Cold wave
Why in news?
India Meteorological Department forecasts a cold wave hitting Delhi from Friday, December 5, 2025, with temperatures dropping by at least 2°C over three days amid poor air quality.
What is Cold Wave?
- A cold wave is a rapid and significant drop in air temperature over a short period, often measured as a departure from normal temperatures, affecting large areas.​
- In India, the IMD declares it when plains minimum temperature drops to ≤10°C or is 4.5–6.5°C below normal.​
Formation Process
- Cold, dense polar or Arctic air masses form in high latitudes (e.g., northern Canada, Siberia) under strong high-pressure centers during winter.​
- Upper-level jet stream amplifies into a ridge-trough pattern, creating "waves" that steer cold air equatorward.​
- Cold air moves southward due to density (sinking toward warmer air) and upper-level winds; subsidence and convergence strengthen surface highs.​
- Clear skies, light winds, snow cover, and cyclones enhance cooling via radiation and advection.​
Causes and Impacts in India
- Cold waves form due to northwesterly winds from the Himalayas carrying dry, cold air southward, intensified by high-pressure ridges over northwest Asia and clear skies enhancing radiative cooling.​
- Snowfall in the Himalayas and subsidence of cold air contribute, while western disturbances temporarily alleviate them.​
- These events cause health issues like hypothermia and frostbite, agricultural damage to crops, livestock stress, and transport disruptions from fog.​
- Northern states like Punjab, Uttar Pradesh, and Rajasthan see the highest frequency (6-8 days annually on average).​
