For many locations in the Northern Hemisphere, the coldest day of the year, climatologically speaking, is around the 20-21 of January. In other words, if we average the daily mean temperatures over many years, we see that it tends to be coldest around this date. Obviously does not apply if you are in the Southern Hemisphere. The coldest time of year comes nearly a month after the winter solstice, which is the day when the sun's radiation at the top of the atmosphere reaches a minimum over the course of the year; because of the low sun angle we receive, on average, the least amount of solar radiation on that day. But the land surface (and water) continue to lose more energy than they gain each day for another month, so air temperatures continue to cool until around Jan 20-21. Happy heating season mid-point! PS - forecast here call for temps in the low 40s today...coldest weather (teens) to come next week.