Reflected UVA: snow, sand, water and concrete

Almost everyone thinks of ultraviolet as arriving from above. A substantial share of it arrives from below — bounced off whatever you are standing on. That reflected component is why the UVA Index calculator asks you to pick a ground surface, and why it reaches skin that direct sunlight physically cannot: the underside of the chin, the nostrils, the eyes beneath a hat brim.

UV albedo by surface

Albedo is the fraction of incoming radiation a surface reflects. The critical thing to understand is that UV albedo is not visible albedo — a surface's appearance to your eye is a poor guide to what it does with ultraviolet.

Where visible brightness misleads you

• Grass looks bright green and reflects almost no UV. Chlorophyll and leaf tissue absorb ultraviolet strongly. A lawn is one of the lowest-UV-albedo surfaces there is.
• White paint is often a poor UV reflector. Most white paints get their whiteness from titanium dioxide — which is a potent UV absorber, and is used as a physical sunscreen filter for exactly that reason. A whitewashed wall can be dazzling in the visible and unremarkable in the UV.
• Water is the trickiest case. Looking down at a calm sea, diffuse UV reflectance is genuinely low — a few percent. But reflection off water is largely specular, and specular reflectance climbs steeply as the sun approaches the horizon: near grazing incidence a smooth surface reflects a large fraction. Add chop, wave facets and whitecaps and you get bright glints from many angles at once. Boaters and swimmers are also, of course, wet, which cancels no UV at all while making the skin feel cool and the sun feel mild.

Why an 80% albedo does not mean 80% more exposure

Here is the point that most explanations skip, and it is the one that reconciles those dramatic albedo numbers with the modest factors used in the model. Reflectance and dose enhancement are two different quantities. There are two distinct routes by which reflected UVA reaches you, and they behave very differently.

1. Downwelling enhancement (modest). UV that bounces off snow travels up, some of it is scattered back down by the atmosphere, and it adds to what arrives from above. This multiple-scattering loop is real but lossy: for a horizontal surface under a clear sky, even fresh snow typically adds only about 10–20% to downwelling UV, not 80%. That is what a flat-plate sensor — and this site's model — measures.
2. Upwelling flux (large, and body-shaped). The other 80%-odd goes straight back up and hits you from below. It never appears in a downwelling measurement at all, but it lands squarely on your chin, nose, eyelids, ears and the underside of your jaw. For a standing person, this can roughly double total body exposure over fresh snow.

Snow, and why winter mountains are the extreme case

Fresh snow is a genuine outlier — nothing else in ordinary experience reflects UV like it. Combine it with the things that travel with it and you get the highest-risk UVA setting outside a tropical noon:

• Snow lying on the ground, reflecting 80–90% back up.
• Altitude, adding several percent per kilometre of thin, clean air.
• Cold, dry, low-aerosol mountain air, which attenuates less than hazy air at sea level.
• Snow-covered slopes facing you as well as beneath you, so the reflecting surface fills much more of your field of view than flat ground would.
• Low temperatures removing every sensory cue that you are being irradiated.

This is why photokeratitis — snow blindness, effectively sunburn of the cornea — is a mountain condition rather than a beach one, and why wrap-around eye protection matters more on snow than anywhere else: the light is coming from below the frame of ordinary sunglasses.

Beaches, shade, and the umbrella problem

Sand at 15–25% is far below snow, but beach exposure has its own geometry. Reflected UV from sand strikes the underside of a beach umbrella and is scattered down onto whoever is beneath it, and a large share of UVA is diffuse skylight that an umbrella does not intercept at all. Measurements of beach umbrellas typically find they cut ultraviolet by only about half to three-quarters — useful, but nowhere near the near-total protection people assume they are getting when they move into shade.

The same logic applies to the shade of a single tree, a parasol or a patio umbrella. Shade blocks the direct beam. It does relatively little about the sky, and nothing about the ground.

How the UVA Index models albedo

The calculator's ground-surface selector applies a multiplier to the modelled UVA irradiance:

Those multipliers look small next to the albedo table above, and now you know why: the model reports downwelling UVA on an open horizontal surface, so it captures route 1 and not route 2. The large upwelling flux that actually causes chin and eye burns is a property of your body's geometry, not of the sky, and is explicitly out of scope.

Two practical notes on using the selector. Concrete and paving are not options — at 0.08–0.15 they sit between grass and sand, and grass is the closer approximation for a city street. And "Water" at ×1.00 describes standing beside or above water with a reasonably high sun; with a low sun, or on a bright choppy sea, the real reflected component is higher than the model's flat treatment suggests.

What to take from this

• Set the surface honestly before reading your UVA Index — snow is not a rounding error.
• Apply sunscreen upward-facing. Under the chin and nose, the underside of the jaw, the ears, the eyelids. On snow and water these are the exposed surfaces, not the sheltered ones.
• Wrap-around eyewear on snow and water, not just dark lenses.
• Do not count shade as protection over a bright surface. Over sand it is worth roughly half, not all.
• Remember the sensory trap: cold air and cool water remove the heat that normally tells you to seek shade, while the ground beneath you is doubling your dose.