UV rays become stronger on cloudy days.

Broken-Cloud Effect >

Ultraviolet wavelengths can pass through clouds and

rather On days when clouds cover about half the sun, ultraviolet rays are reflected by the clouds Increases the UV index.

According to a study conducted in six U.S. cities in 1994, cumulus clouds increase surface UVB by 25%, and

According to a 2004 Australian study, UVB rays were found to be up to 40% stronger in partially cloudy skies.

source: https://www.hidoc.co.kr/healthstory/news/C0000510120

On cloudy days, since there is less visible direct sunlight, people tend to neglect UV protection.

I need to work harder than usual.