The UV Index turns invisible ultraviolet radiation into a number you can act on. Read the value for the right time, then use shade, clothing, eye protection, and sunscreen when the index reaches 3.
A current reading describes conditions now. A daily forecast usually reports the highest value expected around solar noon. Check which one you are viewing before you plan time outdoors.
What Is the UV Index?
The UV Index is an international scale for the strength of ultraviolet radiation from the Sun at a particular place and time. The World Health Organization (WHO) and the World Meteorological Organization (WMO) standardised the categories so that the same number means the same risk worldwide.
The index is weighted towards the wavelengths most likely to damage skin. It is therefore a guide to the risk of sunburn and to longer-term harm to the skin and eyes, not a measure of visible brightness or of how warm the air feels.
The scale starts at 0 and has no fixed upper limit. A higher number means stronger UV and a greater risk from unprotected exposure. Values of 11 or more are classed as extreme. In high mountains near the equator, the index can rise well above 11.
UVA, UVB and UVC
Ultraviolet radiation is split by wavelength. Only some of it reaches the ground, and only part of that drives the UV Index.
UVC
UVC covers wavelengths from 100-280 nanometres. The atmosphere absorbs it before it reaches the surface, so it does not contribute to the outdoor UV Index.
UVB
UVB covers 280-315 nanometres. It is the main cause of sunburn. Stratospheric ozone absorbs a large share of it, so the amount of ozone overhead has a strong effect on UVB at the ground. UVB also drives most of the skin’s vitamin D production.
UVA
UVA covers 315-400 nanometres. More of it reaches the surface than UVB. It penetrates deeper into the skin and contributes to ageing and to some longer-term damage. Ozone absorbs UVA much less than UVB. The UV Index weighting peaks in UVB and extends into UVA, so it is a sunburn guide, not a complete UVA dose.
UV Index Levels
The five categories use the same numerical ranges worldwide. WHO advice is to use sun protection from 3 upward, with extra care as the index rises.
- 0-2, Low: Protection is usually not needed for brief exposure. Sunglasses can help on bright days, especially near snow or water.
- 3-5, Moderate: Seek shade around solar noon. Cover exposed skin, wear eye protection, and use sunscreen.
- 6-7, High: Reduce time in direct sun around solar noon. Use shade, clothing, a hat, sunglasses, and sunscreen.
- 8-10, Very High: Plan outdoor activities away from the strongest midday UV where possible, and use all forms of protection.
- 11 or higher, Extreme: Avoid direct midday sun where possible, and use all forms of protection when outdoors.
How quickly skin burns is not a fixed countdown. It varies with skin type, UV strength, exposure history, medicines, altitude, reflected radiation, clothing, and sunscreen use. Darker skin can take longer to burn, but it can still suffer UV damage, including to the eyes.
Some medicines and skin products can increase sensitivity to sunlight. Read the label and ask a doctor or pharmacist about extra precautions. Do not stop a prescribed medicine unless the clinician who manages it tells you to.
What Affects UV Levels?
The UV Index can be high even when the air feels cool. Temperature is not a UV meter. Several factors determine how much ultraviolet radiation reaches the ground.
Time of day
UV is generally strongest during the four hours around solar noon, when the Sun is highest and its path through the atmosphere is shortest. Morning and late-afternoon levels are usually lower. Solar noon is not always 12:00 on the clock. It depends on longitude, the time of year, and whether clocks are on summer time. Sunrise and Sunset: Times, Twilight and Day Length explains how solar timing shifts through the year.
Season and latitude
In the middle latitudes, UV is usually stronger in summer than in winter because the Sun sits higher and the day is longer. Closer to the equator, high levels can occur throughout the year. At high latitudes in winter, the Sun stays low and UVB can fall so far that vitamin D synthesis in skin becomes negligible. That seasonal drop does not make summer UV safe.
Altitude
UV increases with elevation because there is less atmosphere above you to absorb it. WHO guidance is that UV typically rises by about 10-12% for every 1,000 metres of altitude. Mountain locations and ski areas can therefore have strong UV even in cold weather.
Cloud cover
Thick cloud often reduces UV, but clouds do not block it completely. Thin high cloud may cut only a small fraction. Broken cover can still allow substantial exposure, and bright cloud sides can reflect extra UV. Cloud Cover Explained: Percentages, Oktas and Ceilings describes how cover amount is reported. A high cloud percentage does not by itself mean a low UV Index.
