What is the UV index?

The ultraviolet index (UVI) measures how intense UV radiation is at any given moment. It ranges from 0—meaning minimal risk—to 11 and beyond, which represents extreme danger from unprotected sun exposure. The higher the UVI, the stronger the sun's UV rays and the quicker skin damage can happen. This is why taking protective steps becomes critical when the UV index climbs.

Stay informed about UV radiation in your area with our UV monitoring tool. Simply enter your zip code or city and state to get hourly ultraviolet radiation forecasts tailored to your location.

UV index scale

The UV index scale provides a standardized way to understand how intense UV radiation is and what level of risk it poses to your skin.

The sun delivers vital warmth and light, yet its ultraviolet (UV) radiation poses real risks to your skin. While the ozone layer offers some protection by filtering UV rays, several factors—including ozone depletion, seasonal changes, and weather conditions—affect how much radiation reaches the surface.

Understanding the UV index is essential for safeguarding your skin. This helpful tool lets you assess UV risk in your area and take the right protective steps.

UV index range

The UV index scale ranges from 1 to 11+, with each level indicating progressively greater risk from sun exposure.

0-2 (Low): little to no risk of sunburn or skin damage.
3-5 (Moderate): increased risk of sunburn with extended exposure.
6-7 (High): significant risk of burns and skin damage in a short time.
8-10 (Very high): severe risk of painful burns and long-term skin damage with minimal sun exposure.
11+ (Extreme): extreme danger of severe burns and irreversible skin damage that increases melanoma risk.

Protection recommendations

Below are protective clothing options suited to different UV index levels:

Level	Recommendations
Low (0-2)	Sunglasses and a wide-brimmed sun hat offer excellent protection during low UV index days. Apply sunscreen with at least SPF 15 if you plan to spend extended time outdoors, even when UV levels are minimal.
Moderate (3-5)	Seek shade near midday, wear protective clothing (e.g., long sleeves, pants), a wide-brimmed hat, and sunglasses. Consider SPF 30+ sunscreen for fair skin.
High (6-7)	When UV levels are high, limit time in direct sunlight between 10 a.m. and 4 p.m., when UV rays are strongest. Apply broad-spectrum SPF 50 sunscreen generously to all exposed areas and reapply every two hours or after swimming. Opt for sun-protective swimwear designed to block UV rays, and always wear a wide-brimmed hat and quality sunglasses that offer UV protection.
Very High (8-10)	At very high UV levels, take extra precautions by staying indoors or in shade during peak hours from late morning through mid-afternoon. When you must go outside, apply SPF 50+ broad-spectrum sunscreen liberally and reapply frequently. Layer up with long-sleeved protective clothing, a wide-brimmed hat, and UV-blocking sunglasses to shield your skin from intense rays that can cause rapid and severe damage.
Extreme (11+)	During extreme UV conditions, prioritize staying indoors or in deep shade whenever possible. If outdoor exposure is unavoidable, apply SPF 70+ broad-spectrum sunscreen to all exposed skin and reapply every hour. Wear tightly woven, long-sleeved clothing, long pants, a wide-brimmed hat with neck protection, and sunglasses that block 100% of UV rays. These precautions are essential to prevent severe burns and long-term skin damage.

When UV levels are moderate, timing is everything. Schedule outdoor activities during early morning or late evening hours when the sun's intensity is lower. Choose breathable, lightweight fabrics in long sleeves and pants to stay cool while protecting your skin. Complete your sun-safe outfit with a wide-brimmed hat and UV-protective sunglasses, and don't forget to apply SPF 30+ sunscreen to any exposed areas, particularly if you have fair or sensitive skin that burns easily.

The UV index scale is a practical tool for understanding your daily sun exposure risk. By following protection guidelines matched to the forecasted UV intensity, you can minimize painful sunburns and prevent long-term skin damage that may result in skin cancer. The scale offers clear recommendations for the protective steps to take at each risk level.

How is the UV index calculated?

The UV index is determined through a sophisticated calculation that measures the intensity of UV radiation at Earth's surface. Scientists and environmental agencies track multiple variables to predict and forecast UV levels for the coming day.

Sun angle
Cloud cover
Ozone layer thickness
Ground albedo (ability of a surface to reflect sunlight)

These individual measurements are combined into a single calculation to produce the final UV index value. The result is expressed as a whole number ranging from 0 to 11+, where higher values represent greater risk of sun damage.

What factors influence UV index levels?

