It’s a valid question after all. On a cloudy day, you won’t feel the sun beating on your skin, and therefore, why would you need sunscreen? But, you might be surprised to find out just how much UV radiation can penetrate cloud cover. Here, we tap leading dermatologists to weigh in on the question we frequently get from readers, our family members and beyond: Do you need sunscreen when it’s cloudy? Or what about when the UV Index score is lower than 3? This is also a valid question. Let’s discuss.
Featured Experts
- Dr. Deborah Longwill, a board-certified dermatologist based in Miami
- Orit Markowitz, MD, a board-certified dermatologist based in New York
How Is the UV Index Score Determined?
According to New York dermatologist Orit Markowitz, MD, the U.S. National Weather Service calculates the UV Index. “It uses a computer model that relates the ground-level strength of solar ultraviolet (UV) radiation,” she explains. Miami dermatologist Dr. Deborah Longwill says the World Meteorological Organization, Environmental Protection Agency and World Health Organization are also involved in determining the Index.
“The UV Index is a standardized measure of the strength of ultraviolet (UV) radiation from the sun at a particular place and time,” Dr. Longwill explains. “Several factors contribute to the UV Index Score, including 1.) Ozone Levels. This is the concentration of ozone in the atmosphere, which absorbs much of the sun’s harmful UV radiation. 2.) Solar elevation angle, which is the height of the sun in the sky, affecting the intensity of UV radiation. 3.) Cloud cover, which is the amount and type of cloud cover, which can reduce UV radiation. 4.) Altitude, as higher altitudes receive more intense UV radiation due to the thinner atmosphere. 5.) Surface Reflection. Surfaces like water, sand and snow can reflect UV radiation, increasing exposure.”
The time of day also affects UV radiation. “It is typically strongest between 10 a.m. and 4 p.m. because there is more direct sunlight and less atmospheric scattering,” says Dr. Longwill. “UV radiation peaks at midday when the sun is highest in the sky, and lessens in the early morning and late afternoon,” Dr. Markowitz adds. Latitude is a factor in UV radiation as well. “UV rays are strongest at the equator and decline toward the North Pole and South Pole,” adds Dr. Markowitz.
The UV Index Scores, Explained
Here is a breakdown of the UV Index and what each “score” means. You can find the UV Index Score on your weather app or a national weather website.
- Low exposure (green): 1-2
- Moderate exposure (yellow): 3-5
- High exposure (orange): 6-7
- Very high exposure (red): 8-10
- Extreme exposure (violet): 11+
Do You Still Need Sunscreen When It’s Cloudy?
Yes, says Dr. Longwill. “The UV Index score is typically lower on cloudy days because clouds can block and scatter UV radiation, reducing its intensity,” she says. However, UV radiation, especially UVA rays, can penetrate clouds and cause skin damage, even sunburn. “Clear skies allow virtually 100 percent of UV rays to pass through,” Dr. Markowitz explains. “Scattered clouds transmit 89 percent of UV rays, broken clouds transmit 73 percent, and overcast skies transmit 31 percent.” Most people think they don’t need sunscreen on a cloudy day (data supports this), but these numbers prove otherwise. “The majority of UV damage occurs on these types of days when people do not realize they need to protect and wear sunscreen,” adds Dr. Markowitz.
Do You Still Need Sunscreen When the UV Is Low?
Dr. Markowitz says we still need sunscreen to prevent UV exposure, even during a low-risk day. “Sun damage can occur at low UV levels due to cumulative exposure over time,” Dr. Longwill explains. “Even low levels of UV radiation can contribute to skin damage, premature aging and increased risk of skin cancer. Consistent use of sunscreen helps protect against long-term damage.”
