The UV index measures the strength of ultraviolet radiation from the sun on a scale from 0 to 11+. It determines how much sun protection you need — both for your skin and in your outfit choices. A UV index of 3 means casual sun protection; a UV index of 9 means you should avoid direct sun exposure entirely.
Understanding the UV Index Scale
- 0-2 (Low): No protection needed. You can be outside without sunscreen or special clothing.
- 3-5 (Moderate to High): Sunscreen (SPF 30+) recommended. A hat and sunglasses are useful.
- 6-7 (High): Seek shade during midday (10 AM-4 PM). Wear protective clothing, hat, and sunglasses.
- 8-10 (Very High): Avoid sun exposure. Wear UPF clothing, hat, and sunglasses. Limit outdoor time.
- 11+ (Extreme): Dangerous sun exposure. Avoid all unnecessary outdoor time.
Why Clouds Don't Stop UV
A common misconception is that clouds block UV rays. They don't. Up to 80% of UV radiation passes through clouds, which means you can get sunburned on a cloudy day just as easily as a sunny day. The difference is that cloudy days feel cooler, so you might not notice sun damage happening until it's too late.
Clothing and UV Protection
Not all fabrics provide equal UV protection. Dense, dark fabrics like denim or wool block UV better than light, thin fabrics like linen. UPF-rated clothing is designed to block ultraviolet radiation — UPF 50 blocks 98% of UV rays.
What to Wear at Different UV Levels
UV Index 0-2: Any outdoor clothing works. Standard summer wear is fine.
UV Index 3-5: Choose light colors (they reflect UV), wear sunglasses, and consider a hat. Long-sleeve shirts aren't necessary unless you'll be out for hours.
UV Index 6-7: Wear a wide-brimmed hat (minimum 3-inch brim), long sleeves or a rash guard, and UV-blocking sunglasses. Light colors are better than dark ones, which absorb heat.
UV Index 8+: Plan outdoor time for early morning or evening. If you must be out during peak UV hours, wear full coverage — long sleeves, long pants, hat, and sunglasses. Consider UPF-rated athletic wear.
The Relationship Between Temperature and UV
High UV doesn't always mean hot. On cool, sunny days with a clear sky, UV can be dangerously high while the temperature is mild. This is especially true at high elevations or near water (which reflects UV). You might wear a light sweater and still get burned.
Time of Day Matters
UV strength peaks between 10 AM and 4 PM, regardless of temperature. An afternoon walk at 75°F might have higher UV exposure than a midday walk at 85°F if clouds develop. Outdoor workout timing should consider UV index, not just temperature.
Checking the UV Index
Most weather apps show the UV index. Check it along with temperature to plan your outfit fully. A 72°F day with UV index 9 requires very different clothing than a 72°F day with UV index 2.