Why is the level of UVA radiations on the earth surface the same throughout the day, while the level of UVB significantly varies?
UVA rays are very penetrating and are not weakened by the ozone layer, so are much the same throughout the day. UVB rays are of shorter wavelength, much less penetrating, and are weakened by passing through the atmosphere. At different times of day there are thicker or thinner layers of atmosphere to pass through. Near sunset, the UVB rays will be oblique, and therefore have a greater thickness of atmosphere to pass through, so will be consequently weakened. UVC rays are of even shorter wavelength and are mostly blocked by the ozone layer.