When is UV strongest during the day?

Around solar noon, across the 10 a.m. to 4 p.m. window, when the sun is highest. A quick check: if your shadow is shorter than your height, UV is strong.

What time of year has the highest UV?

Near the summer solstice, when the midday sun climbs highest. That peak comes before the hottest weeks, so early summer often carries higher UV than late summer.

Does altitude increase UV?

Yes. Ground-level UV rises about 10% for every 1,000 meters (3,300 feet) of elevation, because thinner air filters less. Mountain sun burns faster than the same latitude at sea level.

Can you get UV exposure through clouds?

Yes. Up to 80% of UV passes through light cloud, and haze can scatter extra UV. Because UV does not feel warm, a cool overcast day can still cause sunburn.

Does snow increase sun exposure?

A lot. Fresh snow reflects as much as 80% of UV back at you, nearly doubling exposure, which is why snow sports cause fast burns. Sand reflects about 15% and water around 10%.

Is UV higher near the equator?

Yes. The closer to the equator, the more directly the midday sun strikes, so UV runs higher year-round. Of surface UV, about 95% is UVA and 5% UVB.

When Is UV Strongest? Time of Day, Season, and Latitude

UV is not steady through the day or the year. It peaks when the sun is highest, so the strongest exposure comes at solar noon and across the 10 a.m. to 4 p.m. window. Time of day is the single biggest lever on how much UV reaches you.

Across the year, UV peaks near the summer solstice, not in the hottest month — the air keeps warming into late summer even as the sun starts dropping. Latitude, altitude, clouds, and reflective ground then push the local figure up or down from there. The EPA forecast already accounts for sun angle and elevation.

Time of day: the midday window

Most of a day's UV arrives in the hours around solar noon, when the sun is highest and its rays pass through the least atmosphere. CDC uses a shadow rule of thumb: when your shadow is shorter than you are tall, the sun is strong and UV is high. Planning outdoor activity before 10 a.m. or after 4 p.m. avoids the worst of it.

Season and latitude

UV is strongest near the summer solstice because the midday sun climbs highest then. That peak runs ahead of the temperature peak, so June often carries higher UV than a hotter, hazier August. Latitude sets the baseline: the closer you are to the equator, the more directly the sun strikes and the higher UV runs year-round.

The ozone layer absorbs most incoming UVB, and thinning ozone raises ground-level UV. Of the UV that does reach the surface, about 95% is UVA and roughly 5% is UVB, the burn-and-vitamin-D band.

Altitude and reflection

Thinner air at altitude filters less UV: ground-level exposure rises by roughly 10% for every 1,000 meters (about 3,300 feet) of elevation. Reflective surfaces add more on top. Fresh snow bounces back as much as 80% of UV, dry sand about 15%, and open water around 10%, which is why mountain and beach days burn fast.

Clouds don't always help

Cloud cover is unreliable shade. As much as 80% of UV passes through light or broken cloud, and bright haze can even scatter extra UV toward you. Because UV does not warm your skin the way infrared does, an overcast, cool day can still deliver a serious burn. The reliable signal is the UV Index, not how warm or bright it feels.

Check your local UV index

UV is driven by latitude, elevation, and clear skies, so it runs hardest in the Sun Belt and the high mountain West. These metros see some of the country's strongest UV — check the current forecast for each, or look up any US city on the UV index hub:

Honolulu, HI UV index — A tropical latitude keeps the UV Index in the Very High to Extreme range for much of the year.
Miami, FL UV index — Low latitude and strong sun push midday UV to Extreme through the long summer.
Phoenix, AZ UV index — Desert sun and reliably clear skies produce some of the highest UV readings in the country.
Las Vegas, NV UV index — Clear desert skies keep midday UV very high from spring through fall.
El Paso, TX UV index — High-desert elevation and clear air raise UV well above the national average.
Albuquerque, NM UV index — Mile-high elevation and clear skies make for intense midday UV.

Frequently asked

When is UV strongest during the day?: Around solar noon, across the 10 a.m. to 4 p.m. window, when the sun is highest. A quick check: if your shadow is shorter than your height, UV is strong.
What time of year has the highest UV?: Near the summer solstice, when the midday sun climbs highest. That peak comes before the hottest weeks, so early summer often carries higher UV than late summer.
Does altitude increase UV?: Yes. Ground-level UV rises about 10% for every 1,000 meters (3,300 feet) of elevation, because thinner air filters less. Mountain sun burns faster than the same latitude at sea level.
Can you get UV exposure through clouds?: Yes. Up to 80% of UV passes through light cloud, and haze can scatter extra UV. Because UV does not feel warm, a cool overcast day can still cause sunburn.
Does snow increase sun exposure?: A lot. Fresh snow reflects as much as 80% of UV back at you, nearly doubling exposure, which is why snow sports cause fast burns. Sand reflects about 15% and water around 10%.
Is UV higher near the equator?: Yes. The closer to the equator, the more directly the midday sun strikes, so UV runs higher year-round. Of surface UV, about 95% is UVA and 5% UVB.