UV Index

A 0–11+ scale measuring the strength of ultraviolet radiation from the sun. Tropical South Asia routinely reaches UV Index 10–13 — extreme.

measurement

What is UV Index?

The UV Index (UVI) is a standardised international measurement of the intensity of ultraviolet radiation reaching the Earth’s surface from the sun. It was developed by the World Health Organization (WHO), the World Meteorological Organization (WMO), the UN Environment Programme (UNEP) and the International Commission on Non-Ionising Radiation Protection (ICNIRP) in 1994, and is now used globally.

The scale ranges from 0 (no UV — night) to 11+ (extreme). Each unit represents 25 milliwatts per square metre of UV-affecting radiation. Higher values mean greater risk of:

Skin damage — sunburn, premature ageing, skin cancer.
Eye damage — cataracts, corneal damage.
Immune suppression — UV-induced reduction of skin immune response.

For tropical South Asia, the UV Index is a daily health concern that is often underappreciated. Delhi, Mumbai, Chennai, Karachi and Dhaka routinely record UV Index 10–13 between March and October — the Extreme category.

UV Index categories and risk

The WHO defines five exposure categories:

UV Index	Category	Burn time (fair skin)	Recommended action
0–2	Low	60+ min	Minimal precautions
3–5	Moderate	30–45 min	Sunglasses, hat, SPF 15+
6–7	High	20–30 min	SPF 30+, hat, shade midday
8–10	Very High	15–20 min	SPF 50+, full skin cover, avoid 11 AM–3 PM
11+	Extreme	< 15 min	Full protection; avoid outdoors midday

For darker skin types (common across South Asia), the visible sunburn threshold is roughly 2–4× longer, but subsurface damage occurs at similar rates — the long-term cancer and ageing risks are similar across skin types.

UV Index in South Asia

Because South Asia sits close to the Tropic of Cancer, solar elevation at noon during summer is nearly 90° — sun directly overhead. UV Index regularly reaches the Extreme category:

Typical noon UV Index in summer (March–October):

Delhi, Lucknow, Patna: 10–12
Mumbai, Pune, Ahmedabad: 11–13
Chennai, Bengaluru, Hyderabad: 11–13
Kolkata, Dhaka: 10–12
Karachi, Lahore: 10–12
Sri Lanka, Maldives: 11–14 (Extreme year-round)
Himalayan high-altitude (Leh, Manali, Pokhara): 8–12, with higher UVB intensity due to thinner atmosphere

Winter UV Index (November–February) drops considerably:

North India (Delhi, Punjab): UV 5–7 — High but manageable.
South India (Chennai, Bengaluru): UV 8–10 still — Very High year-round.

This is why sunscreen is needed year-round in South India but only critically in summer for north India.

Factors that change UV Index

UV Index depends on:

Solar elevation — Higher sun = higher UV. Peaks at solar noon (around 12:00–13:00 local solar time).
Cloud cover — Heavy clouds reduce UV by 50–90%. Thin or scattered clouds can actually increase UV by 25% (cloud reflection).
Altitude — UV rises ~10% per 1,000 m elevation. Leh and Skardu see extreme UV despite lower temperatures.
Ozone column — Lower stratospheric ozone = higher UV. The “ozone hole” effect is strongest at high latitudes but applies globally.
Surface reflection — Snow reflects 80% of UV; sand 25%; grass 3%. Hill stations and beaches double effective UV exposure.
Air pollution — Surprisingly, heavy haze and smog can reduce UV by 10–30%. Delhi’s winter smog blocks UV but doesn’t make outdoor exposure safer for other reasons.

Health protection at UV 6+

Skin protection:

Broad-spectrum sunscreen SPF 30+ on all exposed skin.
Reapply every 2 hours, after swimming, or after sweating.
Wear lightweight long sleeves, hats with brims (not caps).
Seek shade between 11 AM and 3 PM (the highest UV window).

Eye protection:

Wear UV-blocking sunglasses (UV400 standard).
Cataract risk doubles after lifetime cumulative UV exposure above WHO threshold.

Special populations:

Children: Skin damage accumulates from birth. Use kid-friendly sunscreen; cover arms and legs.
Outdoor workers: Most at risk for occupational skin cancer in tropics.
People on photosensitising medications (some antibiotics, anti-inflammatories): Extra caution.
People with vitiligo, fair skin, or family history of skin cancer: Strict avoidance midday.

Vitamin D balance

UV-B radiation (a sub-band) is needed for the skin to produce vitamin D. South Asians are often vitamin D deficient despite abundant sun, primarily because:

Cultural clothing covers most skin.
Indoor lifestyles in air-conditioned offices.
Pollution blocks UV-B more than UV-A.
Darker skin produces vitamin D more slowly than lighter skin.

WHO recommendation: about 15–20 minutes of incidental sun exposure on arms and legs, 2–3 times a week, before 10 AM or after 4 PM (when UV is moderate), is sufficient for vitamin D in South Asia. No need to deliberately expose yourself at high-UV midday.

Mausam Online and UV reporting

Mausam Online displays the current and forecast UV Index alongside temperature and humidity for every South Asian city. The value is colour-coded by category — useful for daily planning of school runs, sports, outdoor work and travel.

Frequently asked questions

Why is UV Index so high in South Asia compared to Europe? South Asia sits between latitudes 5°N and 35°N — close to the equator. Sun is nearly overhead at noon. Europe’s typical latitude is 45–60°N — sun is much lower in the sky, so UV travels through more atmosphere and is weaker. London’s summer noon UV is ~7; Mumbai’s is ~12 — nearly double.

Is UV Index higher on cloudy days? Usually no. Thick cloud reduces UV by 50–90%. But thin scattered clouds can reflect UV downward and raise the effective dose by 10–25%. The dangerous case is “partly cloudy” days when people don’t realise UV is still high.

Can I get sunburn through glass? UV-A (longer wavelength) passes through window glass and contributes to skin ageing. UV-B (sunburn wavelength) is blocked by glass. So you can age but not burn through a window.

Do I need sunscreen if I’m only outside briefly? For UV Index 8+ and skin types II–III, even 15 minutes a day adds up. Cumulative UV exposure causes most long-term damage. Daily sunscreen on the face is a sensible habit in tropical South Asia.

Where can I see UV forecast for my city? Mausam Online shows live UV Index and 7-day forecast on every city page. See Mumbai, Chennai, Delhi, Bengaluru, Karachi, Dhaka.