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© Meteorologist Steve Frazier © Meteorologist Steve Frazier

Although we sort of got a late start to it, summer is now in full force. A lot of us are running, not walking, to the out of doors. Why wouldn't we? I mean after months of gray, cold and snow and now it's as bright as can be outside. Cramming in as much as possible in our short allotment of summer, we can easily find ourselves running around like a bunch of a bunch of hula dancers in a grass fire. Also, like the hula dancers, we need to do our best to keep from getting burned.

Other parts of the country have more practice with the bright sun than we do here in Minnesota. It seems that it just pops out in late spring, says "Hi", and then runs away until next spring. Since we're way up north, relatively speaking, our geographical position gives us slightly shorter days in the winter and slightly longer days in the summer. Hence, the common expression around June, "Hey, can you believe its after ten o'clock and its still light outside". Therefore, we have the opportunity to stay outside a bit longer. Staying out that long give us a greater chance of soaking up too many rays. So, as we head in to mid-summer, I thought it might be fun to explore the ABCs of the UV index, thanks in part to the NWS and the EPA. How's that for a mouthful of abbreviations?

First of all, what is the UV index?

You most likely see it every day on the TV weather maps, a little UV index number and a blurb about burn time. This index provides an indication of the level of risk or intensity of ultra-violet radiation on a scale of 1 (low) to 11+ (extremely high).

So, how is that UV number calculated? The calculation starts with measurements of current total ozone amounts over the entire globe, obtained via two satellites operated by the National Oceanic and Atmospheric Administration. The data collected from these satellites is then used to forecast ozone layers in the upper atmosphere. Other computer models then used that same data to calculate the intensity of the radiation at the earth's surface at various times of the day, and for various locations across the US. Depending on where you live and what time of year it is, the angles of direct sunlight vary. You have all heard about the ozone. It's a layer in the upper atmosphere, made up mostly of Oxygen that acts as a filter from some of the sun's harmful rays. The more intense the sun, the greater your exposure to UV rays. The amount of UV that will reach you depends on the following:

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The Time of Day and Type of Weather — the sun is at it's highest in the sky around noon. Therefore, UV is at it's strongest in the hours just before and after noon, roughly 10 am to 2 PM. Keep in mind that this is the time when the rays are most intense, you can still get burned later in the day, it just may take a bit longer. Clouds can filter some of the harmful rays but not all of them. So don't get lured into a false sense of security on a cloudy day.

Tis The Season—While UV exposure is the greatest in the summer (May—August) in the United States, it is important to remember that UV rays reach Earth every day. Here in Minnesota we have a lot of snow cover in the winter and snow can reflect up to 90% of the sun's UV rays. So, that bright January day you spend cross country skiing can burn you just as bad as a day at the beach.

Altitude—the air is cleaner and thinner at higher altitudes, so UV exposure is greater in the mountains than in the valleys. Keep that in mind if you're off hiking in the mountains to escape the winter cold. While we don't have mountain tops here in Minnesota, we do have cleaner drier air, which is less of a filter for UV rays.

Location—UV is strongest at the equator and gets weaker as you move towards the poles. Going down south to escape the cold? Be prepared and take your sunscreen with you!

Exposure Time—the longer you are out in the sun, the more UV rays you receive. Remember, you are exposed whenever you're out: A picnic at Lake Harriet, weekends in the garden, a game at Target Field, and even in the car on a long road trip.


Ultraviolet radiation is composed of three wavelengths: UVA, UVB and UVC.

UVA rays make up about 95% of the sun's rays that reach the earth. They have the longest wavelength and can penetrate deeper into the skin. These rays are around all year round and can penetrate clouds and glass; therefore most of us are exposed to them.

UVB rays have a shorter wavelength than UVA rays. They are not absorbed as deeply into the skin and tend to cause most damage on the surface of your body. They are most plentiful in the peak hours around noon, between 10 am and 2 PM.

UVC rays have even a shorter wavelength than UVA and UVB rays. Besides, they are completely absorbed by the Ozone and they are not a main concern for damage to human skin.


Now that you know the UV index number and the ABC's of UV Rays, it is now time to put up a defense. Wouldn't you know it; there are more numbers and letters. When you go to buy sunscreen, you will see the letters SPF (Sun Protection Factor), followed by a number, usually 15, 30, 50, etc. The higher the number on the bottle, the higher the protection. Latest studies show that adults need at least a SPF of 15 and a SPF of 30 is recommended for young children. Now hold on, we may get a bit technical, but its really simple. Sunscreens come with a combination of "Absorbers" and Blockers". The more "Absorbers" and "Blockers" that sunscreens have, the higher their SPC number, and the thicker they become. Absorbers are organic compounds that absorb the sun rays and release them as heat. Blockers, are metal particles, yes metal, that blocks or scatters the harmful rays. The most commonly used blocker used is zinc oxide. Hey at least you won't rust! Smearing on a greasy cream is not your only defense. You can wear protective clothing, large brim hats, and quality sunglasses to protect yourself from the onslaught of the sun. You can even just hang in the shade, or limit outdoor activities to early morning or late evening.

Now that you are prepare, get out, and enjoy the limited bright and warm weather that we call summer. Frazier

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