The same variables that determine UV index calculations also shape UV levels in your area. Understanding each factor helps explain why UV intensity varies day to day and location to location.

Ozone layer density

The stratospheric ozone layer serves as a vital shield, blocking approximately 90% of the sun's ultraviolet radiation. When ozone becomes depleted due to pollution, more UV rays pass through and reach Earth's surface.

Researchers continuously track changes in ozone levels to forecast their impact on daily UV index values. A reduction in ozone thickness results in elevated index readings. Weather organizations collect stratospheric data through satellites and incorporate ozone depletion information into their UV index calculations.

Cloud cover

Clouds function as natural barriers that block UV rays from reaching the surface. Weather forecasters incorporate cloud predictions into UV index models. Cloudy days typically show lower UV readings compared to clear, sunny days.

Altitude and latitude

Locations at higher elevations and closer to the equator experience stronger UV radiation due to more direct sun exposure. For example, high-altitude Denver receives more intense UV than sea-level New Orleans. Similarly, moving toward the equator increases UV index values year-round as the sun's rays strike the surface more directly.

Sun angle

How high the sun sits in the sky determines how much UV exposure a location receives at any moment. During summer when the sun climbs higher, UV rays strike the ground at a steeper angle, producing elevated index values. Your location's longitude and latitude are key factors in calculating this sun angle.

Ground reflection

Reflective surfaces such as snow and white sand bounce back more than 80% of UV rays, which can nearly double your exposure. Less reflective surfaces like grass, soil, and water bounce back less than 10%. UV index calculations account for ground reflection to reflect these geographic differences. A fresh layer of snow can cause UV index values to jump significantly.

Pollution and particulates

Fine particles in air pollution can absorb and scatter UV radiation. On days with very high pollution, the UV index may drop slightly, but hazy conditions still permit substantial direct radiation to pass through. Understanding how pollution affects UV index calculations remains an active area of scientific research.

What are the different types of UV radiation?

There are three main types of UV radiation:

UVA rays comprise the bulk of UV radiation reaching Earth's surface. These rays penetrate deeply into skin layers and are associated with premature aging, wrinkles, and skin cancer development.
UVB rays don't reach as far into the skin as UVA rays, but they're the main culprit behind sunburns and skin redness. These rays can damage DNA in skin cells, increasing cancer risk.
UVC rays carry the highest energy of all three types but are completely blocked by the ozone layer before reaching Earth's surface. Artificial UVC rays are utilized in sterilization applications for equipment and surfaces.

What is the normal UV index?

There's no single "normal" UV index—ultraviolet levels shift constantly. Rather than a fixed number, the UV index represents a dynamic spectrum shaped by multiple changing factors.

Seasonal shifts: The amount of UV radiation reaching the ground varies significantly throughout the year. Winter brings a lower sun position in the sky, which creates a 2-3 point shift in UV index values compared to summer levels.

Geographic differences: UV index values vary significantly based on location, latitude, and elevation. Florida typically maintains a year-round index of 6-8, while Alaska sees summer peaks around 3-5 and winter readings barely reaching 1.

Atmospheric dynamics: The atmosphere plays a key role in determining UV transmission. A depleted ozone layer allows more rays to penetrate, while clouds provide natural blocking. Surface reflection can amplify exposure, and airborne pollution and particles also affect UV intensity.

What is the highest UV index in the world?

The highest UV index ever documented reached 43.3 in Bolivia on December 29, 2003, at the Licancabur volcano—an extreme level that creates severe danger from unprotected sun exposure.

The most intense UV levels globally are found in tropical regions and at high altitudes.

Several regions around the world experience dangerously high UV index levels:

The Altiplano plateau spanning Chile, Argentina, and Bolivia sits at elevations exceeding 12,000 feet, where UV index values regularly surpass 20 during summer months.
Sub-Saharan Africa, particularly Namibia, experiences extreme UV exposure due to its high elevation and equatorial location. UV index levels often exceed 15 during midday hours.
Australia experiences intense UV radiation in certain regions due to ozone layer depletion above the continent. Some areas record peak UV index levels exceeding 12 on a daily basis throughout summer.
High-altitude equatorial regions including Kenya, Indonesia, and Ecuador consistently experience UV index values of 14 or higher during midday hours.

If you're traveling to high-risk UV areas during peak sun hours, take protective measures against excessive exposure. Wear protective clothing, avoid spending time outdoors during midday, and apply sunscreen regularly. Prolonged unprotected skin exposure in these regions can rapidly cause sunburn, permanent skin damage, and increase skin cancer risk.