The “Dead of Winter” is an old saying that refers to the coldest part of the winter season. For most of the northern hemisphere, that usually means the month of January.
While actual daily weather varies, historical average temperatures typically reach their lowest point of the year between January 10 and February 10. In the US, the western half of the country, with the exception of high elevation areas, tends to see its coldest day before the eastern half. This is because the mountainous areas of the west and much of the eastern US tend to be snow covered in winter, reflecting much of the sun’s heat. Here in New York City, mid to late January is usually the coldest part of the winter season.
This traditional cold period does not begin on the winter solstice, the day we receive the least amount of solar energy, because of a phenomenon known as seasonal temperature lag.
Air temperature depends on both the amount of energy received from the sun and the amount of heat lost or absorbed by the oceans and continents. From the start of winter through mid-February, both the oceans and land are losing more heat than they gain.
These few frosty weeks are the opposite of the “Dog Days of Summer.”