Being outside is great for our physical, emotional and mental health. But when you’re heading outdoors, it’s essential to take steps to protect your skin from the sun, even when it’s cloudy.

For some of us, this may seem counterintuitive, and we might be tempted to skip sun protection when we can’t see the sun through the clouds. But the sun’s ultraviolet (UV) radiation, while invisible, is still very powerful. UV radiation can still harm your skin on cloudy days, causing sunburn and skin damage that increases your risk of skin cancer.

So, you might be thinking, it can’t be that bad, right? Well, it turns out that a whopping 90% of UV rays are capable of passing through light cloud cover. What's more, if the clouds are very light, in addition to giving little protection, they can actually enhance UV levels. That's because of a phenomenon called the "broken cloud effect," where light clouds reflect and scatter UV radiation.

What Is UV Radiation?

Ultraviolet radiation is part of the electromagnetic spectrum emitted by the sun. UVC rays, with wavelengths of 100 to 280 nanometers, are absorbed by the atmosphere's ozone layer, but most radiation in the UVA range (315 to 400 nanometers) and about 10 percent of the UVB rays (280 to 315 nanometers) are able to make it to the Earth’s surface.

UVA and UVB rays are both extremely important to human health. According to the WHO, small amounts of UV radiation are necessary for the production of vitamin D in our bodies, but overexposure can result in serious health impacts to the skin, eyes and immune system. In the short term, that overexposure takes the form of sunburn, which can be painful, and, if severe enough, can be considered a first-degree burn. In the long-term, overexposure leads to an increased risk of developing skin cancer, which can potentially be fatal.

What Is The UV Index?

The UV Index is ranked on a scale from 0 to 11-plus, with 0 being the lowest and 11-plus being the most extreme. Those numbers are grouped as follows:

The Environmental Protection Agency offers some tips on how to stay safe from UV rays:

-Minimize sun exposure between 10 a.m. and 4 p.m.

-If outdoors, seek shade and wear protective clothing, a wide-brimmed hat and UV-blocking sunglasses.

-Generously apply broad spectrum SPF 30 or higher sunscreen every two hours, even on cloudy days, and after swimming or sweating.

-Watch out for bright surfaces, such as sand, water and snow, which reflect UV rays and increase exposure.

Surfaces Make a Difference

Just as broken clouds can intensify your UV exposure, so too can various surfaces. Grass, soil and water reflect less than 10% of UV radiation, while sand reflects around 15% and seafoam reflects around 25% (this is part of why sunscreen at the beach is such an absolute must). Fresh snow is the biggest culprit for increased UV exposures: It almost doubles the amount of UV exposure by reflecting so many rays.

That’s why it’s so important to seek shade often and use sun protection all year long.

Senior writer Chris DeWeese edits Morning Brief, The Weather Channel’s newsletter.