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
The WMO uses the following standard 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 (200 m visibility, 60 m cloud base) — most airports.
- CAT II (50–200 m visibility, 30 m cloud base).
- CAT III-A (50–200 m visibility).
- 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 to 1–3 km.
- 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 loses an estimated ₹500–1,000 crore annually 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 at 800 m.
- Jay Prakash Narayan (Patna): CAT I; major winter disruptions.
- Chaudhary Charan Singh (Lucknow): CAT I.
- 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.” Extremely rare but documented in extreme Himalayan or Antarctic conditions.
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.