Why do areas of high altitude (e. g., on the Tibetan plateau) have greater than expected UV intensity and areas of constant cloud cover (e. g., Congo Basin) have less than expected?

Explanation:

UV (ultraviolet) levels are high in the areas of high altitudes. Tibetan plateau being the highest plateau of the world is more exposed to the UV radiations as the atmosphere gets thinner in the areas of higher altitude. So it can not filter the maximum of ultraviolet radiations. Tibetan plateau is on 4,500 meters height and the UV level increase by 10-12% upon every 1000 meter elevation.

          However in those areas having the constant cloud cover like in Congo Basin the UV level is low as the clouds scatter the most of the UV radiations and they particularly stop the UV-B radiations. Thats why the cloudy areas have less UV level then the areas having a cloudless clear sky.