I live in the northern hemisphere and in my country we see a difference in UVB rays across the year, peaking in summer. So why does UVA stay the same over the year when they both come from the sun?
From the Skin Cancer Foundation
‘Ultraviolet B (UVB) rays, which are mainly responsible for sunburn, are the strongest in the summer.
Ultraviolet A (UVA) rays, which lead to tanning, dark spots and wrinkles, remain constant throughout the year and can penetrate through clouds and fog.’
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You’ve answered your own question there.
UVB rays can’t penetrate Clouds, so UVB ray levels will fluctuate based on the Seasons, ie in Fall and Winter you’re more likely to get cloud cover, so your exposure to UVB is reduced and with clearer skies in Spring and Summer it’s increased.
UVA rays can penetrate Clouds so remains constant throughout.
There are actually more types of UV waves than UVA and UVB. There are also UVC and UVD. The reason our exposure varies at different times of the year relate to the length of the waves. UVA waves are long enough that our exposure continues despite our distance/angle from the sun. UVB waves are shorter. In temperate climates, for example, one might only be able to receive adequate UVB exposure about half the year. UVC and UVD are even shorter.
They are of two different wavelengths the UVA have the longest wavelength and UVB have a shorter wavelength in comparison.
UVA is about 90-95 % of the uv radiation from the sun while UVB is about 5%. That reaches the surface of the earth as UVB can be absorbed by the Ozone layer, while UVA doesn’t.
UVA have lower energy and penetrates the skin deeper than UVB.
UVB can be blocked by clouds and windows, UVA doesn’t usually.
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