The winter solstice does not coincide with the coldest time of the year due to the tilt of the Earth and the planet’s heat retention.
The darkest time of the year is the winter solstice, a day with the least sunlight and the longest night, but the coldest period usually occurs about a month after the winter solstice. The reason lies in the tilt of the Earth and the planet’s heat retention, according to Live Science.
Simulation of the Earth’s tilt. (Photo: Video: Britannica).
The axis of the Earth, an imaginary line connecting the North and South Poles, is tilted at an angle of 23.4 degrees. This means that one day each year, the North Pole is tilted farthest away from the Sun. On another day, the South Pole is tilted farthest away from the Sun. Those days are known as the winter solstice in the northern and southern hemispheres, respectively, according to Christopher Baird, an associate professor of physics at West Texas A&M University. The more the Earth’s surface tilts away from the Sun, the shorter the daylight hours. The winter solstice in the northern hemisphere, the shortest day in the Northern Hemisphere, occurs on December 21, 22, or 23 each year, while the winter solstice in the southern hemisphere usually falls on June 20, 21, or 22.
The Earth receives most of its warmth from the Sun, leading us to mistakenly think that the winter solstice is the coldest day of the year in each hemisphere, according to Baird. However, the coldest temperatures in each hemisphere typically lag about a month behind the solstice, according to Nick Bassill, director of the Weather Risk Communication Center at the University of New York, Albany. In the Northern Hemisphere, this period is typically mid-January.
Conversely, the summer solstice, the longest day of the year, occurs on June 20, 21, or 22 in the Northern Hemisphere and December 21, 22, or 23 in the Southern Hemisphere, and is often not the hottest day of the year, according to Jason Steffen, an assistant professor of physics and astronomy at the University of Nevada, Las Vegas. For example, the Mojave Desert experiences its hottest temperatures in late July, well after the summer solstice, while the Florida Gulf Coast sees its highest temperatures in August.
The gap between the solstice and the coldest or hottest temperatures of the year is due to objects such as lakes, oceans, and land not responding immediately to colder or warmer temperatures, Bassill explains. In winter, this means they retain heat from autumn and summer longer than the air. The closer a location is to the ocean, the more these seasonal temperature fluctuations are moderated, as it takes four times more energy to raise the temperature of water by one degree Celsius compared to land. Additionally, because oceans circulate heat, even when nights are long, ocean currents can keep those areas warm. For example, many locations along the Pacific coast of the United States have their coldest winter temperatures warmer than other places at the same latitude due to this effect.
