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).
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