The UV index, explained simply

Raydrop · July 19, 2026

The UV index measures how strong ultraviolet radiation is at ground level, at a given place and time. The scale starts at zero and technically has no ceiling: in the tropics in the middle of summer, it regularly climbs past 11. According to the World Health Organization, the rule is simple: the higher the UVI, the greater the potential for damage to skin and eyes, and the less time it takes for harm to occur. Raydrop leans on that exact same index, in real time, to decide when to nudge you to slather up.

What the UV index actually measures

The UV index isn't a temperature reading or a "feels like" sunny-day score: it quantifies the intensity of UV radiation, the stuff that burns skin and damages eyes without you necessarily feeling it happen. A hazy sky or a cool day can still post a high index. That's exactly what makes the index useful: it gives you an objective number where gut feelings mislead, and that's the number Raydrop watches for you, not the "feels like" forecast.

WHO and the health agencies that follow its standard, like the U.S. Environmental Protection Agency, sort that number into bands. In everyday language, you'll see five levels: low, moderate, high, very high, extreme. But the line that should actually change your behaviour comes down to three zones:

• 1 to 2: broadly low risk for most skin types, no particular precaution needed.
• 3 to 7: WHO recommends seeking shade during the hottest hours, covering up skin and eyes, and wearing sunscreen.
• 8 and above: unprotected exposure gets genuinely risky, and shade stops being a nice-to-have. It becomes the rule.

Why 3 is the threshold that matters

That number isn't arbitrary. It's exactly the forecast level, 3 or higher, at which health authorities start issuing sun-protection alerts. In plain terms: under 3, most skin types can be outside without much fuss. From 3 up, the risk of skin damage stops being theoretical. That's why Raydrop wakes up at that threshold: below it, the app stays quiet, it won't nag you to protect yourself from sun that doesn't warrant it yet.

That threshold of 3 also changes the question you're asking. Below it, you're wondering whether you feel like sunscreen. Above it, the question isn't "if" anymore, it's "how often", and that's exactly where a timer stops being decorative and starts being useful.

A number that shifts across the day

The UV index isn't flat over a day: it climbs through the morning, peaks around solar noon, then eases off in the late afternoon. Specifically, WHO calculates the reported index as the maximum level expected over the four-hour window centred on solar noon, the moment the sun sits highest in the sky, which isn't necessarily 12 pm on your watch. That's exactly why an 11 am plan and the same plan at 2 pm don't carry the same risk, even under an identical sky. A single "daily" index hides a real curve, rising then falling.

And a number that shifts with season and latitude

Two more factors carry real weight, beyond the hour. Season, first: the sun sits higher in summer, its rays travel through a thinner slice of atmosphere, and the index climbs noticeably compared with winter, even at the same latitude. Latitude, next: the closer you get to the equator, the higher the sun sits year-round, and the higher the baseline index stays, even in winter in some regions, while further from the equator the index drops off sharply outside summer. Altitude and reflection matter too: thinner mountain air filters out less UV, and snow or calm water bounce some of the radiation back onto your skin. That's how people end up with sunburns they never saw coming, up a mountain in winter or out on the water in summer.

How Raydrop uses it

Raydrop doesn't run on a seasonal average or a rough guess: the app queries Apple's weather service for the real UV index at your location, for the hour you're actually outside. Below the threshold of 3, it stays quiet. Above it, it tightens the sunscreen reminder cadence as the index climbs through the middle of the day, and eases off as it drops in the late afternoon. That's the whole gap between a fixed two-hour timer, the same whether it's overcast or blazing, and a timer that actually follows today's sky, hour by hour.

The rest comes down to the sunscreen itself: how often to reapply once you're above the threshold, and how skin type changes that pace from one person to the next in the same family. That's the subject of the next article.