Seasonal lag pushes temperature minimum into January Heat stored in land and oceans during summer releases slowly, so average temperatures keep falling after the December 21 solstice and bottom out in mid‑January [1][2]. This lag explains why the coldest daily highs and lows occur weeks after the shortest day. Both outlets note that the phenomenon is consistent across long‑term climate records.

Polar jet stream intensifies, delivering Arctic air southward Early winter patterns cause the polar jet to strengthen and become more variable, allowing frigid Arctic air masses to plunge into lower latitudes [1][2]. These incursions deepen the cold spell across the continental interior. The jet’s behavior is identified as a key driver of the January temperature dip.

Snow cover and high albedo reinforce the chill Widespread snow in January reflects solar radiation, reducing surface heat absorption and enhancing radiational cooling at night [1][2]. The increased albedo limits ground warming even when daylight returns, keeping overnight lows especially low. This feedback loop helps sustain the coldest averages.

Geography and elevation magnify the lag effect Continental interiors and higher elevations experience stronger seasonal lag because land heats and cools faster than water [1][2]. Mountainous regions retain snow longer, further delaying spring warming. These geographic factors cause regional variations in the depth and duration of the January cold.