)
A term coined as the ‘broken cloud effect’ reveals that certain clouds can actually create higher UV levels than a perfectly cloudless day. When compared with completely clear skies, studies have shown that partially cloudy skies have raised the UV-B rays by 25% and increased DNA damage up to 40%! (source)
- They have few ideas surrounding this phenomenon. The first explanation is the fact that dense clouds may reflect UV rays, which in turn send them back down to the earth’s surface. (source)
- The second theory is the idea of UV rays being redirected as they pass through thin wisp-like clouds. The combination of refraction and reflecting between varying clouds is resulting in a significantly greater UV strength at ground level. (source)
Up to 80% of UVA rays can penetrate through clouds (NB: can't seem to find this anywhere else, and they don't exactly cite a source) (source)

Latitude

Annual doses of UVA radiation decrease much less with increasing latitude than annual doses of UVB. (source)
- UVA is actually higher at 60° N in peak summer than at equater / 0°
- Ozone depletes UVB bands more than UVA bands
- Ozone depletion is higher at higher latitudinal values, because it comes in at a higher angle, so it has to go through more ozone

Winter

https://ultrasuninternational.com/wp-content/uploads/grigalavicius-et-al-2015_daily_seasonal_and_latitudinal_variations_in_solar_ultraviolet.pdf
- Lots of graphics, showing the effects of time of year on UVA and UVB radiation throughout the year
- One graphic shows how at 60°N latitude, UVA and UVB rays are much lower
  - UVB rays basically gone, but UVA still there but much lower

Shade / Diffuse

tl;dr: Depending on the environment, somewhere between 5%-50% UV in diffused environments make their way to the final target. UVA diffused more, so weaker by the end, than UVB. Roughly 15% in urban environment, 5-10% in tree shade in "outdoor" environment

The sensors located in tree shade indicated that over 5% of UV radiation was detected in the shade. The sensors located in sun-umbrella shade showed that greater than 17% of UV radiation reached the shade. The sun sensors used in our study collected UV radiation data relevant to UV index; however, they did not differentiate between UVA, UVB, visible, and infrared light. The amount of UV radiation detected in the shade is not zero, thus regular sunscreen use and other sun protective practices should be followed to reduce the risk of sun damage and skin cancer. (source)
Umbrellas are a great option for UV protection, but there are some limitations. UV rays are small and some UV may penetrate between fabric fibers. (source)
No matter how high the fabric’s SPF or UPF is, the amount of UV present under the umbrella can be up to 84 percent of that in the sun depending on the levels of indirect UV that can come in from around the shade. (source)
For a typical umbrella, the true SPF value is approximately between 10 -15 when used in midday sunlight, when the sun is the most intense.[4] Therefore, umbrellas are effective at blocking midday sunlight. However, they are not as effective in blocking UV earlier in the morning or late in the day when the UV rays are more diffuse in the atmosphere. (source)
The percentage diffuse UV was higher for the shorter 320-400 nm range (UVB) than for 280-320 nm (UVA). The percentage diffuse UVB ranged from 23 to 59%, whereas the percentage diffuse UVA ranged from 17 to 31%. (study in Australia under trees) (source)
The percentage diffuse UV was lower at noon than in the morning and afternoon with the difference more pronounced for the UVB. The average percentage diffuse UVB over all the measurements in the tree shade for the morning, noon and afternoon was 62, 58 and 71%, respectively, and the average percentage diffuse UVA was 52, 51 and 59%, respectively. (source)
Up to 50% of UV can be diffused
Environment matters (e.g. nearby pool, trees, buildings)
- Water scatters UV rays different than concrete, sand, etc.
Urban studies see around 10-18% UV rays making it to street level (depending on the city -- some studies are based on 20 ft. story high buildings and a certain ratio of width of street to height of buildings) (source)
https://escholarship.org/content/qt4wc0f6tw/qt4wc0f6tw.pdf?t=pvm1tk

