Behind the Forecast: The burning truth of UV rays on cloudy days

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Published: May. 28, 2021 at 10:13 AM EDT
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LOUISVILLE, Ky. (WAVE) - On sunny days it’s easy to remember the dangers of ultraviolet (UV) radiation. However, it’s important to consider UV rays even when there are clouds overhead.

Ultraviolet radiation is broken into three sub-categories: UV-A, UV-B, and UV-C. UV-C radiation (wavelengths less than 280 nm) is very dangerous to animals and plants but is completely absorbed by ozone in the stratosphere and doesn’t reach the surface. Ozone also absorbs much of the UV-B radiation but some radiation still reaches the ground. UV-A radiation can easily make it through the ozone layer. UV-B rays can cause sunburn by damaging DNA. While we are all vulnerable, those with darker skin are equipped with much more melanin which helps to block the radiation and minimize DNA damage. UV-A rays do not cause sunburns but they can lead to long-term damage and potentially cancer.

The UV Index explains how much UV radiation will reach the earth’s surface on a particular day. The typical UV Index forecast is given for when the sun is highest in the sky and the time of highest fluctuation in UV radiation. The UV index forecasts distributed by the National Oceanic and Atmospheric Association are comprised of weather satellite observations about snow, clouds, and ozone. All of these reflect solar radiation into the atmosphere.

The measure of ultraviolet radiation that reaches the ground is a tally of the amount of radiation that is directly coming from the sun and the amount that is scattered, refracted, and reflected by various objects in the atmosphere such as clouds, air molecules, water droplets, ice crystals, and dust.

The amount of UV radiation reaching the surface is also dependent on the time of day. When the sun is at a lower angle (think closer to sunrise and sunset), less direct UV radiation reaches the ground since it has to travel through more of the atmosphere and is absorbed by stratospheric ozone. As the sun rises higher overhead, the amount of UV radiation increases. Keep in mind that peak UV radiation is not always seen at the hottest part of the day. Solar noon (when the sun reaches the highest point in the sky) is when the most direct UV radiation is seen. However, the day’s high temperature may not be reached until three to four hours later. By that point, the amount of UV radiation reaching the ground could be cut almost in half. In the United States, the strongest UV rays are typically seen between 10 AM and 4 PM Daylight Saving Time (9 AM to 3 PM Standard Time).

Clouds are much better at blocking visible light than ultraviolet radiation. On average, clouds do decrease the amount of UV-A and UV-B radiation that reaches the ground. Several studies since the 1960s have shown that clouds can sometimes enhance the amount of UV radiation reaching the Earth’s surface as much as 50% above what’s seen on a day with just blue skies. Research suggests that cumulus clouds can help to enhance UV radiation when it strikes and reflects off of them. The type of cloud and its thickness also can play a role in how much radiation is reflected, refracted, and scattered. UV radiation can easily penetrate thinner clouds while overcast skies (especially with thick lower-level clouds) significantly reduce the amount of UV radiation reaching the surface. Studies have found that a mixture of thin cirrus clouds and lower-altitude cumulus clouds could be perfect for enhancing UV radiation.

There’s also the consideration that many tend to put off applying sunscreen on cloudy days. That may enhance your chances of getting a sunburn on a cloudy day.

Hazy skies and dust reduce the amount of UV radiation reaching the planet’s surface since they scatter the radiation quite easily.

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