Do You Need Sunscreen on Cloudy or Cold Days?
TL;DR
- Clouds reduce UV radiation by only 0-69%, meaning 31-100% still reaches your skin depending on cloud type and thickness
- Cold temperatures have no effect on UV intensity—a clear winter day delivers the same UV as a clear summer day at similar solar angles
- Snow reflection can nearly double your UV exposure, making winter sun protection especially important at altitude
The Myth
A common assumption about sun protection goes something like this: "I don't need sunscreen on cloudy days because the sun isn't out. And definitely not in winter—it's cold! The sun isn't strong enough to cause damage when it's overcast or when temperatures are low."
This myth makes intuitive sense. We associate sun damage with feeling warm, seeing bright sunlight, and the sensations of summer. When skies are gray and temperatures are chilly, the whole experience feels different. The connection between sun and damage seems to disappear along with the warmth.
The confusion arises because we conflate visible light and heat (infrared radiation) with ultraviolet radiation. These are all part of the electromagnetic spectrum coming from the sun, but they behave differently and are perceived differently by our bodies.
The Reality
Understanding UV Radiation
Ultraviolet radiation is invisible to human eyes. We cannot see it or directly feel it the way we feel warmth. When you feel the sun warming your skin, that sensation comes from infrared radiation, not UV. When you see bright sunlight, that is visible light. UV radiation accompanies these but operates independently.
This matters because clouds and temperature affect these different types of radiation differently:
Infrared (heat): Clouds block infrared radiation quite effectively, which is why overcast days feel cooler. Temperature is also directly related to infrared energy. Cold days mean less infrared reaching you.
Visible light: Clouds diffuse and block visible light significantly. Overcast days are dimmer. This is what you see with your eyes.
Ultraviolet radiation: Clouds provide much less protection against UV than most people assume. UV radiation scatters and penetrates clouds more effectively than visible light or infrared.
What Clouds Actually Do
The amount of UV that reaches the ground through clouds depends on cloud type, thickness, and coverage:
Thin, high clouds (cirrus): These may block virtually no UV radiation. Under thin cirrus cover, you may receive 90-100% of the UV you would get under clear skies.
Moderate cloud cover: Typical overcast conditions reduce UV by roughly 25-50%. This means 50-75% of UV radiation still reaches you.
Thick, dark storm clouds: Heavy cloud cover can reduce UV by 60-70%. However, this still leaves 30-40% getting through—enough to cause damage over extended exposure.
Broken clouds: Patchy cloud cover can actually increase UV exposure at certain moments. When the sun appears in breaks between clouds, you receive direct UV radiation plus UV scattered from nearby cloud edges, potentially exceeding clear-sky levels briefly.
The bottom line: even on very overcast days, a substantial amount of UV radiation reaches your skin. A full day spent outside under clouds exposes you to meaningful cumulative UV, particularly during midday hours when the sun is higher.
Cold Weather Does Not Equal Low UV
Temperature and UV intensity are independent variables. Temperature depends on many factors including air mass, wind, humidity, and infrared radiation. UV intensity depends primarily on:
Solar angle: How directly the sun's rays hit the atmosphere. Higher sun angles mean shorter paths through the atmosphere and more intense UV.
Altitude: Higher elevations mean less atmosphere to filter UV. UV intensity increases approximately 10-12% per 1,000 meters of elevation gain.
Latitude and season: These affect solar angle. Summer has higher UV because the sun is higher in the sky.
Ozone levels: Atmospheric ozone absorbs UV-B. Ozone varies by location and season.
Surface reflection: Snow, water, sand, and concrete reflect UV back up toward you, increasing total exposure.
Notice that temperature does not appear on this list. A clear day in January delivers the same UV intensity as a clear day in July at the same solar angle. If you are skiing at altitude on a clear winter day, you may receive more UV than someone at a summer beach at sea level.
The Winter Sun Trap
Several factors make winter sun exposure deceptive:
Lower perceived threat: Cold air and reduced infrared radiation mean you don't feel the warmth that usually warns you about sun exposure. Your body's heat-based warning system fails you.
