Some people assume that we record our coldest temperatures around the time of the Winter Solstice in late December. After all, that's when the sun is at its lowest angle above the horizon and we have the fewest hours of daily sunlight. In fact, the coldest average temperatures occur several weeks later around the third week of January. Why? It takes several weeks for the land to react to the increase of solar energy coming in over the horizon. The land surface holds on to the cold it's stored up in the months leading to the Solstice. In the week's following the Solstice, the increasing sun must work to defeat the surface cold. Eventually, the surface begins to absorb enough of that increased solar energy to begin radiating some of it back into the air, warming it.
It's this radiation of solar energy from the land that begins are gradual warm-up through late winter and on into the spring.