Fog: Types, Formation, and Impacts
22.12.2025
Fog: Types, Formation, and Impacts
In late 2025, the India Meteorological Department (IMD) issued a Red Alert for dense to very dense fog over Uttar Pradesh and several parts of North and East India, highlighting the seasonal severity of this phenomenon.
Definition: Fog is a meteorological phenomenon where tiny water droplets or ice crystals remain suspended in the air near the Earth’s surface. Technically, it is defined by a reduction in horizontal visibility to below 1 km due to the scattering of light by these particles.
How Fog is Formed? Fog formation occurs when the air becomes saturated (reaches 100% relative humidity). This happens through two primary paths:
- Cooling: The air temperature falls to its dew point.
- Moistening: The moisture content in the air increases until it can no longer hold the water vapor.
- Favorable Conditions: Calm winds, high humidity, long winter nights, and temperature inversions (where warm air traps cold air near the ground).
Fog is classified based on the physical process that leads to its formation:
- Radiation Fog: Forms on clear, calm nights as the ground loses heat through radiation, cooling the air immediately above it. It typically "burns off" after sunrise.
- Advection Fog: Occurs when warm, moist air moves horizontally over a significantly colder surface (like snow or cold ocean currents).
- Valley Fog: Cold, dense air sinks into valleys due to gravity, becoming trapped and cooling further to form dense, persistent fog.
- Upslope Fog: Created when moist air is forced up a mountain slope, cooling adiabatically as it rises.
- Freezing Fog: Consists of supercooled liquid droplets that freeze immediately upon contact with any solid surface, creating "rime" or ice coatings.
- Evaporation (Steam) Fog: Forms when cold air moves over warm water; the water evaporates into the cold air, saturating it instantly.
- Hail Fog: A rare, patchy fog occurring after hailstorms as melting hail rapidly cools the moist air near the surface.
Impacts on Local Weather and Society
- Transport Disruption: Near-zero visibility is the leading cause of massive delays and accidents in road, rail, and aviation sectors.
- Temperature Suppression: Dense fog reflects incoming solar radiation, preventing the ground from warming and leading to "Cold Day" conditions.
- Health & Air Quality: Fog acts as a lid, trapping particulate matter (PM2.5) and pollutants near the ground, creating Smog (Smoke + Fog), which exacerbates respiratory illnesses.
- Technological Integration: Improving CAT-III Landing Systems at airports and installing fog-pass devices in trains to navigate low-visibility zones.
- Early Warning Systems: Enhancing IMD’s satellite-based "Nowcasting" to provide real-time alerts to commuters.
- Public Safety: Implementation of automated speed-limit signs on expressways during "Red Alert" periods to prevent pile-ups.
While fog is a natural atmospheric process, its interaction with anthropogenic pollution (Smog) and the modern transport network makes it a significant socio-economic hazard. Robust forecasting and technology-driven infrastructure are essential to mitigating its impact on India’s winter connectivity.