What is the UV Index (and Why Should I Care About It)?

iStock
iStock

We’re so used to running for cover when we hear a rumble of thunder during the summer that it’s easy to forget that we’re just as exposed to the elements under a beautiful, cloud-free sky. Whenever the Sun's out (and even when it isn't), we're at risk for sunburns, which are caused by the Sun's ultraviolet, or UV, radiation. But just how serious is that risk on a given day? That's where the UV Index comes in.

Ultraviolet radiation falls next to visible light on the electromagnetic spectrum. The two wavelengths of UV rays that can cause injury are called UVA and UVB rays. Both types are dangerous over long periods of time, but UVB rays are thought to be the most dangerous. Excessive exposure to UVA and UVB rays can cause cell death, leading to painful sunburn, or, over time, some forms of skin cancer.

Earth’s atmosphere does a pretty good job protecting us from harmful UV rays, but it doesn’t completely block them out. Most ultraviolet radiation is absorbed by the ozone layer. Ozone is gas that’s usually situated more than twice as high above sea level as the cruising altitude of a commercial airliner. In the 1970s and 1980s, there was a concerted effort to eliminate the use of chemicals that deplete the ozone layer as people began to understand what its depletion would mean: overexposure to dangerous UV rays for humans, animals, and plants.

UV Index Forecast
A map of the UV Index on June 9, 2017.
NOAA/NWS/EPA

The UV Index was developed to tell us the level of solar radiation on a particular day, and thus, how susceptible we are to a sunburn. The UV Index is an open-ended scale that starts at 0, indicating the lowest level of danger, with the risk gradually increasing through a UV Index of 10. Any value above 10 indicates an extreme risk for harm from UV rays, with sunburns and eye damage occurring in just minutes.

The index is compiled daily using factors like ozone concentration in the atmosphere, expected cloud cover, and the elevation of a certain point above sea level. Greater ozone concentrations, more clouds, and a lower elevation will help prevent UV rays from reaching the ground. A bright, hot, sunny day can easily cause the UV Index to soar above 10.

Risk from UV rays doesn’t end with sunshine pouring out of the sky. Ultraviolet radiation can also be reflected, meaning that the danger for sunburns and eye damage increases around bright surfaces like a beach or the shimmering waters of a swimming pool. If your eyes have ever hurt after looking at snow on a sunny day, that painful condition is directly caused by UV rays reflecting off of the white surface.

One easy way to protect yourself from harmful UV radiation is to cover up. But clothing and hats don't deflect all UV rays. That's where sunscreen comes in. The best sunscreen to use is one that’s rated for both UVA and UVB protection with an SPF (sun protection factor) of at least 15 or higher, according to the FDA. (The American Academy of Dermatology suggests one with an SPF of 30 or higher.) A sunscreen’s SPF doesn’t relate to the amount of time you can spend in the sun, but rather the amount of protection offered by a particular formula. An SPF of 15 blocks 93 percent of UV rays, while SPF 50 is said to block 98 percent. (Some argue that sunscreens over SPF 30 are no more effective.) Whichever SPF you choose, don't forget the sunglasses—specially rated pairs can protect your eyes from radiation, and also help you channel your inner celebrity. Sounds like a win-win to us.

7 Wintery Facts About Ice, Freezing Rain, and Sleet

Razvan Socol via Flickr | CC BY-SA 2.0
Razvan Socol via Flickr | CC BY-SA 2.0

Whether you’re trying to fly across the country or you just want to buy groceries, a winter storm can have a significant impact on your life. But how can you tell if the ice, sleet, or freezing rain will prompt a winter weather advisory or a snow day from work? Here are a few facts about winter storm weather to help you prepare.

1. Freezing rain and sleet are a winter storm's silent hazards.

Ice in the form of freezing rain and sleet is just as big of a threat as snow, and often result in a winter weather advisory being issued for the affected region. Ice is arguably more dangerous than the fluffy white stuff. Snow is generally manageable: You can shovel it and plow it, and while others are doing the work, you can enjoy a snow day with a cup of hot cocoa. You can’t do that with ice.

For the most part, frozen water becomes solidly affixed to any exposed and untreated surface. There comes a point when ice is entirely unmanageable. Even a giant vehicle with four-wheel drive is useless when it can’t grip the surface it’s sliding on. Ice—mostly from freezing rain—is not only dangerous because of the associated travel hazards, but also because of the damage it can cause.