Reflective surfaces
Snow, water, sand, and other bright surfaces can reflect UV towards your skin and eyes. Fresh snow can reflect as much as 80% of UV, which is why ski fields can give intense exposure to the face and eyes. Dry sand and sea foam reflect less than snow but still add to the total. Water reflects a smaller fraction, more so when the Sun is low. Shade that leaves you exposed to a bright surface is only partial protection.
Ozone
Stratospheric ozone absorbs much of the incoming UVB. When the ozone column overhead is thinner, more UVB reaches the ground and the UV Index rises. This is the ozone layer, not the ground-level ozone that forms in polluted air. Do not treat a smog forecast as a UV forecast.
Shade, Temperature and Vitamin D
A cool breeze does not weaken UV. Temperature Explained covers how air temperature is measured in the shade. Direct sun can heat skin and surfaces while the official air temperature stays modest, and the reverse also occurs: a hot, hazy, or windy day is not automatically a high-UV day.
Shade reduces direct UV, but open shade still leaves you under a large sky that scatters ultraviolet radiation. Trees, beach umbrellas, and light canopies block only part of that scattered and reflected load. Deep shade, closely woven clothing, and a brimmed hat reduce exposure more than a small patch of shadow on bright ground.
UVB in skin can produce vitamin D, but that is not a reason to skip protection once the index reaches 3. You do not need to burn, or to tan, to obtain vitamin D. Diet and supplements can supply it without the skin and eye risks of overexposure. In winter at higher latitudes, outdoor UVB is often too weak for reliable synthesis anyway. Treat vitamin D as a nutrition question, not as a UV-Index exception.
How to Protect Yourself
At a UV Index of 3 or above, combine several forms of protection rather than relying on sunscreen alone.
Seek shade, especially around solar noon. Remember that trees and umbrellas do not block all scattered or reflected UV.
Wear closely woven clothing, a wide-brimmed hat, and wraparound sunglasses that protect against UVA and UVB.
Apply broad-spectrum SPF 30 or higher sunscreen generously to skin that clothing does not cover. Follow the product instructions and reapply at least every two hours, as well as after swimming or sweating.
Do not use sunscreen to extend the time you spend in direct sun.
Give children extra protection because UV damage accumulates over a lifetime.
If your shadow is shorter than you are, the Sun is high and UV is likely to be stronger. It is a useful prompt to find shade, but the local UV Index is the better guide.
How to Read a UV Forecast
Decide first whether you are looking at a current-time estimate or a daily maximum. The daily maximum is usually the value around solar noon. It is the right number for planning a midday outing. It is the wrong number for an early-morning walk, when the index is often still low.
If an hourly series is available, read the rise, the peak, and the decline. Protection from 3 still applies in the shoulder hours if the index has already reached that level. A single daily figure cannot tell you when the index will cross 3.
Forecast models combine solar elevation, ozone, cloud, altitude, and surface reflectivity to estimate the index. How Weather Forecasts Work explains why local predictions can change as the input data change. Cloud timing is often the largest short-term uncertainty. A clearer or more broken sky than expected can leave UV higher than a cloudy-looking forecast suggested.
The UV Index is a model estimate or a nearby measurement, not a personal burn timer. Skin type, time already spent outdoors, reflection, and clothing change the outcome for the same number. Use the category to choose protection, then keep that protection in place while the index remains at 3 or above.
How Airpult shows the UV Index
The UV Index card on an Airpult forecast page shows a model estimate for the current time, not the forecast daily maximum. Airpult derives it from GFS UV radiation, rounds the number to the nearest whole value, and assigns the standard category: 0-2 Low, 3-5 Moderate, 6-7 High, 8-10 Very High, or 11 and higher Extreme.
The international scale has no fixed upper limit. Airpult’s data pipeline caps its input at 15, and the coloured indicator reaches the end of its bar at 10. Read the number and category together. This is a model estimate, not a measurement at your exact position.
The card’s short advice label is a summary. At 11 or higher, avoid direct sun around solar noon where possible and use all forms of protection outdoors. The category is not a blanket instruction to stay inside all day. Find a location to check the current-time estimate.
