Your light box doesn’t care about the weather.

Overcast skies and snow cover have zero effect on a properly used light therapy lamp — the lamp itself is the light source, not the sky.

The confusion comes from thinking about “effective illuminance” like outdoor light. But a 10,000 lux lamp at the right distance (usually 16–24 inches) puts out the same amount of light regardless of whether it’s cloudy or sunny outside. Snow cover actually brightens the outdoors (reflects up to 80% of UV), but again, that doesn’t touch your lamp.

Where this can trip people up: if you’re relying on a window for light therapy or positioning the lamp near a window expecting outdoor boost, then yes, overcast will reduce