Bottom two-stroke ice radical — the contracted form of
冫 (and ultimately of
冰 ice). Two dots picturing frozen droplets. Indexes
冬 firmly in the cold family:
冰 (ice),
冷 (cold),
凉 (cool),
冻 (freeze),
凝 (congeal). The radical tells the reader: this season is about cold. Combined with the closing-foot above, the whole character pictures 'year-end with ice underfoot' = winter.