Water and sand both reflect a meaningful amount of UV radiation back upward, which is part of why beach sunburn — including in shaded spots and under umbrellas — is so common.
Reflection percentages
Sand reflects roughly 15-25% of UV radiation, water around 10-25% depending on angle and surface conditions — both add real additional exposure on top of direct overhead sun, striking skin from below and the sides, not just above.
Why shade doesn't fully protect at the beach
An umbrella blocks direct overhead UV, but reflected UV bouncing off sand and water still reaches skin from other angles — which is why sunburn under an umbrella at the beach, while less severe than direct exposure, remains genuinely possible.
Snow reflects even more
For comparison, fresh snow reflects up to 80% of UV — far more than sand or water — which is why snow environments carry an even more pronounced reflection risk.
Does sitting in the shade at the beach eliminate sunburn risk?
It significantly reduces it but doesn't eliminate it, due to reflected UV from sand and water — SPF is still worth applying even when spending most of the time in beach shade.