The Tropics, the Sun, and the Siesta

I started spending at least one season each year working on projects in the tropics a decade or so ago. Aiming for the dry season there usually means heading South in November after a long cool summer in the Pacific Northwest.. By late Autumn I am a blank canvass for the tropical sun to do it's worst. Even so, the joy of finding lush, sun baked summer alive and well is like the smell of fresh baked bread to a starving man - I can't tear myself away from it. Like everyone who has this experience, it led to some truly painful, red-faced sunbu
s. After a few years of being over optimistic and suffering for it, I have learned to be a much better judge of how much sun I am standing under and a lot more aware of how important it is to use some caution.

There are three main factors that affect the amount of sun, and the amount of UV radiation, that you are getting at any time: how high the sun is in the sky, how far above sea level you are, and how much light is being reflected by the ground, the water, snow or clouds around you. I am going to focus on just the first part of that here, and try to explain why the quick burn you can get in the tropics has nothing to do with the heat.

The first thing to understand is that when the sun is low in the sky, the UV radiation reaching you is less intense. As the sun gets higher, there is less atmosphere to protect you from the harmful rays that cause your skin to burn and damage skin cell DNA (the reaso
UV exposure can lead to cancer). You can see this filtering of the light happening easily enough at sunrise and sunset. This is when the path that the sun's light has to take through the atmosphere is longest, and it has the most filtering, so the light grows red and dim. You can also see it in the winter here in the US; as the days get short the sun never gets very high. Even if it wasn't freezing cold, you would have a very hard time getting a sunbu
in December because the light is just too weak because of the low angle. Its as if it was almost sunset all the time.

The reverse is also true, then: when the sun is very high in the sky, there is much less natural protection from our atmosphere.

The closer you get to midday and the equator, the higher up in sky the sun will be. On any map or globe, you'll see 2 lines showing the tropical zone at 23 degrees north and south of the equator. These lines show the farthest away from the equator that the sun can ever be directly overhead. If you ever do get to visit a place on the equator itself, there the sun is always close to to directly overhead at midday, all year long.

The difference is easy to feel and see. First, the sun comes up quickly in the tropics. In the Northern U.S., you'd only need to be careful between 10AM and 2:30PM, while in the tropics the sun is already intense enough to cause a sunbu
in just 15 minutes by 8:30AM and doesn't ease back until very late in the afte
oon, around 3:30PM. At midday, the sun's intensity in the tropics is about double even the sunniest midsummer day in New York or Chicago. And those big, puffy white clouds reflect enough enough light to add another 20% to the amount of UV hitting your skin. Add that to the reflection off white sand and surf, and it's no surprise so many of us look like cooked lobsters within just a couple hours.

I have learned all this the hardest way and, being something of a knucklehead, it took a few lessons before it really sunk in. It takes more than just sunblock, too; I use a quality swim shirt or rash guard when I swim and especially when I snorkel . I also wear a good, wide brimmed hat. Of course there is an upside to the ferocious sun besides the welcome break from Winter; it's a great reason for everyone to take a siesta in the shade in the middle of the day.