Notes

There are two types of UV rays: UVA rays and UVB rays," Dr. Nichols says. "The shorter UVB rays are what causes sunburns, and they dwindle in the colder months as they are less able to cut through clouds and reach the Earth’s surface. However, the longer UVA rays persist all year, and actually penetrate deeper into the skin to cause the dark spots and wrinkles associated with sun damage. By continuing to wear your SPF all year round, you’ll protect your skin and more effectively prevent common signs of aging. (source)
The effectiveness of a base tan has been studied before, and it only offers SPF of 3 or less. That is an incredibly low level of protection. Therefore, dermatologists recommend using sunscreen as sun protection and not base tanning. (source)
You should apply 1 shot glass full (about 1 to 1.5 ounces) of sunscreen to cover your entire body. This works out to about a teaspoon for your face and 1 teaspoon for each arm/hand. (source)
- An ounce of sunscreen is about as much as you can hold in your palm
- For the face, two fingers full. Meaning a strip of cream down your index and middle finger
Creams, lotions and sprays are equally as effective in theory. However, in practice it’s a different story. In the real world, people tend to apply spray sunscreen as if its “spray and walk away” cologne. As a result, people get less protection. As a rule, you should spray five seconds’ worth of sunscreen on each general body part, then rub it in so that you get adequate protection. (source)
Remember to put sunscreen on your ears! Skin cancer on the ears can be more difficult to treat and require surgery that, depending on the location, can leave the ears looking like they have been nibbled on. The lips are also susceptible to UV damage, so always look for lip balm containing SPF 30. (source)
most people do not apply a thick enough layer, so their SPF 30 may be more like an SPF 20. I usually recommend patients use an SPF 50+ so that they are covered if they do not apply their sunscreen very well. (source)
Get "broad spectrum" suncreens. These protect against both UVA and UVB rays
UVB rays are responsible for producing sunburn. The UVB rays also play the greatest role in causing skin cancers, including the deadly black mole form of skin cancer (malignant melanoma). (source)
UVA rays also play a role in skin cancer formation. In addition, the UVA rays penetrate more deeply into the skin and play a greater role in premature skin aging changes including wrinkle formation (photoaging). There are approximately 500 times more UVA rays in sunlight than UVB rays. (source)
Reapply every two hours, or immediately after being in the water
Skin cancer is the most common cancer in the United States and worldwide. 1 in 5 Americans will develop skin cancer by the age of 70. (source)
When detected early, the 5-year survival rate for melanoma is 99 percent. (source) (Melanoma is the most common and fatal form of skin cancer)
Apply extra SPF to freckles or dark spots — they're signs of sun damage. (source)
While being bronzed does give you some level of protection, it’s only the equivalent of SPF 4 at best (source)
There is no such thing as a safe tan. The increase in skin pigment, called melanin, which causes the tan color change in your skin is a sign of damage. (source)
Sunscreen degredation, when to reapply
- Reapplication time is highly variable, and depends on a lot of things
- 2 things that can degrade sunscreen: UV radiation and physical
  - UV: Spending time in sun
  - Physical: Touching face with hands, dirt, wind, hair, clothes, tissue, pillow
- Some sunscreens are more organic, have better "half lives", so they last longer
  - If half life is poor, it degrades very quickly, and mostly from UV radiation
In short: Yes, you should wear sunscreen every day. If you don't do so, says Manno, "You're going to accumulate damage in the skin, which can lead to developing cancerous skin lesions later in life." (source)
Sun damage can lead to wrinkles, uneven skin tone, loss of firmness, and aging signs (source)
Elbuluk noted that leaving residual makeup, dirt and/or oil behind makes it hard for products like sunscreen to be effective. “Those products can’t penetrate as well if your skin isn’t clean because you have a coat of something on it,” she said. “You decrease the efficacy of those things.” (source)
The Ultra Violet Radiation Index (UVI) has been around for more than 20 years and gives a measure of the intensity of the sun, using both UVA and UVB. (source)
Both UVA and UVB rays can cause sunburn and tanning, which damage the DNA in your skin cells and increase your risk for skin cancer. (source)
UVA rays are the main type of light used in tanning beds. (source)
UVA rays cause an immediate tanning effect, and sometimes a sunburn. (source)
UV exposure is highest in areas on or near the equator, where UV rays have less distance to travel before reaching the ground. (source)
UV rays are more powerful at higher elevations because they have less distance to travel. (source)
The ozone layer provides protection from UV rays. But greenhouse gases and pollutants have caused the ozone layer to thin, increasing UV intensity. (source)
UV rays reflect off surfaces such as snow, water, sand, and pavement. This can increase UV exposure. (source)
Also, being near water can reflect enough to burn the fuck out of you. Was nearby a pool, under cover, burnt as hell. (source)
It is possible to get sunburned through your clothing. (Got a second-degree burn on my chest once that way.) It depends on the type of clothing, of course, but a plain white cotton t-shirt is about the equivalent of SPF-8 sunscreen. (source)
Definitely wear sunscreen when snow is out -- super reflective
IIRC, the UV index measures UBV, not UVA. (source)
Yes, a broad spectrum is used, but the UV index we see is heavily dictated by UVB because one of its main components is based on erythema (or sunburn), which is specially caused by UVB rays. (source)
Ultraviolet B (UVB) rays (the ones your body needs to make vitamin D) (source)
The UVB:UVA ratio is maximal at noon (source)
Annual doses of UVA radiation decrease much less with increasing latitude than