2 days in the same week so the sun would be the same distance from Earth, no clouds anywhere, how can one day have a a different UV danger rating to the other?
In: Planetary Science
Just because the sun and clouds are the same from day to day does not mean the overall air quality is the same from day to day. There might be higher or lower humidity, ozone in the upper atmosphere, or particulate matter (local pollution) in the air. There are a bunch of factors that affect UV danger other than just sun and clouds
Sunniness doesn’t really correspond to uv index exactly. The UV index is based on how close the earth is so the sun and how clear the sky is. however, sunniness is just based on how clear the sky is.
There’s lots of other things in the air other than just clouds; e.g. plumes of industrial or volcanic gases, particulates like ash, soot, dust, pollen, etc. Some of these absorb or reflect light in the UV wavelengths. Changes in humidity, temperature and so on can also subtly change the way that different wavelengths of light travel through the atmosphere. As different air currents and stratified layers change through time the intensity with which different wavelengths reach the ground changes.
Moisture in the air will play a factor as UV radiation, just like the visible light, are being filtered through them.
While I can’t speak specifically for your situation, there’s an important thing to keep in mind here.
UV radiation is a type of light that is invisible to our eyes. So, while the way a day looks tells us the conditions for visible light, there’s no easy way for our eyes to tell the conditions for UV light. Materials behave differently to different wavelengths of light, which is likely what leads to this discrepancy. (You can see an easy visible light example of this in the blue sky; the sky looks blue because one wavelength is scattered more than others).
The most likely thing that happened was that, on one of those days, there was something in the atmosphere that affects only UV light, like a thin dust or a denser ozone layer
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