How to read the UV index, and when to change your plans

Every weather app carries a small number labeled UV, and almost nobody uses it. Temperature decides what people wear and whether they bring a jacket; the UV index rarely changes anyone's plans. It should, because it is the one number in the forecast that predicts how quickly unprotected skin will burn that day, and unlike heat, ultraviolet exposure cannot be felt until the damage is already done.

The index is a standardized measure, developed with the World Health Organization, of the strength of sunburn-producing ultraviolet radiation reaching the ground, reported on a scale that starts at 0 and is open-ended above 11. Several things drive it: how high the sun climbs (which depends on the season, the time of day, and your latitude), the ozone overhead, cloud cover, and altitude. The value in the forecast is typically the day's peak, which arrives around solar noon, roughly 1 p.m. during daylight saving time, not at the hottest hour of the afternoon. That mismatch matters: UV peaks earlier than heat does, so the pleasant late-morning hours are often stronger than they feel.

The thresholds are worth memorizing because they map to action. At 0 to 2, exposure is low and most skin tolerates ordinary time outside. From 3 to 5, protection becomes sensible for anyone spending extended time outdoors: sunscreen, sunglasses, and shade at midday. At 6 and 7, unprotected fair skin can burn in well under half an hour, and the full toolkit earns its keep, a hat included. From 8 to 10, burns arrive in as little as 15 minutes and the practical advice is to shift outdoor plans earlier or later in the day. At 11 and above, which is routine in summer at low latitudes and high altitudes, unprotected skin burns in minutes. The Environmental Protection Agency publishes a plain-language version of the scale at epa.gov, and it is a two-minute read that outlasts every sunscreen trend.

A few things the number does not capture. It assumes level ground: sand, water, and especially snow reflect ultraviolet light back at you and can add meaningfully to the dose, which is how people burn under an umbrella at the beach. Thin cloud passes most UV, which is why a gray sky is not protection, a point covered in sunburn myths that make it worse. And the index describes the sun, not you: fair skin, childhood skin, and skin on photosensitizing medications all burn at lower doses than the population average the scale imagines. If any of those apply, mentally bump the index up a category.

There is also a serviceable backup when you have no forecast at all: the shadow rule. If your shadow is shorter than you are, the sun is high and UV is strong; if it is longer than you are, exposure is easier to manage. It is crude, but it is the same physics the index formalizes.

The practical routine is simple. Check the UV peak the way you check the temperature, treat 3 as the line where sunscreen becomes non-negotiable and 8 as the line where midday plans deserve rescheduling, and apply the strictest version of all of it to children, whose burns carry the most lifetime weight. And if the forecast wins anyway, the playbook for the first 24 hours of a sunburn is the place to start.

Related reading: A daily routine for sun-prone and sun-damaged skin and How sunburns raise your skin cancer risk.