It may be surprising to learn that cloud cover does not significantly reduce UV exposure.
UV risk can be high even on hazy or overcast days. This is because UV is invisible radiation, not visible light, and can and does penetrate clouds.
Both UVA (long-wavelength) and UVB (short-wavelength) rays can pass through clouds and cause eye damage even on overcast days.
So, sunglasses that block 100 percent UV are as important for eye protection on partly cloudy and overcast days as they are on days of full sun.
Because UV rays are invisible, they are not affected by the tint color or density of sunglass lenses. It's the lens material or other additives to the lenses not the tint that determines the level of UV protection sunglasses provide.
UV Map Index: See today's local UV levels.