Bengaluru’s recent spell of cold mornings and dense fog has once again drawn attention to how weather behaves in ways that often surprise the public. While such conditions are commonly associated with northern India, their occurrence in a tropical city has raised questions about what is driving these changes. Scientists say the answer lies not in anomalies, but in basic atmospheric physics.
Weather may appear unpredictable, but it follows well-defined physical laws. Bengaluru’s elevated inland location, combined with clear winter skies and calm winds, allows rapid night-time heat loss from the surface. This cooling becomes strongest just before sunrise, leading to colder mornings and the formation of fog.
As the ground cools after sunset, the air in contact with it also cools. When this air reaches saturation, moisture condenses into tiny droplets suspended near the surface, forming fog. Fog is essentially a cloud at ground level and does not require rainfall or storm systems to develop. Urban aerosols further influence its density by providing surfaces on which water vapour can condense, causing visibility to drop sharply.
The persistence of fog is linked to the formation of a temperature inversion during winter nights. Colder, denser air becomes trapped near the ground beneath warmer air above, suppressing vertical mixing. With little wind to disturb this arrangement, fog can persist for several hours into the morning, especially in low-lying and open areas.
Local terrain also plays a role. Cold, dense air tends to flow downslope at night and collect in depressions, making certain areas more prone to dense fog. Built-up regions may remain slightly warmer, but higher pollution levels can make the fog that does form more opaque. Fog usually clears after sunrise, when solar heating weakens the inversion and allows air to mix vertically.
As droplets evaporate, visibility improves rapidly. Such episodes highlight how fundamental physical processes govern everyday weather and explain why scientific understanding is essential to interpret sudden changes in temperature and visibility.
Fog, visibility and safety
Winter fog typically forms during late night and early morning hours under clear skies and calm winds, when rapid cooling near the ground causes air to reach saturation. Visibility can reduce suddenly to a few hundred metres or less, particularly close to sunrise, and may persist for one to three hours before clearing after sunrise.
During foggy conditions, reduced visibility increases the risk of road accidents. Driving at lower speeds, maintaining safe distances and using low-beam or fog lights can improve safety. High-beam headlights should be avoided, as they reflect light back and worsen visibility. Sudden braking, overtaking, and reliance solely on vehicle taillights should be avoided. Pedestrians and cyclists should remain cautious and visible, especially near highways and open stretches.
Fog generally dissipates rapidly once sunlight warms the surface and breaks the temperature inversion, restoring visibility within a short period. Awareness of these patterns can help commuters plan travel and reduce risk during winter mornings.
— Kamsali Nagaraja, Professor, Dept of Physics, Bangalore University
— Chanabasanagouda S Patil, Scientist, Meteorological Centre, Bengaluru