Fog
Topic: Water in the Atmosphere: Humidity, Condensation, Clouds and Precipitation · NCERT: Class 11, Ch 10 "Water in the Atmosphere"
Meaning
Fog is a cloud whose base is at or very near the ground. It forms when moist air cools suddenly and its vapour condenses on tiny particles of dust, smoke or salt. In fog, visibility drops below 1 km. It is common where warm and cold air, or warm and cold ocean currents, meet. Fog mixed with smoke is called smog. It harms health and disrupts flights, trains and road traffic.
Example
Dense fog covers the Indo-Gangetic plain, from Punjab to Bihar, on many winter mornings. When it mixes with smoke over Delhi, it becomes smog. London had a deadly smog in 1952.
Don't confuse with
- Mist: it is thinner, with visibility of 1–2 km. Fog cuts visibility below 1 km.
- Dew: its drops form on surfaces such as grass. Fog droplets float in the air.