Chillai Kalan: How Cold Temperatures Shape Weak Layers in Kashmir’s Snowpack

The 40-day period of Chillai Kalan (December 21–January 31), the harshest winter phase in Kashmir, has a profound impact on snowpack dynamics and avalanche conditions in regions like Gulmarg. 

Marked by extreme cold, heavy snowfall, and strong winds, Chillai Kalan fosters the development of faceted snow layers, redistribution of snow, and highly variable mechanical snowpack structures, creating a challenging environment for avalanche forecasting and backcountry travel.

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Faceting of the Snowpack: Temperature Gradients at Work

The extreme cold during Chillai Kalan drives the formation of faceted grains, that can form weak layers in the snowpack, which persist for long periods, until they heal or are flushed away by avalanche cycles.

Key Processes for faceting:

1. Temperature Gradient Formation:

   • Surface temperatures drop dramatically at night, often below -10°C, due to radiative cooling.
   • The ground remains relatively warm (~0°C), creating a steep temperature gradient (>10°C/m) within the snowpack.

2. Formation of Faceted Grains:

   • This gradient causes water vapor within the snowpack to migrate upward. As the vapor moves around snow crystals, they recrystallize into angular, poorly bonded faceted grains.
   • These grains lack cohesion, 
   • They do not revert back into round crystal forms readily. infact the process of faceting is lot quicker than the process of rounding/healing
   • These form persistent weak layers (PWLs) that are prone to failure under loading. 
     Why? : because the irregular shapes of facets don't let them bond well. They also allow empty void space between other crystals. The snow structure becomes less dense. This snowmass made of faceted crystal may hold vertical compressive force well, until a directional shear force is applied. They are not very tolerated of shear forces. 

3. Enhanced Faceting in Shallow Snowpacks:

   • Early-season snowpacks, which are shallow and less insulated, experience rapid faceting. These weak layers often persist into the late winter and spring, buried under successive snowstorms.

4. Surface Hoar Formation:

   • Clear nights also favor the growth of surface hoar, another potential weak layer, which can get buried during storms and add to the snowpack's instability. The process of surface hoar formation is different from faceting. I'll write another post on SH, and usually areas it can be seen at. But Surface Hoar development will occur most readily during Chillai Kalan days as opposed to other times of the season. 

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Role of Winds: Snow Redistribution and Slab Formation

Strong winds during Chillai Kalan are a dominant factor in shaping snowpack variability.

Key Impacts of Wind:

1. Wind Slab Development:

   • Dry, cold snow is easily transported by winds, forming dense wind slabs on lee slopes.
   • These slabs often overlie faceted layers or surface hoar, creating highly unstable avalanche-prone conditions.
   • Slab zones can have deeper snowpacks, where effective temp gradient is reduced. but that doesn't mean the snowpack must readily heal due to reduction in gradient.

2. Temperature Effects:

• Redistribution exposes previously insulated weak layers to the cold, thinning the snowpack in windward areas and thus stimulating the faceting process in those zones.  

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Chillai Khurd and Stabilization

Following Chillai Kalan, the 20-day Chillai Khurd (February 1–20) brings milder temperatures and reduced temperature gradients:

1. Grain Rounding:
   • Warmer conditions allow faceted grains to round and bond, improving snowpack stability.
2. Residual Risks:
   • Deeply buried faceted layers from Chillai Kalan remain a significant hazard, especially during heavy snowfalls or warming events.

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Implications for Avalanche Forecasters and Backcountry Enthusiasts

Snowpack Monitoring:

• Regular snow-pit analyses are essential during Chillai Kalan to track the depth, reactivity, and stability of weak layers.
• Observations should focus on wind-loading patterns, slab density, and signs of propagation potential.

Terrain Management:

• Avoid steep slopes with convex rollovers, wind-loaded aspects, and terrain traps.
• Be cautious in areas with shallow snowpacks, which are prone to faceting and slab formation.

Localized Forecasting:

• Keep track of local avy forcasts. For Gulmarg see the GULMARG AVALANCHE ADVISORY