2. A winter storm with Freezing rain is dangerous.

Freezing rain is rain that freezes when it comes in contact with an exposed surface like a tree or a sidewalk. A small amount of freezing rain can leave a thin glaze of ice on just about any surface, creating a situation where surfaces that look wet are really icy instead. A steadier freezing rain will allow a crust of solid ice to form on trees and power lines, weighing them down to the point of breaking. Extreme ice accretions—over an inch—can cause significant damage and disrupt life for weeks at a time.

3. Freezing rain is actually melted snowflakes.

Freezing rain forms when there’s an inversion layer present during a winter storm. An inversion layer occurs when a layer of warm air gets sandwiched between two colder air masses. Snowflakes fall through the warm layer and completely melt before reentering the subfreezing air near the surface. This newly formed raindrop can’t freeze back into ice because it doesn’t have a nucleus around which to freeze, so the raindrop becomes supercooled, meaning it remains in liquid state even as its temperature drops below freezing. Once the supercooled raindrop reaches the ground, the water instantly freezes into ice.

4. All that ice from freezing rain is extremely heavy.

If you’ve ever had to carry a case of bottled water up a flight of stairs, you know that even a little bit of water is extremely heavy. Imagine even more weight on a much more fragile surface, and that’s what you get during an ice storm. Damage to trees can begin with just a quarter-inch of ice, with more damage to bigger and sturdier trees as the crust of ice grows thicker. The Weather Channel points out that just a half-inch of ice accretion on a standard power line can add 500 pounds of extra weight to the line and the poles supporting it. Extreme ice storms can cause as much damage as an intense tornado, as even a couple of inches of ice adds enough weight to crumple the tall steel transmission towers that carry high-voltage power lines—and those take a while to repair.

5. Sleet is freezing rain's annoying cousin. 

A close relative to freezing rain is sleet. Sleet, also known as ice pellets, forms through the same process as freezing rain. Snowflakes destined to become sleet also fall through a warm layer of air, but one that isn’t deep enough to melt the snowflake completely. Once the partially melted snowflake enters subfreezing air, there are still a couple of ice crystals left in the raindrop that allow the raindrop to freeze into a little ball of ice before reaching the ground. The result is an ice pellet about the size of half a grain of rice that makes a distinctive tinking noise as it bounces off cars, vegetation, and roofs.

6. Sleet is like snow that freezes solid. 

Sleet looks like snow and it accumulates like snow. It’s easy to mistake sleet for snow if you’re not a hardcore weather geek, but with enough accumulation, even the casual observer will know something is different pretty quickly. Sleet has a nasty habit of freezing into solid ice within a few hours of falling, especially if the Sun comes out or if temperatures briefly rise above freezing once the precipitation stops. Once this hardening occurs, it can be next to impossible to remove it from sidewalks, driveways, and roads until there’s a major thaw. In the southeastern United States, sleet is particularly common (and problematic), since the region is prone to warm air intruding on its winter storms and many municipalities don’t have enough snow equipment to clear the roads before that sleet freezes solid.

7. When a winter storm warning is issued, join the grocery lines.

Everyone makes fun of the throngs of panicked shoppers before a snowstorm, but stocking up on groceries before a winter storm is a pretty good idea for even the biggest cynic. If freezing rain knocks out power for an extended period of time, stores and restaurants will be forced to close until power is restored and they get fresh shipments of food. If that happens, you’re pretty much on your own for food and drink until conditions improve. Before a storm arrives, make sure you get plenty of food and beverages that you don’t have to cook or keep fresh.

Lake Michigan Has Frozen Over, and It's an Incredible Sight

Scott Olson, Getty Images
Scott Olson, Getty Images

A polar vortex has brought deadly temperatures to the Midwest this week, and the weather is having a dramatic effect on one of the region's most famous features. As the Detroit Free Press reports, parts of Lake Michigan have frozen over, and the ice coverage continues to grow.

The Lake Michigan ice extent has increased rapidly throughout January, starting around 1 percent on the first of the month and expanding to close to 40 percent by the end of the month. Yesterday was the coldest January 30 in Chicago history, with temperatures at O'Hare Airport dropping to -23°F. Even though it's frozen, steam can be seen rising off Lake Michigan—something that happens when the air above the lake is significantly colder than the surface. You can watch a stream of this happening from a live cam below.

Lake Michigan's ice coverage is impressive, as these pictures show, but it's still far from breaking a record. Though Lake Michigan has never frozen over completely, it came close during the winter of 1993 to 1994 when ice reached 95 percent coverage.

Midwestern states like Wisconsin, Michigan, Illinois, and Indiana aren't the only places that have been hit hard by the cold this winter. At the United States/Canada border, Niagara Falls froze to a stop in some spots, a phenomenon that also produced some stunning photographs.


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[h/t Detroit Free Press]

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