The greatest horizontal distance at which a prominent object can be clearly identified. Reduced by fog, haze, smog, rain, dust storms and air pollution.
MeasurementWhat is Visibility?
Visibility in meteorology is the greatest horizontal distance at which a prominent dark object can be clearly identified against a contrasting background. It is reported in metres or kilometres at every weather station.
Visibility is one of the most safety-critical weather measurements:
- Aviation — Determines whether aircraft can take off or land.
- Road transport — Highway speed limits, fog warnings.
- Rail transport — Train signaling, especially in winter fog.
- Maritime navigation — Ports and shipping lanes.
- Public health — Visibility is a proxy for air-pollution levels.
The World Meteorological Organization (WMO) specifies a standardised method for visibility observation. Modern airports use automated visibility sensors that measure light scattering by aerosols and droplets in a small air sample, calibrated against international standards.
Visibility categories
A rough guide to visibility categories:
| Visibility | Category | Cause |
|---|---|---|
| < 50 m | Dense fog / blizzard | Saturated atmosphere |
| 50–200 m | Thick fog | Dense fog |
| 200–500 m | Fog | Moderate fog |
| 500–1,000 m | Moderate fog | Light fog |
| 1,000–2,000 m | Mist or heavy haze | Light fog or smog |
| 2,000–5,000 m | Mist or haze | Air pollution, light rain |
| 5,000–10,000 m | Moderate visibility | Light haze |
| 10,000+ m | Good visibility | Clear conditions |
For aviation, the critical thresholds:
- CAT I (800 m visibility or 550 m runway visual range, 60 m decision height) — most airports.
- CAT II (300 m runway visual range, 30 m decision height).
- CAT III-A (runway visual range not less than 175 m).
- CAT III-B (50 m visibility, no cloud base limit) — Delhi IGI, Chennai, Mumbai, Bengaluru.
What reduces visibility
Water droplets (saturated air):
- Fog — densest reduction; below 1 km.
- Mist — light reduction; 1–5 km.
- Heavy rain — variable; 1–10 km.
Dry particles:
- Smog/haze — pollution-driven; common in Delhi, Lahore, Patna in winter.
- Dust storms — common in May–June across Rajasthan, Sindh, Pakistan.
- Smoke — crop residue burning in Punjab/Haryana in November.
- Volcanic ash — rare in South Asia.
Other:
- Sandstorms — Thar Desert, Rajasthan summer.
- Spray — coastal storms and surf.
- Falling snow — Himalayan winter.
Visibility in South Asia
South Asia experiences some of the world’s most extreme visibility variations:
Winter (Nov–Feb):
- Delhi: Repeated dense-fog mornings; visibility often 50–200 m.
- Lahore, Faisalabad: Same as Delhi.
- Patna, Lucknow, Kanpur, Varanasi: Frequent fog forcing airport closures.
- Bangladesh (Dhaka, Rajshahi): Heavy winter fog.
Pre-monsoon (Apr–Jun):
- Rajasthan, Sindh, Balochistan: Dust storms reduce visibility.
- Coastal Tamil Nadu, Kerala: Light haze.
Monsoon (Jun–Sep):
- Country-wide: Heavy rain occasionally reduces visibility to under 500 m.
- Mumbai, Chennai, Kolkata: Monsoon storms; visibility variable.
Post-monsoon (Oct–Nov):
- Punjab, Haryana: Stubble-burning smoke + still air → severe haze.
- Delhi: November-December AQI peaks; visibility often 500–2,000 m.
Aviation impact
Indian aviation suffers losses due to fog-related cancellations and delays. Major affected airports:
- Indira Gandhi International (Delhi): CAT III-B equipped; can operate at 50 m visibility.
- Lal Bahadur Shastri (Varanasi): CAT I; closes when visibility is below 800 m and runway visual range below 550 m.
- Jay Prakash Narayan (Patna): CAT I; major winter disruptions.
- Chaudhary Charan Singh (Lucknow): CAT III-B.
- Bagdogra (Siliguri): Frequent mist closures.
Bangladesh’s Shahjalal International (Dhaka) also faces fog-related disruptions, particularly January.
Road and rail
Road visibility hazards are even more deadly. India records roughly 12,000 fog-related road deaths annually — concentrated on the Yamuna Expressway, GT Road, NH-48 and similar high-traffic corridors. Pile-ups of 30+ vehicles are not uncommon during winter morning fog.
Railway disruptions: Northern Railway cancels or runs hours late during dense fog. The Anti-Collision Device (ACD) and the new KAVACH automatic train protection system are slowly reducing fog-related accidents.
Measuring visibility
Modern instruments:
- Transmissometer — measures light attenuation over a known baseline.
- Forward-scatter sensor — measures light scattered forward by particles in a small sample volume.
- Visibility meter (manual) — observer identifies visibility markers at known distances.
Most automatic weather stations now use forward-scatter sensors that update every minute.
Frequently asked questions
Can fog reduce visibility to zero? Effectively yes — in the densest fog, you cannot see your own outstretched hand. Such conditions are recorded as “visibility less than 10 m.” It happens in North India’s winter fog: IMD reported 0 m visibility at Bhatinda (Punjab) and Agra (Uttar Pradesh) at 08:30 IST on 28 December 2023.
Is haze the same as fog? No. Haze is dry particulates (dust, smoke, pollutants); fog is suspended water droplets. They reduce visibility through similar mechanisms but are different physically. Haze can persist for days; fog usually lifts within hours of sunrise.
Why are CAT III-B landings safe at 50 m visibility? The aircraft uses an Instrument Landing System (ILS) Category III-B receiver to receive precise lateral and vertical guidance signals from the runway. The autopilot lands the plane. Specially trained pilots monitor the autoland. Modern CAT III-B systems are statistically safer than visual landings.
What is “smog visibility” and how does it compare to fog? Smog (smoke + fog) combines water droplets with pollution particles. Visibility reduction can be similar to fog, but smog is much more harmful to health — every breath delivers a dose of PM2.5. North India’s winter “fog” is technically smog.
Where can I see live visibility for my city? Mausam Online displays current visibility on every city page — useful for early-morning travel and flight planning. See Delhi, Lahore, Patna, Lucknow, Dhaka.