How the UV Index Works and How to Read It
UV Protection
How the UV Index Works and How to Read It
A single number that tells you how strong the sun's ultraviolet is right now. What each band means, why Indian cities sit high on the scale for much of the year, and what to do about it.
In this article
- What the number measures
- What each band means
- Why Indian cities read high
- What the index means for your eyes
- Using the index in practice
- Frequently asked questions
What the number measures
The UV Index is a standardised scale describing the intensity of ultraviolet radiation reaching the ground at a particular place and time. It starts at zero and has no formal upper limit, though readings above 12 are uncommon.
It is not a temperature reading and it does not track how bright the day feels. A cool, hazy morning can carry a higher index than a hot afternoon in winter. This disconnect between how the day feels and how much ultraviolet is present is the single most useful thing the index corrects.
What each band means
| Index | Band | What it means in practice |
|---|---|---|
| 0 to 2 | Low | Minimal risk for most people |
| 3 to 5 | Moderate | Protection advisable around midday |
| 6 to 7 | High | Protection needed, seek shade at midday |
| 8 to 10 | Very high | Unprotected exposure causes harm quickly |
| 11+ | Extreme | Take every precaution, avoid midday sun |
The bands are set by international convention, which means a reading of 8 carries the same meaning in Chennai as it does anywhere else. That comparability is the point of having a scale at all.
Why Indian cities read high
Four factors drive the index upward, and much of India carries several of them simultaneously.
Latitude. Ultraviolet intensity is highest near the equator, where sunlight travels the shortest path through the atmosphere. Southern India sits close enough to feel this strongly year round.
Sun angle. The index peaks when the sun is highest, which is why the middle of the day matters far more than total hours outside. In much of India the peak window runs roughly from late morning to mid afternoon.
Altitude. Ultraviolet increases measurably with elevation as there is less atmosphere to absorb it. Hill stations and Himalayan destinations carry a higher load than their comfortable temperatures suggest.
Reflection. Surfaces bounce ultraviolet back upward. Water, sand and snow are the strongest reflectors, which is why beaches and high-altitude snow produce exposure from below as well as above.
What the index means for your eyes
Most public guidance about the UV Index concerns skin. The eye is affected too, and in some ways more directly, because there is no equivalent of a tan and no adaptive response.
Short-term, high exposure can cause photokeratitis, effectively a sunburn of the cornea, which typically appears hours after exposure as pain, watering and light sensitivity. It is most common in high-reflection environments such as snow and open water.
Long-term, cumulative ultraviolet exposure is associated with cataract and with growths on the surface of the eye. This is the exposure the index helps you manage, because it accumulates on ordinary days rather than dramatic ones.
Using the index in practice
Most weather apps show the UV Index alongside temperature, and it is worth learning to glance at it rather than judging by warmth.
At index 3 and above, ultraviolet protection is worth having. Given that much of India sits at or above this for the majority of daylight hours across most of the year, the practical conclusion for most people is straightforward: wear 100% UV400 protection whenever you are outdoors in daylight, rather than trying to judge it day by day.
Two things matter beyond the lens itself. Coverage, because ultraviolet reaching the eye from the sides and from below is not filtered by a lens sitting flat in front of you. And consistency, because the damage the index tracks is cumulative and a protective pair left at home protects nothing.
Frequently asked questions
Key takeaways
