Western disturbances driving dryness this winter | Number Theory

.

Winter 2025-26 – the season runs from December to February according to the India Meteorological Department (IMD) – is a dry one in northern India. Forecasts this winter by IMD suggest that the reason could be that the storms that bring winter rain to the region are not intense. However, IMD doesn’t publish data that could help in quantifying this. Fortunately, a late January update to a dataset of these storms – called western disturbances -- by Kieran M Hunt, a climate scientist at the University of Reading and the National Centre for Atmospheric Science, can help us do just that. Here is what the data shows

People enjoying the sun on a winter morning at Lodhi Garden in New Delhi. (Vipin Kumar/ HT Photo)

Western disturbances driving dryness this winter

• 
  This winter has been dry so far in northern India

  IMD considers the 1971-2020 average as the Long Period Average (LPA) currently for tracking precipitation’s performance, with a deficit of 20% or more from the LPA classified as “deficient”. The entire northern hilly region — Jammu & Kashmir, Ladakh, Himachal Pradesh, and Uttarakhand — is in this category this winter. While states like Haryana and Punjab or Delhi are not rain deficient, other metrics — such as the number of days it rained -- show they are also dry. As HT reported earlier, these places — they also have lower LPA than regions further north -- accumulated rain from just a few days of rain starting January 23, a month and a half after winter started. In December, Delhi, Haryana, Punjab, and Uttarakhand received no rain at all. This was the first time since 2022 that dryness was this widespread in northern India this December.
• 
  This might have to do with weak western disturbances

  Winter precipitation trends in northern India are almost always driven by western disturbances. An analysis of Hunt’s dataset suggests that this was the case this December too. Here is how. Since the data tracks the entire path of these storms from 1950 to 2025 (the data for the 2023-25 period was updated on January 21), HT filtered those that came close to India. This shows that there were 12 storms that could potentially affect India in December 2025. This frequency is the lowest since 2020, when there were 10, and the 20th lowest since 1950. To be sure, the count itself would not matter as much if the storms were intense. For example, just one western disturbance around January 23 filled up a lot of the precipitation deficit in places like Haryana and Delhi. However, even the intensity of these storms was low this December. The average of the maximum intensity seen in these storms close to India was the 12th lowest for December since 1950 and the lowest since 2022. The median—it is the middle value in an ordered list of numbers — of the maximum intensity was the sixth lowest for December. Clearly, weak western disturbances were responsible for a dry December.
• 
  So far these storms have not become weak long-term in winter…

  Given western disturbances were weak and few in number in quick succession in December of 2020, 2022, and 2025, it might be tempting to attribute this to a long-term winter trend. However, that is not the case. For example, the 30-year rolling average of the intensity of these storms in winter does not show a clear trend yet. In terms of frequency—the number of these storms affecting India in winter—however, there is indeed a long-term trend. The 30-year rolling average of the winter frequency of these storms has increased consistently. Both these trends are consistent with Hunt’s research on western disturbances.
• 
  …but they appear to weaken in simulations of the future

  While western disturbances may not have weakened in winter so far, simulations of the future in the business-as-usual scenario—where we keep emitting greenhouse gases as we are—suggest that they would. In such simulations performed by Hunt, they appear to weaken in intensity in winter (taken by Hunt as the December-March period) and also in frequency. This has serious implications for India, where winter precipitation from western disturbances is important for the snowpack on the Himalayas, where rivers serving India’s plains originate.