Albedo and Reflected Radiation

Albedo is the plainest formula on this shard and the one with the largest consequences. A surface reflects the fraction α of the shortwave radiation that lands on it and absorbs the rest, so reflected is αG and absorbed is (1 − α)G. Under 800 W/m² of sun, grass at α = 0.25 bounces 200 W/m² back and keeps 600. Fresh snow at 0.85 bounces 850 W/m² of a full 1,000 and keeps only 150, which is why deep snow can persist in bright sunshine and why snow blindness is a real injury rather than an exaggeration. The word is Latin for whiteness, and Johann Heinrich Lambert brought it into optics in 1760.

Typical values are worth carrying in your head, because they span more than an order of magnitude. Fresh asphalt sits near 0.05 and aged asphalt near 0.10 to 0.15. Bare soil runs 0.10 to 0.25 depending on how wet it is; wet soil is markedly darker than dry. Grass and most crops fall between 0.20 and 0.26, deciduous forest near 0.15 to 0.18, and conifer forest lower still at 0.08 to 0.15 — a dark forest canopy absorbs almost as greedily as tarmac. Water is the awkward one: near 0.05 with the sun overhead, but rising steeply toward 0.6 and beyond at grazing incidence, which is why a lake dazzles at sunset and not at noon. Fresh snow reaches 0.80 to 0.90 and decays to 0.4 or lower as it ages and dirties. The planetary average is about 0.30.

The mistakes are mostly bookkeeping. Albedo is a SHORTWAVE property, not the longwave emissivity used one page over, and the two are genuinely independent: white paint has a high albedo and a high longwave emissivity at the same time, which is exactly the combination a cool roof wants. Albedo also varies with sun angle, with cloudiness, with wetness and with season, so a single tabulated number is a daily average at best. And it is a proportion, never a percentage in disguise: 25 and 0.25 are not the same input unless you pick the % unit.

The absorbed complement is the term that matters for everything downstream, so this page computes and reports it alongside the reflection. That absorbed flux is what warms the ground, drives evaporation, and sets the daytime side of a microclimate — and it is why an urban surface of dark roofs and asphalt runs hotter than the countryside beside it, before you have said a word about waste heat.

The feedback that makes albedo interesting rather than merely arithmetic is the one snow provides. Snow reflects, so the ground beneath it stays cold, so the snow lasts; melt it and you expose dark ground with an albedo four times lower, which absorbs far more, which melts the snow around it faster. That runaway works in both directions and it is the reason polar regions warm and cool faster than anywhere else. The same loop operates on a garden scale in spring: a patch of bare dark soil among snow will clear itself in a day, while the shaded white beside it sits there for a week.