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The following quotes seem to be contradictory.

  1. From skincancer.org, the amount of UVA radiations during daylight hours stays constant throughout the year:

    UVA accounts for up to 95 percent of the UV radiation reaching the earth. These rays maintain the same level of strength during daylight hours throughout the year. This means that during a lifetime, we are all exposed to a high level of UVA rays. UVA can penetrate windows and cloud cover.

  2. From https://www.washingtonpost.com, the sun angle varies over the course of the year:

    We have seasons because the sun angle varies over the course of the year, and it varies because the Earth's plane of rotation is tilted by about 23.5 degrees from the plane of its orbit around the sun.

  3. However, according to biointeractive.org (mirror), the angle of the sun impacts how much UVA one receives (which means that how much UVA one receives depends on the day of the year and the lattitude):

    How do you explain the relationship between the UV Index and latitude? (In other words, why does UV intensity change with latitude?)

    The answer has to do with the angle of Earth relative to the sun. Latitudes at the equator receive direct sunlight year-round. Latitudes toward the poles receive sunlight at an oblique angle, which means that the same amount of radiation is spread out over a larger area than at the equator.

These quotes confuse me as they seem to be contradictory to me (quote 1 contradicts quotes 2+3). Does the amount of UVA radiations one receives depend on the day ot the year? I don't mean the total amount accumulated over the day, but instead sometime during daylight.

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  • $\begingroup$ Summer days are longer and winter days are shorter. $\endgroup$
    – Spencer
    Sep 7, 2021 at 1:54
  • $\begingroup$ @Spencer sure, I didn't mean the total amount accumulated over the day, but instead sometime during daylight, like the quote "These rays maintain the same level of strength during daylight hours throughout the year". $\endgroup$
    – Franck Dernoncourt
    Sep 7, 2021 at 1:56

2 Answers 2

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UVA passes through the atmosphere without losing much intensity, so per quote one it's intensity doesn't change, nor does it's magnitude as a percentage of total insolation vary, much, during the year (and what variance there is happens at source). Total ground level insolation intensity in $\mathrm{Wm^{-2}}$ does change with latitude and seasonal angle of incidence though. As such while you're getting the same proportion of UVA in your sunlight year round you're getting less total solar radiation during the winter, so you get less UVA per hour on a winter day than in the summer, all else being equal.

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  • $\begingroup$ Thanks, why did you write "you're getting the same proportion of UVA in your sunlight year round" since while the intensity of each UV type will vary throughout the year based on their wavelength? (Eg UV-B doesn't penetrate the atmosphere as well as UVA) $\endgroup$
    – Franck Dernoncourt
    Sep 7, 2021 at 2:17
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    $\begingroup$ @FranckDernoncourt They don't though, the local atmospheric conditions have an effect on what UV wavelengths are absorbs or scattered before they reach the ground but if you take two days of equal cloud cover and absolute humidity you'll have the same ground level UV readings. Therefore "all else being equal" you get the same UVA dose per unit of insolation. $\endgroup$
    – Ash
    Sep 7, 2021 at 2:27
  • $\begingroup$ Got it, thanks! In other words, quotes 2 and 3 are correct, and quote 1 would be correct if by "[UVA] rays maintain the same level of strength during daylight hours throughout the year. " they meant the same as your previous comment. $\endgroup$
    – Franck Dernoncourt
    Sep 7, 2021 at 2:32
  • $\begingroup$ The "total solar radiation" per day is only relevant if you spend from dawn until dusk outdoors with bare skin facing the sun (not in the shade), right? So it would be accurate to say the UVA dose on your face per hour of being outside is near constant? Assuming the different sun angle and presumably different outdoor activity and headgear aren't confounding factors. (Again assuming equal cloud cover, or with lower sun angle, equal optical depth of cloud or something.) $\endgroup$
    – Peter Cordes
    Sep 7, 2021 at 7:23
  • $\begingroup$ @PeterCordes The units are Watts per square metre, one Watt is one Joule per second, so the total solar radiation is measured in Joules per second per square metre that is what changes as you change the angle of incidence, the amount of energy being intercepted by a given piece of ground every second, and thus your per hour dose of solar radiation changes with the seasons. $\endgroup$
    – Ash
    Sep 7, 2021 at 22:14
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Reddit user and r/EarthScience mod Halcyon3k pointed me to the following visualization that nicely illustrates Ash's great answer on the fact that "total ground level insolation intensity in $\mathrm{Wm^{-2}}$ does change with latitude and seasonal angle of incidence":

enter image description here

(image source)

Halcyon3k's explanation:

Solar energy is measured in watts per square meter but it will vary with the angle with respect to the sun. This is best understood with [the image above]. It’s talking about latitude but this is the same thing that’s going on near sunrise vs noon vs near sunset. This is also a good interactive illustration of what’s going on: https://engaging-data.com/solar-intensity/

Also, from this 2001 study {1} that looked at the UVA irradiance data for a Southern Hemisphere, subtropical site (Toowoomba, Australia, 27.6°S, 151.9°E):

enter image description here

References:

  • {1} Sabburg, J. and Parisi, Alfio and Wong, J. C. F. (2001). Effect of cloud on UVA and exposure to humans. Photochemistry and Photobiology, 74 (3), 412-416. ISSN 0031-8655. [GScholar] [PDF]
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    $\begingroup$ But note that as a human, you're usually not lying on the ground. Parts of your skin may be directly "facing" the sun. The sun angle may change which parts of you are getting the most sun per area (e.g. the side of your face or neck, or nose, instead of your arms, if sitting with bare arms on a table for example, if the sun is at 90 degrees overhead vs. at 45 degrees). Of course shallow sun angle is correlated with cold weather so you'd probably be wearing more clothes, too. $\endgroup$
    – Peter Cordes
    Sep 7, 2021 at 22:28
  • $\begingroup$ @PeterCordes Thanks good point. Personally I'm fully covered outdoors.stackexchange.com/q/27268/2667 :) $\endgroup$
    – Franck Dernoncourt
    Sep 7, 2021 at 22:32

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