People warm themselves at a bonfire during morning cold weather in Mysuru.
| Photo Credit: M.A. SRIRAM
Several districts in north interior Karnataka and Bengaluru experienced a steep drop in temperatures in the past week between December 8 and 14. What contributed to this? An India Meteorological Department (IMD) and a Bangalore Professor have decoded this.
They have cited an interplay between global La Niña conditions, synoptic advection from the Siberian High and local radiative physics for this phenomenon.
Kamsali Nagaraja, Professor at the Department of Physics, Bangalore University, and IMD Scientist Chanabasanagouda S. Patil said that to understand why a region geographically closer to the Equator experienced such biting cold, one must look thousands of kilometres away to the Pacific Ocean, where the 2025 winter unfolded under the shadow of La Niña.
The duo said that this global atmospheric phenomenon, characterised by cooler-than-average sea surface temperatures, acted as a conductor for the planetary weather orchestra.
Why harsher
It strengthened the trade winds and modified global circulation patterns, effectively loading the dice for a harsher winter over the Indian subcontinent. While La Niña set the stage, the primary actor was the Siberian High, a massive, semi-permanent collection of cold, dry air that accumulates over northern Asia. Typically blocked by the Himalayas, a specific pressure configuration this December unlocked the gates, allowing this continental air to escape its usual boundaries, the duo said.
They added that a perfect synoptic storm facilitated this southward migration of cold air, which resulted in Vijayapura district and Bidar district recording 7 °C and 7.4 °C with a departure of nearly 8°C from the normal. Besides, Bengaluru, which is usually insulated by its urban heat, saw airport temperatures drop to 12.9 °C.
Prof. Nagaraja and Mr. Patil said that the winds, however, only brought the cold air to Karnataka. They said that the extreme severity of the freeze was dictated by the microscopic physics of the boundary layer, specifically a process known as “nocturnal radiative cooling”.
‘Open window’ effect
The atmosphere during this week was exceptionally dry, particularly at the mid-tropospheric level, effectively stripping the earth of its natural blanket. Water vapour and clouds usually trap heat radiating from the ground, but with clear skies and arid air, the earth lost its warmth directly to space at a rapid rate after sunset. This “open window” effect caused the mercury to crash by 15 degrees or more from day to night, turning pleasant afternoons into biting nights, Prof. Nagaraja and Mr. Patil said.
Published – December 14, 2025 09:13 pm IST
