An Odd Description of Skin Cancer Cause from UV Radiation

[Buzz Bloom]

TL;DR Summary

Quotes from a recent Scientific American will be in the main text. The primary quote is about New Zealand having a 40% stronger UV radiation exposure than places in the northern hemisphere. My effort at understanding the math produces a limit of only a 6.9% stronger UV exposure.

Scientific American
June 2, 2022
Vol 32 Number 6
Page 62
Title: Skin Cancer around the World
Two Quotes:
“The main cause of skin cancer is the exposure to the sun’s ultraviolet rays…”
“UV radiation is about 40% stronger in New Zealand than it is at corresponding latitudes in the Northern Hemisphere. Because of Earth’s tilt, the Southern Hemisphere is closer to the sun than the north is during its own summer. “

I am unable to find any basis for this difference other than that the distance of the sun to the Earth is greater when the Earth is at aphelion (152.1 x 10^6 km) than it is at perihelion (147.1 x 10^6 km).

The amount of sun UV radiation hitting the Earth is related to the Earth’s distance from the sun. If there are two distances being compared, say D1 > D2, then the ratio R of these two radiation densities is (roughly) approximately (D1/D2)^2, which is greater than 1. The larger these distances, then the more precise will be this approximate ratio.

Letting D1 be the aphelion distance and D2 the perihelion distance gives the result:
R = (152.1 / 147.1)^2 = 1.069.
This means that the Earth at perihelion has a radiation density 6.9% greater than the radiation density at aphelion. Seeing this 6.9% result I have not been able to understand how it is possible that the quote “UV radiation is about 40% stronger…” can possibly be correct.

I am hopeful that someone here at the Physics Forums will be able to educate me regarding my apparently incorrect understanding.

 

[Drakkith]

The increased amount of UV is due to a combination of several factors. Below is an incomplete list:

• Reduced ozone concentration in the upper atmosphere over New Zealand vs comparable latitudes in the north.
• Decreased distance between the Sun and the Earth during New Zealand summers.
• Possibly reduced UV absorbing aerosols compared to northern latitudes.
• Increased UV optical depth (less absorption of UV) at lower altitudes.

Quote from the 2nd of my references below:
Peak UV intensities in the New Zealand site (45 degrees S, alt. 0.37 km) exceed those at comparable latitudes and altitudes in North America by 41 +/- 5%, and are more comparable with those over 1 km higher and 5 degrees closer to the equator.

From the third reference:
UV in New Zealand is relatively high compared with corresponding northern latitudes, due to its lower summer ozone amounts, closer Earth-Sun separation in summer, and unpolluted air.

Refs:
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2008GL037141
https://pubmed.ncbi.nlm.nih.gov/16520871/
https://aip.scitation.org/doi/pdf/10.1063/1.4975499

 

[Laroxe]

The increased amount of UV is due to a combination of several factors. Below is an incomplete list:

• Reduced ozone concentration in the upper atmosphere over New Zealand vs comparable latitudes in the north.
• Decreased distance between the Sun and the Earth during New Zealand summers.
• Possibly reduced UV absorbing aerosols compared to northern latitudes.
• Increased UV optical depth (less absorption of UV) at lower altitudes.

Quote from the 2nd of my references below:
Peak UV intensities in the New Zealand site (45 degrees S, alt. 0.37 km) exceed those at comparable latitudes and altitudes in North America by 41 +/- 5%, and are more comparable with those over 1 km higher and 5 degrees closer to the equator.

From the third reference:
UV in New Zealand is relatively high compared with corresponding northern latitudes, due to its lower summer ozone amounts, closer Earth-Sun separation in summer, and unpolluted air.

Refs:
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2008GL037141
https://pubmed.ncbi.nlm.nih.gov/16520871/
https://aip.scitation.org/doi/pdf/10.1063/1.4975499

I would suggest peoples behaviour and lifestyle may be more important than all of these. In Australia there was a very visible campaign to use sun screen, wear hats and avoid over exposure, this might be why the rate is now higher in New Zealand.

 

[Drakkith]

I would suggest peoples behaviour and lifestyle may be more important than all of these. In Australia there was a very visible campaign to use sun screen, wear hats and avoid over exposure, this might be why the rate is now higher in New Zealand.

While perhaps true, this doesn't explain the increase in UV radiation itself, merely another reason why rates of UV related skin diseases are higher.

 

[Laroxe]

It is a bit strange but the differences may be less significant than we think. There have been a number of explanations, the favourites appear to be low levels of air pollution and air coming off Antarctica during the summer months which is low in ozone. I thought this site was interesting, even though it doesn't seem very impressed with the ozone idea, but this does seem to be mentioned in most other sources.
https://www.ehinz.ac.nz/indicators/uv-exposure/daily-uv-levels/