Fog
Kartavya Desk Staff
Source: HT
Subject: Geography
Context: The India Meteorological Department has issued a Red Alert for dense to very dense fog over Uttar Pradesh, with similar conditions forecast across north and eastern India.
About Fog:
What it is?
• Fog is a meteorological phenomenon in which tiny water droplets or ice crystals remain suspended near the Earth’s surface, reducing visibility to below 1 km due to scattering of light.
Types of Fog:
• Radiation fog: Forms during clear, calm nights when the Earth rapidly loses heat by radiation, cooling the air near the surface to the dew point. It is common in winter over plains and usually dissipates after sunrise.
• Valley fog: A special type of radiation fog where cold, dense air flows downslope and accumulates in valleys. The trapped air cools further, making valley fog denser and longer-lasting than fog over flat terrain.
• Advection fog: Occurs when warm, moist air moves horizontally over a colder surface such as snow, land, or cold ocean currents. Cooling from below causes condensation, and the fog can persist even during daytime.
• Freezing fog: Composed of supercooled liquid droplets that freeze instantly upon contact with surfaces. It creates a thin ice coating on roads, trees, and power lines, making travel extremely hazardous.
• Evaporation (mixing) fog: Forms when water vapour added by evaporation mixes with cooler, drier air until saturation is reached. Common examples include steam fog over warm water bodies and frontal fog during rainfall.
• Upslope fog: Develops when moist air is forced to rise along hills or mountain slopes. As the air ascends, it cools adiabatically, condenses, and forms fog, often covering large elevated areas.
• Hail fog: A rare fog that forms after heavy hailstorms when melting hail cools the warm, moist air near the ground to its dew point. It is usually shallow, patchy, and short-lived.
How fog is formed?
• Fog forms when air temperature falls to the dew point or when moisture content increases enough for saturation.
• Key mechanisms include radiational cooling, horizontal movement of moist air, evaporation, or orographic uplift.
• Calm winds, high humidity, long winter nights, and temperature inversions favour fog formation.
Impacts on local weather and society:
• Visibility reduction: Near-zero visibility disrupts road, rail, and air transport, increasing accident risk.
• Temperature moderation: Fog can suppress daytime heating, prolonging cold conditions.
• Air quality deterioration: Fog traps pollutants near the surface, worsening smog and respiratory issues.