Authors: Sanjeev Kumar Singh, Yogesh Kant, D. Mitra and R. P. Singh

Western Disturbances (WDs) are eastward-moving extratropical synoptic-scale weather systems that significantly influence the winter weather of north and northwestern India, particularly during the period December to March (Hunt et al., 2025). These systems are responsible for cloudiness, rainfall, snowfall over the Western Himalayas, cold wave conditions over the Indo-Gangetic plains, and occasional hailstorms.

WDs originate over the mid-latitude regions, primarily the Mediterranean Sea and adjoining West Asian region (Madhura et al., 2015), within the belt of prevailing westerlies (Figure 1). They form as upper-air short-wave troughs or as small-scale instabilities embedded in the Subtropical Westerly Jet (STWJ) in the upper troposphere (Singh et al., 1981). The development of a WD is closely associated with baroclinic instability (Vellore et al., 2016), arising from strong horizontal temperature gradients between cold polar air and warmer subtropical air masses. WDs generally have total lifetimes in the range of approximately 2-12 days (Hunt et al., 2025), with a typical lifespan of about 3-7 days and weather impacts over the Indian region lasting around 2-3 days. WDs generally propagate eastward along the STWJ, affecting Iran, Iraq, Afghanistan, Pakistan, and India, and occasionally extending their influence to Nepal, Bangladesh, and China (Pisharoty and Desai, 1956; Rao and Srinivasan, 1969).

During its eastward journey, the WD gradually gets moisture from the Mediterranean Sea, Black Sea, Caspian Sea, Red Sea and Persian Gulf. Upon reaching the Indian subcontinent, additional moisture influx from the Arabian Sea at lower levels enhances precipitation potential. The interaction of the WD with the Himalayan orography leads to orographic lifting, resulting in widespread snowfall over the Western Himalayas and rainfall over the adjoining plains. The winter rainfall associated with WDs significantly contributes to water security (Thayyen and Gergan, 2010) and agricultural productivity (Yadav et al., 2012). Over climate timescales, the contribution of WD-driven winter precipitation is essential for glacier mass balance in a region known for its anomalous glacial advance (Dimri et al., 2015; Javed et al., 2022).

Duing 22-23 January 2026, a strong WD was observed, bringing significant rainfall and snowfall to the western Himalayas. A second, moderate WD occurred during 26-28 January 2026, producing another round of precipitation and snowfall across the region, with peak activity on 27 January. Both events were part of a series of WDs that influenced winter weather over India in late January.

Figure 1: Schematic representation of a western disturbance and its associated trough and cyclonic circulation.

Brightness Temperature (BT) explains cloud characteristics by translating the infrared radiation measured by the satellite into an equivalent temperature. INSAT-3DS Thermal Infrared (TIR) channels provide important observations for the cloud structure and its evolution. For the 22-23 January WD event, the peak activity occurred on 23 January over the India. INSAT-3DS TIR-1 (10.8 µm) BT imagery at 00UTC on 23 January typically shows extensive west-east oriented cloud bands with lower BT values (< 220 K), indicating the presence of high, cold cloud tops produced by large-scale ascent ahead of the upper-tropospheric trough (Figures 2(a)). The spatial extent and temporal evolution of low BT regions enable continuous tracking of the eastward propagation, intensification and precipitation potential of the WD across the region. Complementing TIR, Water Vapour imagery is particularly useful for identifying upper-tropospheric features of WDs. It highlights moist and dry air patterns in the mid-to-upper troposphere. INSAT-3DS Water Vapour (6.8 µm) imagery at 00 UTC on 23 January shows dry air subsidence appearing as darker regions on the western side of the system and enhanced moisture and ascent ahead of the trough appearing brighter (Figures 2(b)).

In the upper troposphere (~200-300 hPa), a WD is characterized by a well-defined trough and enhanced westerly winds within the STWJ. These winds, located near the tropopause, represent the westerly jet stream that largely steers the movement and trajectory of the WD. At this level, the troughs associated with the disturbance are clearly identifiable, allowing for early detection before surface impacts occur. Jet-level divergence ahead of the trough induces large-scale ascent, enabling the intensification of the system. The strong upper-tropospheric trough embedded within the STWJ associated with the first WD is well depicted in the 300 hPa analysed wind field obtained from the Global Forecast System (GFS) model at 00 UTC on 23 January, 2026 (Figure 2(c)). As the WD propagates eastward, it extends downward into the mid-troposphere (around 500 hPa), where it appears as a cyclonic circulation or trough.

Figure 2: (a) TIR-1 (10.8 µm) brightness temperature imagery, (b) Water vapour (6.8 µm) imagery from INSAT-3DS and (c) Analysed wind fields at 300 hPa pressure level from GFS  model during the western disturbance event over India on 23 January, 2026.

The second WD event was moderate with peak impact on 27 January over India. The TIR-1 BT observations form INSAT-3DS associated with second WD is shown in Figure 3(a) on 27 January. The BT imagery illustrations the west-east oriented cloud bands with lower BT values (< 220 K). This indicates the presence of high, cold cloud tops ahead of the upper-tropospheric trough over the northern India and Himalayan region. The dark-bright contrast pattern in INSAT-3DS water vapour imagery provides clear evidence of dry air subsidence behind the trough and enhanced moist ascent ahead of it Figure 3(b). The upper-tropospheric trough embedded within the STWJ associated with the second WD is clearly depicted in the 300 hPa analysed wind field from the GFS at 00 UTC on 27 January 2026 (Figure 3c).

Figure 3: (a) TIR-1 (10.8 µm) brightness temperature imagery, (b) Water vapour (6.8 µm) imagery from INSAT-3DS and (c) Analysed wind fields at 300 hPa pressure level from GFS  model during the Western Disturbance event over India on 27 January, 2026.

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