Many people assume a cloudy sky provides complete protection from the sun. However, the energy that causes sunburn and skin damage is not fully blocked by cloud cover. The answer to whether you can still get sunlight on a cloudy day is unequivocally yes. This invisible ultraviolet (UV) radiation penetrates the atmosphere, and while a heavy cloud layer reduces its intensity, a significant amount still reaches the ground, presenting a risk to unprotected skin.
How Clouds Filter and Scatter Ultraviolet Light
The light spectrum reaching Earth includes visible light and invisible ultraviolet (UV) radiation, which causes tanning and burning. Clouds, composed of water droplets and ice crystals, interact with UV radiation primarily through scattering and absorption. The extent of this interaction depends heavily on the cloud’s composition, thickness, and altitude.
Clouds do not provide full UV protection because they affect UV-A and UV-B differently. UV-A radiation has a longer wavelength and penetrates clouds more easily, reaching the ground in greater proportion. UV-B rays, which have a shorter wavelength, are more readily scattered and absorbed by cloud particles.
Cloud particles scatter UV light in multiple directions, a process that can sometimes lead to an unexpected increase in UV levels at the surface. When the sky is partly cloudy, the UV radiation reflected off the sides of the clouds can be scattered downward, increasing the total amount of UV hitting the ground, an effect known as the “broken-cloud effect.” Up to 90% of UV radiation can still pass through thin or scattered cloud cover. Even on a completely overcast day, clouds generally allow between 50% and 80% of UV rays to transmit to the ground.
The Real-World Impact on Sun Safety and Vitamin D
The high percentage of UV radiation that penetrates cloud cover affects sun safety. Unprotected skin can still sustain damage since the rays that cause sunburn are present even when the sun is not visibly shining. Light cloud cover offers little defense, and certain cloud conditions can even increase total UV exposure compared to a clear sky, making sunscreen a year-round necessity.
UV-B radiation, which causes sunburn, is also required for the body to synthesize Vitamin D. While brief, unprotected exposure is often sufficient on a sunny day, the diminished UV-B on a cloudy day can still facilitate synthesis, though less efficiently.
The amount of Vitamin D produced depends on cloud thickness, time of day, and latitude, since these factors determine how much UV-B successfully reaches the skin. While heavy cloud cover significantly reduces UV-B, thin or broken clouds may allow enough to pass for synthesis to occur, especially around midday. Dermatologists suggest obtaining Vitamin D through diet and supplements rather than relying on sun exposure, due to the persistent risk of skin damage.
Environmental Factors That Increase UV Exposure
Several environmental factors influence the total UV exposure received at ground level. The most significant of these is the angle of the sun, determined by the time of day and the season. UV intensity peaks during the four-hour period around solar noon, typically between 10 a.m. and 2 p.m., regardless of cloud cover.
Altitude also plays a role in intensifying UV radiation because the atmosphere is thinner at higher elevations, providing less air mass to absorb and scatter the radiation. For every 1,000 meters increase in altitude, UV levels can increase by approximately 10 to 12 percent. This means that a cloudy day in the mountains poses a greater UV risk than a similar day at sea level.
The reflection of UV light off surfaces further compounds exposure, even on an overcast day. Surfaces like water reflect about 10% of UV radiation, and light-colored sand reflects around 15%. Fresh snow is a particularly strong reflector, capable of bouncing back between 50% and 88% of UV rays, which can nearly double a person’s total exposure. This reflected light bypasses the overhead cloud filter, delivering UV radiation from below and the sides.