Fog

What is Fog?

Fog is a cloud whose base touches the ground, reducing horizontal visibility to less than 1 kilometre. Meteorologically it is identical to a stratus cloud — only its altitude differs. When the same droplets float a few hundred metres above ground, we call them a cloud; at street level, we call them fog.

Fog forms when the air temperature falls to the dew point, the temperature at which the air becomes saturated and water vapour condenses into tiny suspended droplets (typically 5–50 micrometres across). Each cubic metre of fog contains only about half a millilitre of water, but the trillions of droplets scatter light enough to white-out the world.

In South Asia, particularly across the Indo-Gangetic plain (Delhi → Lahore → Patna → Dhaka), the winter fog season from late November to early February is one of the most disruptive weather phenomena of the year — causing flight cancellations, train delays, road accidents and severe air-quality crises.

Types of fog

Meteorologists classify fog by how it forms:

• Radiation fog — The most common type in north India and Pakistan. On clear, calm winter nights, the ground radiates heat to space and cools rapidly. The air just above cools to its dew point and fog forms in low-lying areas. Burns off by mid-morning as the sun warms the ground. Example: Delhi, Punjab, Bihar dawn fog.

• Advection fog — Forms when warm, moist air flows over a cooler surface. Common on coasts (Mumbai, Karachi) when warm sea breezes hit cool land at night.

• Steam fog — Forms when cold air moves over warmer water. Visible above the Ganga, Brahmaputra and Yamuna on cold mornings.

• Upslope fog — Forms when moist air is lifted and cooled by terrain. Common in the Himalayan foothills, the Western Ghats and Darjeeling tea estates.

• Valley fog — Cold dense air pools in valleys overnight, forming persistent fog that may last all day in winter. Common in Kashmir, Himachal, Nepal valleys.

• Frontal fog — Forms along weather fronts when warm and cold air masses meet. Less common in tropical South Asia.

Fog in South Asia

The Indo-Gangetic plain produces some of the most extensive and persistent fog on Earth. A single dense fog event can cover 2 million square kilometres — visible from space as a continuous white blanket from Pakistan to Bangladesh.

Why is north Indian fog so dense?

1. Topography traps cold air — the Himalayas to the north prevent cold air from escaping.
2. Calm winter winds — low-pressure gradients allow fog to stagnate.
3. High humidity — irrigation, rivers and crops keep low-level moisture high.
4. Aerosol pollution — every PM2.5 particle gives a droplet a nucleus to form on. The result is the toxic mix called smog.
5. Temperature inversion — a layer of warm air above cool air at the surface traps fog and pollutants below.

Typical winter fog cities (with visibility regularly below 50 metres):

• Delhi NCR — fog season early December to late January.
• Lahore, Faisalabad, Multan, Bahawalpur — Pakistani Punjab gets weeks of “smog days.”
• Patna, Varanasi, Kanpur, Lucknow — northern UP and Bihar.
• Amritsar, Chandigarh, Jalandhar — Indian Punjab.
• Dhaka, Rajshahi, Khulna — Bangladesh, particularly in January.

Fog disruption to transport

Aviation: Indian airports follow ICAO CAT III instrument-landing protocols. Indira Gandhi International Airport (Delhi) has CAT III-B capability allowing landings with visibility as low as 50 metres, but only specially-trained pilots in CAT III-equipped aircraft can land. Each foggy winter sees hundreds of cancellations and thousands of delays at Delhi alone. Smaller airports (Patna, Lucknow, Varanasi, Amritsar) without CAT III often close completely.

Rail: Northern Railway routinely cancels or runs hours late through Delhi, Punjab, UP and Bihar in late December and January. Many long-distance trains are rescheduled.

Road: Dense fog causes deadly pile-ups on the Yamuna Expressway, Delhi-Mumbai NH-48, and GT Road every winter. India reports roughly 12,000 fog-related road deaths annually. The Punjab–Sindh corridor in Pakistan sees similar tragedies.

Mausam Online displays visibility on every city page — useful for travellers, drivers and airline passengers planning around fog.

Fog vs mist vs smog vs haze

These four words are often used interchangeably but have specific meanings:

The dense winter “fog” affecting Delhi, Lahore and surrounding cities is technically smog — fog mixed with extreme PM2.5 levels. This is far more dangerous than pure fog because the inhalable pollutants damage lungs even during brief outdoor exposure.

Frequently asked questions

When is fog season in South Asia? Late November to early February for the Indo-Gangetic plain (Delhi, Punjab, UP, Bihar, Bangladesh). December and January are peak. Coastal cities (Mumbai, Chennai, Karachi) get occasional advection fog year-round but less severe. Himalayan foothills can see fog any time.

Why is morning fog more common than evening fog? Radiation fog needs the ground to cool below the dew point. This happens most strongly between 3 AM and dawn after a clear night. Sunrise quickly warms the air above the dew point, and the fog “lifts” or burns off within 1–3 hours.

How can I drive safely in fog? Use low-beam headlights or fog lamps (high beams reflect back). Drive slowly. Increase following distance. Use the road’s left edge as a guide. Never stop on the road — pull off completely. If you cannot see the next vehicle, you are following too close.

Why does Delhi fog smell bad? Because it is smog, not pure fog. The mix includes PM2.5 (soot, sulphates), nitrogen dioxide, ozone and aerosols from vehicles, power plants and crop burning. The smell is the cocktail of these pollutants, not the water droplets themselves.

Where can I check visibility for my city? Mausam Online shows live visibility (in km) on every city page — useful for early-morning travel planning. See Delhi, Lahore, Patna, Lucknow, Dhaka.

Related terms

• Visibility
• Dew point
• Mist
• Haze
• Smog
• Humidity