What is the safest time to be outdoors? It turns out that this is not as easy a question to answer as you might think, and part of the reason is something called “albedo.” The term albedo is defined as the percent of sunlight that is reflected off a surface. Fishermen have higher rates of skin cancer than our average outdoorsmen because of sun not only reaching them from above but also because sun bounces off the water and strikes their skin from below. We often say that this gives them a “double dose” of radiation and sun damage. But is the damage really double?
Albedo, the percent of light bouncing off a surface, varies by a number of factors including the type of surface the sunlight is hitting and the angle the sun is at relative to the surface. For example, when the sun is at angles of 40 degrees or higher in the sky relative to the horizon, the albedo of water is fairly constant at around 5%, meaning that only 5% of the sun is bouncing up and potentially hitting you. As the sun drops below 40 degrees, the albedo of water increases dramatically with about 50% of sunlight bouncing off of water when the sun is at an angle of ten degrees and almost 100% of sunlight reflecting off the water when the sun is just above the horizon at an angle of zero degrees. For many latitudes in the northern hemisphere in the winter months, the sun spends all its time below 40 degrees, so a large amount of light is reflecting off surfaces at all times. In Atlanta on Christmas, the solar elevation angle only reaches 32 degrees at noon.
Snow is a different beast all together. We love snow because it is pretty and sparkly… but are those sparkles in fact sinister? (Cue alarming music now). If you are an avid skier who doesn’t want skin cancer, it is useful to know that ice has a higher albedo than water, and snow has an even higher albedo than ice. In fact, snow reflects as much as 90 percent of solar radiation. Some of the worst sun burns that I have ever seen have come after ski trips. We might not think about sun damage in winter months figuring that the sun is less intense. However, many ski trips are in late spring at low latitudes and high elevations, so the snow persists even though the sun is now high in the sky. When it comes to skiing, you really can receive a double dose of UV radiation due to albedo and elevation. The general rule of thumb is that with every 3000 feet increase in elevation, UV radiation increases by 10%.
So what is the safest time to be outdoors? The dermatologist would say that it is safest to ski between 6pm and 6am. The orthopedic surgeon might disagree. The real answer is to protect your skin from light from above and below with proper clothing and sunscreen.
Have fun this winter! And if you or a loved one has a concerning spot or skin care need after your winter fun, consider Premier Dermatology and Mohs Surgery of Atlanta. Dr. Brent Taylor is a fellowship-trained and board certified dermatologist, Mohs surgeon and varicose vein specialist. We look forward to taking care of you.