Which Light Is Stronger?
All sunlight was not created equal. Actually, that’s not true. It all starts out the same way, but by the time it hits your eye, it will have lost a lot of its original potency. And depending on where you are, that level will be different.
Before we face-off mountain vs ocean light, let’s have a brief, very rudimentary science lesson on how sunlight works.
Light is a form of electromagnetic radiation that has properties of waves. The electromagnetic spectrum can be divided into several bands based on the wavelength. From gamma rays to shortwave radio waves, the sun generates radiation across most of the electromagnetic spectrum but typically only emits infrared, visible light, and ultraviolet light—and then shoots those rays at earth at death-defying speeds.
In a vacuum (space), light travels at 186,282 miles per second. So, sunlight is buzzing along in space, and roughly 8 minutes in, it hits a wall (the earth’s atmosphere) and some of those rays are just flat out rejected. Sorry for traveling all that way, but not everyone gets to join Earth’s party.
More than 99% of UVC rays (the most dangerous portion of the spectrum) and approximately 90% of UVB are absorbed by the ozone, as well as water vapor, oxygen, and carbon dioxide. UVA comes on in without much issue, and accounts for 95% of the ultraviolet radiation that reaches earth. Visible light passes right on through as well, which is great because that’s how we get to see all those gorgeous colors we’ve come to know and love.
A very simplistic way of understanding this is thinking of UV as an army attacking a walled city. Some troops are caught by the wall, while other troops manage to get up or around that wall and keep on going until they've wreaked their havoc.
At higher altitudes, the air is thinner. There’s less water vapor, oxygen, and carbon dioxide and this, in turn, means there is less to absorb those rays. Essentially, there is less stuff for them to run into—shorter walls, not as much defense.
That means that when you’re standing on a mountain top, you’re getting a heavier dose of UV exposure than you are when you’re standing on your paddle board in the ocean at the same latitude. UV increases 10-12% for every 1,000 meters of altitude; at 6,000 to 8,000 feet in elevation, you're exposed to 25% more ultraviolet light than at sea level. And if you’re in fresh snow, approximately 90% of that light is reflecting back up again, so you’re getting nailed from all sides. For old snow, the reflection is 50%, which is still double that of the 25% that reflects off sea foam and sand. Albedo (the proportion light or radiation that is reflected by a surface) on the ocean differs depending on the time of day and upon the state of the water. If the sun is high overhead and the water is choppy, less light will be reflected than if the sun is lower in the sky and the water is calm. However, even when light reflection is at its highest while on the ocean, your UV exposure will still be less than while in the mountains.
When surrounded by snow, it’s going to be bright up there on the mountain. Lenses with a visible light transmission (VLT) of 5-10% are recommended for a comfortable view. On the ocean, a low to mid-range light transmission is appropriate (10-18%). In the end, it depends on individual preference, especially as some eyes are more sensitive to light than others.
Ultimately, you should wear sunglasses in both settings. UV can harm your eyes whether you’re mountaineering or surfing, and glare off the water and snow can cause all kinds of issues. Lenses with 100% UV protection should always be essential in your adventures, but above the tree line, they should unquestionably be mandatory.