When the day is hot and the sky is clear, using sunglasses is a no-brainer. But when clouds roll in, you can take them off, right? 

Not so fast. In fact, you may need sunglasses protection more than ever. 

The surprising truth is, when the weather mixes clouds and sunshine, it can actually increase the ultraviolet (UV) rays that hit your skin and eyes. It may seem counterintuitive, but those big white puffy clouds are highly reflective. They bounce radiation from the sun in all directions. 

On such days, the shade between buildings or under a tree may contain more UV rays compared with a cloudless day. In fact, you might feel safe sitting in the shade, only to end up with a surprise sunburn thanks to this phenomenon. 

In addition, all those indirect rays reflecting off clouds, water, sand, or snow can also cause damage to your eyes. Here’s a look at the risks. 

Risk #1: Eye Growths 

“UV exposure from the sun has been correlated with formation of growths on the eye surface,” says Ming Wang, M.D., an ophthalmologist in Memphis, Tennessee.

Dr. Wang says these problems include two of the top causes of vision disability in the United States: early cataract formation, which is marked by a gradual clouding of the eye’s lens, and age-related macular degeneration, which robs people of their central vision. 

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Risk #2: Eyelid Cancers

“UV exposure to the skin around the eye can also lead to certain types of skin cancers that are difficult to treat due to their location on the delicate structures of the eyelids,” says Dr. Wang. 

What’s Your Risk? How to Assess UV Intensity 

Your gut instinct may suggest hotter weather equals greater UV radiation, but that’s only part of the story. Here’s what to consider when evaluating your UV exposure, according to the EPA and the World Health Organization:  

1. Time of year and time of day. The closer the sun is to directly overhead, the more UV waves punch through the earth’s atmosphere, and the shorter the distance they have to travel to reach you on the surface. 
   
   That’s why summer months bring higher UV exposure, especially between 10 a.m. and 4 p.m. 
   
   A simple trick: check the size of your shadow. If it’s shorter than you are, the sun is more directly overhead and your UV exposure is higher. When the sun is low and your shadow is long, UV radiation diminishes.
    
2. Altitude. UV levels jump approximately 10% for each 3,000 feet of elevation gain. That’s because higher altitude translates to less atmosphere available to absorb UV radiation. 
    
3. Reflection. Sand reflects about 15% of UV radiation, and sea foam reflects about 25%. Fresh snow nearly doubles UV exposure. 
    
4. Ozone. The gas that blankets the Earth’s upper atmosphere and ground level is called ozone. The upper level, the stratospheric ozone layer, is a very good absorber of UV radiation. Unfortunately, pollutants have damaged it, causing it to thin out. 
    
5. Latitude. At the equator, the sun is most directly overhead, so UV rays have the least distance to travel through the atmosphere. The tropic zone of the planet that’s nearest to the equator also naturally has a thinner ozone layer, so more UV radiation passes through the atmosphere. 

High- and Low-Tech Ways to Protect Yourself 

To keep your eyes healthy from these dangerous rays, the first line of eye defense is a good pair of sunglasses. 

“The best sunglasses are those that wrap around the face, blocking light from all sides and providing the most protection versus flat-fronted sunglasses,” Dr. Wang says.

He suggests looking for sunglasses that are rated for 100% blockage of UVA and UVB light. Larger frames that cover more of the eye area also up the protection factor. (Learn more about finding the right sunglasses here.)

Next, take notice of local conditions. “Most weather reports will state the UV index, and paying attention to this number will keep you informed as to when the risks of UV overexposure are highest,” says Nancy Akerman, a physical scientist at the Environmental Protection Agency.

The EPA also offers a UV index forecast that you can use as a guide. There are also more modern, high-tech ways to monitor UV radiation, including the EPA’s free smartphone app (SunWise UV Index).

And other apps can do everything except apply your sunscreen. Check the app store for your phone or tablet to find the right one for you. Enter things like skin tone, activity and your location, and the apps do the math to figure out when you need to reapply, or when it’s time get out of the sun for the day.

Major makeup companies have even introduced smart patches that stick on skin or a fingernail. Some simply change color based on UV exposure; others link to a phone app and generate real-time data on sun exposure. Other wearables are designed to clip onto clothes or be worn like a watch.

In fact, with all the information available to you, there’s really no excuse for dangerous exposure to UV.

Of course, it helps to start good habits early. “Wearing sunglasses is especially important early in life, as children often spend much more time outdoors than adults,” Dr. Wang points out. “Baby, infant, and child-size glasses are available, and unfortunately this is often overlooked. 

“Wearing sunglasses at any stage,” he continues, “will help to reduce the cumulative exposure of UV rays throughout life.”