Winter-specific drivers of lahars involve a combination of snow and ice presence, weather patterns, and hydrologic conditions that differ from other seasons. The core idea is that snow, ice, and heavy precipitation interact with eruptive processes to generate lahars more readily in colder months. The following factors help explain why winter eruptions are more conducive to lahar formation:

• Snow and ice availability: Volcanoes with permanent or seasonal snow and ice will have abundant meltwater whenever heat from an eruption or volcanic activity is transmitted to the snow/ice. In winter, these reservoirs are already present and can be rapidly mobilized by heat from explosive activity, pumice flows, or lava interactions, producing large volumes of meltwater that mix with volcanic debris to form lahars. This mechanism is supported by extensive observations of lahars associated with snow-clad volcanoes during eruptions, where meltwater generation is a key contributor to lahar formation [sources describing snow/ice perturbation during eruptions].

• Eruptive melt pathways: Hot volcanic ejecta, pyroclastic flows, and lahars can melt surface snow and ice directly as they advance downslope. The resulting water augments the debris flow, transforming it into a lahar with high sediment concentration and fluidity. This direct melting is a well-documented trigger for lahars on ice- and snow-covered volcanoes during eruptions.

• Crater lakes and meltwater reservoirs: Some glaciers or crater lakes atop volcanoes can supply additional water when heated during an eruption. Melting of lake ice or glacial water adds volume to flows that may coalesce into lahars, particularly when paired with rapid debris entrainment. This mechanism is repeatedly observed in case studies of lahars associated with eruptive activity on snow- and ice-covered volcanoes.

• Weather-driven amplification: Winter precipitation, including rain-on-snow events and rapid snowfall, can rapidly saturate loose volcanic deposits and trigger slope failures or increase fluidity of debris flows. Heavy precipitation during or after eruptions is a common pathway to lahar formation, and winter storms can provide a higher likelihood of such precipitation events in the ensuing period after an eruption.

• Cold lahars and cumulative effects: Not all lahars formed in winter are hot; cold lahars can be triggered by sustained precipitation, snowmelt, or progressive saturation of ash- and tephra-covered slopes. In winter, the combination of wet storms and existing snowpack can mobilize these deposits over time, producing lahars long after eruption onset or during quiescent intervals. These secondary lahars are well described in hazard summaries for volcanic regions.

• Frequency and detectability: Regions with persistent snow and ice on high-altitude volcanoes tend to have well-documented histories of winter-time perturbations leading to lahars, which helps explain observed associations between winter eruptions and lahar activity. This is reflected in reviews that link snow/ice perturbation and lahars to many historical events worldwide, especially at higher latitudes.

Bottom line: When eruptions occur in winter on snow- or ice-covered volcanoes, there is an enhanced potential for rapid meltwater generation, direct melting by hot pyroclastic activity, and additional water input from crater lakes or glaciated systems. These processes combine to produce lahars more readily than in seasons with less available snow/ice and different hydrological conditions.