The UV index around Philly this weekend may be about as high as it gets. Here’s what that means. (2024)

Under that great “beach umbrella” in the sky, temperatures generally will be pleasant, but with the sun as strong as it will be in July, expect a near 100% chance that some folks are going to get a nasty sunburn during the Memorial Day weekend.

The Philadelphia region is approaching its annual peak season for that ultraviolet radiation that, along with the sting and indignity of sunburn, is associated with melanoma, cataracts, and assorted afflictions.


“It’s getting close,” said Laura Ciasto, a meteorologist with the government’s Climate Prediction Center who is responsible for those “UV index” forecasts that aim to capture the potential ultraviolet intensity. They are a National Weather Service-Environmental Protection Agency partnership.

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With clear skies expected Sunday at the Shore, the index could reach the “very high” levels of 9 or 10, forecasters say. The scale ranges from 0, which would be the South Pole about now, to 11-plus, the likes of Miami.

The forecasts are available daily on weather service and EPA sites and also are posted by commercial services such AccuWeather Inc. and the Weather Channel.

And while those index numbers seem simple enough, they are derived through a complex set of variables, not all of them pronounceable, says Ciasto, and they have quite a fascinating history with their roots in Australia’s “slip, slop, slap” campaign of four decades ago.

The process

Humans have several layers of protection from the sun, the grandest of which is the ozone layer — not to be confused with ground-level ozone that sets off air-quality alerts. The good stuff is nine to 18 miles in the stratosphere.

“It acts as Earth’s beach umbrella,” Ciasto said.

The forecast process begins with estimates of ozone levels, via satellites. Ozone concentrations vary and they are not all-shading. Just how much sun gets past that umbrella depends on factors that include cloud cover, time of day, time of season, latitude, and surface. All those go into the forecast equations.

(If you want to try the calculation at home, here is how it works.)

Temperature, by the way, is not a factor: One can get sunburned while skiing because snow reflects so much UV radiation. Sand also is a decent reflector.

Those numbers

At each increment of the scale, the EPA has recommended actions, with wearing sunglasses and limiting sun exposure during the peak sun hours as the leitmotifs.

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When the forecast levels are “high” — that is, 6 to 7, in the average range in summer for the Philly region — or “very high,” 8 to 10, the EPA advises covering up, using sunscreen, and being a glutton for shade.

At “extreme” levels, 11 and up, the agency warns that skin can burn “in minutes.”

Fair-skinned people are more vulnerable, but the government advises that people with darker skin also are “capable of burning.”

Twilight for tanning?

Well-tanned skin was once viewed as a sign of health and vitality, but its popularity has been fading.

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Research that identified the hazards of indoor tanning salons has hurt what was once a booming industry.

In the 1950s and ’60s tanning lotions were heavily advertised on radio, television, and billboards, not to mention purchased.

However, the Australians, who get more than their share of sun, began raising red flags about the health dangers, Ciasto said, when they noticed increased incidences of skin cancer.

Circa 1980, the nation’s Cancer Council, which calls Australia the “skin cancer capital of the world,” developed the “slip, slop, slap” campaign — as in slip on a shirt, slop on some sunscreen, and slap on a hat — credited with raising awareness of ultraviolet-radiation hazards.

The Australians, Ciasto said, “were very instrumental in educating the public to the harmful effects” of excessive sun exposure.

Eventually, researchers in Canada developed a UV index that became publicly available in 1992. The U.S. index debuted in 1994.

What are the peak sun hours?

The EPA recommends limiting sun exposure between 11 a.m. and 4 p.m., but that’s a guideline.

The sun’s power is at its maximum at solar noon, which is about 1 p.m. these days, and thus would be strong for a few hours on either side.

AccuWeather Inc. recommends this measuring stick: If your shadow is shorter than you, then chances are the UV levels are high.

The UV index around Philly this weekend may be about as high as it gets. Here’s what that means. (2024)


The UV index around Philly this weekend may be about as high as it gets. Here’s what that means.? ›

When the forecast levels are “high” — that is, 6 to 7, in the average range in summer for the Philly region — or “very high,” 8 to 10, the EPA advises covering up, using sunscreen, and being a glutton for shade. At “extreme” levels, 11 and up, the agency warns that skin can burn “in minutes.”

What does it mean when UV is very high? ›

A UV Index of 8-10 (Very High) means there is high risk of harm from unprotected sun exposure. Fair skinned people may burn in less than 10 minutes. Minimize as much sun exposure as possible during the midday hours of 10 a.m. to 2 p.m.

