how 2 equally sunny days can have difference UV ratings?

Question

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

Answer 1

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.