Are the Sun’s UV Rays Really Stronger on Cloudy Days? Fact or Myth?


Many people have experienced a sunburn while being outdoors on an overcast day. Is this simply the result of not being as aware of the sun’s damage, and not applying sunscreen at all or as frequently as directed? Or could the sun’s rays actually be stronger on cloudy days? Let’s explore this further…

What is a UV Ray?

There are many types of light invisible to the naked eye, such as x-rays, microwaves, gamma rays, and UV rays, otherwise known as ultraviolet light. These UV rays can be found in man-made products such as tanning beds and welding torches, but they also originate naturally from the sun.

Due to the inability to see these lights, some individuals underestimate their potential to harm. UV rays from the sun are a form of electromagnetic radiation, making their way to the ground as either UV-A or UV-B rays. Even though UV-B rays have greater potency, each type of ray is responsible for harm caused to our skin. Awful sunburns and the production of vitamin D are the results of UV-B rays.

Do cloudy days really create stronger UV Rays?

Many people have heard the claim that UV Rays are stronger on cloudy days, but often this thought is disregarded entirely as a myth. It is unquestionable that human skin can get a sunburned on cloudy days, but are the UV rays actually stronger?

The UV Index is directly proportional to the intensity of radiation one will receive during a particular time of the day and year. Clouds can block up to 70-90% of these UV-B rays during times of extreme overcast. If, however, the forecast presents itself as “partly cloudy,” the situation is entirely different. A term coined as the ‘broken cloud effect’ reveals that certain clouds can actually create higher UV levels than a perfectly cloudless day. When compared with completely clear skies, studies have shown that partially cloudy skies have raised the UV-B rays by 25% and increased DNA damage up to 40%!

So yes! Cloudy days can actually be more dangerous for your skin!

How does this happen?

Although scientists do not have a concrete answer, they have few ideas surrounding this phenomenon. The first explanation is the fact that dense clouds may reflect UV rays, which in turn send them back down to the earth’s surface.

The second theory is the idea of UV rays being redirected as they pass through thin wisp-like clouds. The combination of refraction and reflecting between varying clouds is resulting in a significantly greater UV strength at ground level.

Complicating matters…

On a cloudy day the sun doesn’t feel as hot as it does on a clear day. As a result, it is natural to neglect the application and/or reapplication of sunscreen. Just because you aren’t feeling the heat from the sun, doesn’t mean the UV Rays are not doing damage. Anytime you are outdoors for any period of time – regardless of how sunny it is – you should be protecting your skin with sunscreen. And as it turns out, even more so on those cloudy days!

Get Your Skin Checked Regularly

Dr Gurgen is a dermatologist in The Villages and Leesburg Florida, specializing in skin cancer treatment and Mohs Surgery. It is important to perform regular skin checks yourself and visit a dermatologist every year – especially here in Florida. If you notice changes to your skin or if it is time for your annual skin check, contact our office for an appointment.