What is the UV index in the weather forecast? What to know about staying safe in the sun

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24-05-2025

The summer sun is returning to Phoenix in full force, and with it comes an invisible hazard that can be just as potent as the heat: ultraviolet radiation. As triple-digit temperatures dominate the Phoenix forecast, UV levels are also climbing. Unlike heat, UV radiation can't be seen or felt, but it can damage the skin in minutes and leave lasting health problems. 'We deal with all different types of radiation that comes from the sun,' said Erinanne Saffell, Arizona's state climatologist. 'Ultraviolet radiation helps bring that suntan to our skin, but we all know if we stay out in the sun for too long, it can turn into a sunburn.'
Weather forecasts often include the UV index for the day, a scale ranging from low to extreme. The higher the number, the greater the risk. In 2024, UV levels were very high or extreme on 151 days.Here's everything to know about UV radiation, why it hits harder in Arizona, how it affects human health and ways you can protect yourself:
Ultraviolet radiation is a form of energy emitted by the sun. Although it is just outside the visible spectrum, meaning we can't see it, it has powerful effects on the environment and the human body.There are three types: UVA, UVB and UVC. They vary depending on wavelength — the shorter the wavelength, the more harmful the radiation. UVC is the most damaging type of radiation, but it is completely blocked by Earth's ozone layer. 'But UVA and UVB can make it through,' Saffell said. 'The clouds don't block that radiation, so even when it looks cloudy outside, we still have that exposure.' UVB, the medium wavelength radiation, cannot penetrate beyond the skin's superficial layers. It causes delayed tanning and burning, skin aging and is responsible for most skin cancers. UVA is the longest wavelength, accounting for about 95% of the UV radiation that reaches the ground. This type of radiation can penetrate deeper layers of the skin, leading to immediate tanning and burning and contributing to aging and skin cancer risk. The UV index is categorized by exposure categories ranging from low to extreme. The scale is from 1 to 11+. Low exposure is 1 to 2, moderate is 3 to 5, high is 6 to 7, very high is 8 to 10 and extreme is 11+. Different levels of UV exposure come with various risks. There is less danger during low and moderate exposure, but unprotected skin can burn in minutes with extreme exposure.
UV radiation levels change throughout the year depending on the season, the time of day and geographic location. In Phoenix, late spring and early summer bring the highest UV readings as the sun reaches its highest angle in the sky in the northern hemisphere. Other factors like altitude affect UV intensity. The higher you are, the less atmosphere there is to absorb radiation. UV levels increase by roughly 10% with every 1,000 meters or 3,280 feet of elevation. Various surfaces can increase UV reflection and resulting exposure, like water, sand and concrete. In Phoenix, UV radiation is especially intense due to its southern latitude and high sun angle during the summer. On the summer solstice, the sun reaches its peak and climbs to nearly 88 degrees overhead at noon, meaning its rays hit the ground more directly than in cities farther north. 'If you go up to New York City, they're at a different latitude than we are in Phoenix,' Saffell said. 'The sun angles don't get as high in New York City as they do down here. The closer you are to the equator, the higher your sun angle is going to be, so you get more radiation.' Phoenix's dry air and frequent clear skies only intensify exposure, especially compared to places with thick cloud cover and higher humidity. The atmosphere above Phoenix does little to filter out UV rays. Number of days in each UV exposure category in Phoenix in 2024Extreme - 16 daysVery high - 135 daysHigh - 45 daysModerate - 124Low - 45
Arizonans are familiar with the effects of sun exposure, with sunburns, freckles and tan lines. But doctors say the health risks go far deeper.'There are two main categories of skin damage that come with the sun,' said Dr. Clara Curiel-Lewandrowski, co-director of the Skin Cancer Institute at the University of Arizona. 'One is skin cancer, and the second is photoaging.' Over time, UV exposure breaks down elastic fibers and collagen in the skin, leading to early wrinkling and pigmentation changes. Curiel-Lewandrowski said doctors can even see how the sun affects the skin under a microscope, comparing skin from the face or forearms with protected areas like the lower back with protected areas like the lower back reveals dramatic contrasts. But the biggest risk is skin cancer. UV radiation is a known carcinogen, and skin cancers, including both melanoma and more common non-melanoma types, are sharply increasing. But mortality from skin cancer is decreasing, according to Curiel-Lewandrowski.'When you think about UV exposure and skin cancer, it's no different than thinking about smoking and lung cancer,' Curiel-Lewandrowski said. 'The more sun you put into your skin early in life, the more skin cancers you're going to have later.' Arizona's high UV index puts people at an elevated risk, especially during its long summers and the outdoorsy lifestyles. The most common skin cancers arise from long-term exposure accumulated over years. 'Sun protection is key,' she added. 'Enjoy your time out there early in the morning or later in the day. Cover up, wear protective clothing and apply sunscreen.'
Here are some ways to reduce your risk:
Avoid midday sun - UV is strongest from 10 a.m. to 4 p.m.
Wear protective clothing - long sleeves, brimmed hats and UV-blocking sunglasses can help
Use sunscreen - broad-spectrum SPF 30 or higher, reapplied about every two hours
Seek shade - especially during prime UV hours or when outside for long periods
Living in the desert means living with the sun, as well as understanding how to stay safe. You'll feel the sun's heat before a sunburn, but UV damage can happen just as quickly.
Hayleigh Evans writes about extreme weather and related topics for The Arizona Republic and azcentral.com. Email her with story tips at [e-mail].
This article originally appeared on Arizona Republic: How watching the UV index can help you avoid the risks of sun exposure