Group 1 - The winter solstice marks the beginning of the "Nine Cold Days," with significant temperature drops and snowfall in high-altitude areas of Hangzhou [1] - The temperature in Hangzhou is expected to fluctuate, with a low of 2-5°C in plain areas and -1 to 2°C in mountainous regions, indicating a "cold-warm reversal" [1] - A rapid temperature rebound is forecasted for Tuesday, with highs reaching 18-21°C, followed by a return to cooler temperatures and light rain from Wednesday to Thursday [1][2] Group 2 - The weather changes necessitate attention to temperature fluctuations and appropriate clothing adjustments to prevent colds [2] - A cold air mass is expected to impact air quality, leading to increased pollution levels before a subsequent improvement due to wind and rain [2]