The sun is the main source of electromagnetic radiation on Earth, and only a small part of this radiation is visible to the human eye - the visible spectrum, which includes wavelengths between 380nm and 780nm.
Other types of radiation (UV - IR) can cause significant damage to ocular tissues (eyelids, conjunctiva, cornea, lens and sometimes even the retina).
As most UV radiation can penetrate a layer of cloud, similar precautions should be taken in cloudy weather.
In addition, UV radiation increases with altitude (about +12% per 1000 metres), so it is even more important to protect yourself from UV radiation in the mountains.