What is the UV Index and what does it mean? ›

What is the UV index? The UVI is a measure of the level of UV radiation. The values of the index range from zero upward - the higher the UVI, the greater the potential for damage to the skin and eye, and the less time it takes for harm to occur.

What does UV Index mean in weather forecast? ›

The UV Index represents the amount of skin-damaging UV radiation reaching the earth's surface at any instant of time. The basic UV Index forecast is given for solar noon — the sun's highest point in the sky and the time of the highest fluctuation in UV radiation (under clear sky conditions).

What is the highest UV Index number What does this mean? ›

UVI 1-2 is low, which generally means it's safe to be outdoors unprotected. Other classifications are moderate (3-5), high (6-7), very high (8-10) and extreme (11+). To some extent, the moderate to very high labels tell us little except that UV intensity is increasing.

What is the highest UV ever recorded? ›

The highest UV index ever recorded was 43.3 in Bolivia on December 29, 2003, at the Licancabur volcano.

What state has the highest UV rays? ›

Conversely, states like Alaska and Hawaii show extreme differences, with Alaska displaying a UV index of 1 and Hawaii soaring to an index of 11, reflecting the unique geographic locations and atmospheric conditions of each state.

Which city has the highest UV index? ›

The maximum UVI of 25 is for the grid cell at 13.5° S, 172° W, centred on Cuzco in Peru. The city is at 3360 m a.s.l., with surrounding terrain extending to over 6000 m a.s.l. As the capital of the Inca empire, Cuzco seems to be well sited for sun worship.

Should I wear sunscreen if the UV index is 1? ›

UV 1-2 – Low levels. No sun protection needed. UV 3-7 – Moderate to high levels. Be sun safe and consider protecting your skin.

What UV is best for tanning? ›

The optimal UV index for tanning varies depending on your skin type, but generally falls between 3 and 5 for a balance of effectiveness and safety. People with darker skin may require a slightly higher UV index to achieve the desired tan.

Does rain reduce UV index? ›

On Rainy Days

Just because you can't feel the sun beating down doesn't mean you're not at risk for UV exposure. UVA rays can penetrate through clouds, rain and fog, leaving you exposed. Also keep in mind that summer storms can pass quickly, so when the sky clears, you might be caught unexpectedly in the sun.

Does rain affect UV index? ›

Also the Skin Cancer Foundation, has more than once presented studies that show that ultraviolet rays can penetrate clouds, and they can also reach below the water's surface. So even in a rainy day you're getting hit with UV rays, it's just that you're not getting as much as if the skies were clear.

Does rain increase UV index? ›

Generally speaking, UV radiation is not necessarily more intense during the rainy season compared to other times of the year. In fact, cloud cover and precipitation can actually reduce the amount of UV radiation that reaches the ground by blocking or scattering the rays.

Should I wear sunscreen if the UV index is 0? ›

Sun protection measures, such as wearing sunscreen, should always be taken when the UV index is 5 or above. The UV index is measured on a scale of 0 to 11+. 0 to 2: Low exposure to UV rays is expected for that day. Wear UV-blocking sunglasses on bright days, and cover up your skin if it burns easily.

Is UV index 4 safe? ›

UV Index 3-5 means low risk of harm from unprotected sun exposure. Fair-skinned people, however, may burn in less than 20 minutes. Wearing a hat with a wide brim and sunglasses will protect your eyes. Always use a broad-spectrum sunscreen with an SPF of at least 30, and wear long-sleeved shirts when outdoors.

Is a UV of 6 good for tanning? ›

So how do you decide what is a good UV index for tanning? *This is a general recommendation and varies depending on your location, altitude, and skin tone. Moderate includes UV index ranges from an index of 3 to 5, vs. high which is 6-7, very high which is 8-10 and extreme which is 11+.

What UV levels are safe? ›

The UV Index Scale. UV Index 0-2 means minimal danger from the sun's UV rays for the average person. Most people can stay in the sun for up to one hour during peak sun (10 am to 4 pm) without burning.

What UV index should be avoided? ›

A UV Index reading of 6 to 7 means high risk of harm from unprotected sun exposure. Protection against skin and eye damage is needed. Reduce time in the sun between 10 a.m. and 4 p.m. If outdoors, seek shade and wear protective clothing, a wide-brimmed hat, and UV-blocking sunglasses.

How can UV index be 0 when sun is out? ›

The UV index is reported as a whole number between 0 and 11(+), with 0 indicating absolutely no sunlight (used only at night!) and 11 indicating extreme radiation, when you can burn in less than 10 minutes.


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