system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: In the northern hemisphere, the shortest day of the year is in December. Why are January and February traditionally colder?
gpt
The atmosphere is a pretty big volume. It takes a while to cool down from summer. It's not going to get the coldest on the day of the shortest day, it's gonna take time. 

Then, as you approach spring, it lags behind in warming up, coming out of winter.