Extended exposure: Winter activities like skiing, snowshoeing, and ice skating keep people outside for hours. Without the heat cue, people often stay out longer than they would in summer.
Snow reflection: Fresh snow reflects up to 80% of UV radiation. This means UV hits you from above and below, nearly doubling your exposure. Your face, especially the underside of your chin and nose, receives UV from angles that rarely occur in other seasons.
Altitude: Many winter activities occur at elevation where UV is naturally more intense due to thinner atmosphere.
This combination results in some of the worst sunburns occurring during winter skiing vacations—situations where people felt cold the entire time they were being damaged.
How to Respond
When someone suggests they don't need sun protection on cloudy or cold days, offer this perspective:
Explain the distinction: "The warmth you feel from the sun is infrared radiation, which is different from UV. UV is invisible and can penetrate clouds much better than heat or visible light. You can be receiving significant UV exposure even when you feel cold or see gray skies."
Provide the numbers: "Depending on cloud cover, 30-100% of UV radiation still reaches you on overcast days. That's enough to cause damage during extended outdoor time."
Use winter examples: "Some of the worst sunburns happen during ski vacations. Cold air, no feeling of heat, but hours of UV exposure at altitude with snow reflecting it back up at you. The cold makes people forget about sun protection entirely."
Emphasize cumulative exposure: "Even if each cloudy day delivers less UV than a sunny day, it still adds up. Skin damage and skin cancer risk come from cumulative lifetime exposure, not just beach vacations."
Key Takeaways
- Clouds reduce UV radiation by 0-69% depending on type and thickness—substantial UV still penetrates even on overcast days
- Temperature has no relationship to UV intensity; cold days can deliver just as much UV as warm days
- The absence of warmth on your skin does not indicate absence of UV radiation
- Snow reflects up to 80% of UV, nearly doubling exposure during winter activities
- Higher altitude increases UV intensity by 10-12% per 1,000 meters
- Cumulative UV exposure from "low-risk" days adds up to significant skin damage over time
- Year-round sun protection is appropriate for anyone spending extended time outdoors, regardless of season or weather
Practical Year-Round Protection
Daily routine: Apply sunscreen to exposed skin (face, hands, neck) as part of your morning routine regardless of weather forecast. Weather changes, and UV is present during all daylight hours.
Winter sports: Apply sunscreen before heading out and reapply every two hours or after sweating heavily. Don't forget lips (use SPF lip balm), ears, and the underside of your nose and chin where reflected UV hits.
Overcast outdoor activities: If you'll be outside for more than 30-60 minutes, even under clouds, sun protection is warranted. Gardening, walking, running, sports—all accumulate UV exposure.
Check the UV index: Weather apps and forecasts often include UV index predictions. A UV index of 3 or above warrants sun protection. This can occur even on cloudy days, especially during spring and summer.
Clothing as backup: On days when sunscreen seems like too much effort, sun-protective clothing, hats, and sunglasses provide reliable protection without reapplication.
FAQ
Q: Is there ever a time when UV radiation is truly negligible?
A: At night, UV radiation from the sun is absent—this is the only time of day with zero UV exposure. At very high latitudes during winter months, the sun may be so low that UV is minimal for portions of the day. In most inhabited regions, however, UV is present during all daylight hours year-round, with intensity varying by factors like solar angle and weather.
Q: How can I tell how much UV I'm actually getting on a cloudy day?
A: The UV index forecast provides the best guidance. Weather services predict UV levels accounting for cloud cover, ozone, and solar angle. Mobile apps display real-time UV index for your location. An index of 3-5 is moderate (protection recommended for extended outdoor time), 6-7 is high (protection essential), and 8+ is very high to extreme.
Q: Do car windows and indoor spaces protect me from UV?
A: Standard glass blocks most UV-B (the type causing sunburn) but allows much UV-A (contributing to aging and cancer) to pass through. Extended time near windows or driving in a car still exposes you to UV-A. This is why some dermatologists recommend daily facial sunscreen even for people who spend most time indoors or in vehicles.
