The Northern Hemisphere is witnessing a significant annual astronomical event today, as winter officially begins with the winter solstice. This day is characterized by having the shortest day and the longest night of the entire year, marking the start of the actual cold season according to precise astronomical calculations used by astronomers worldwide.
What is the winter solstice?
The winter solstice occurs when the sun reaches its lowest point in the sky at noon, and its rays are perpendicular to the Tropic of Capricorn in the Southern Hemisphere (at 23.5 degrees south latitude). As a result, the Northern Hemisphere receives the least amount of sunlight, leading to significantly shorter days and longer nights. Conversely, the Southern Hemisphere experiences the peak of summer, also known as the summer solstice.
The difference between astronomical and meteorological winter
It is essential to distinguish between two concepts of winter: the first is "meteorological winter," which typically begins on December 1st and is used by meteorologists to facilitate climate statistics and the comparison of temperatures and weather phenomena across years. The second is "astronomical winter," which begins today and is linked to the Earth's movement around the sun and the tilt of its axis of rotation. This is the date adopted by official astronomical calendars to determine the start of the seasons.
What happens after this day?
Although today is the shortest day, it brings good news for daylight lovers; after the winter solstice, the apparent movement of the sun begins to reverse, so that daylight hours gradually increase minute by minute, and nighttime hours decrease, and this gradual change continues until we reach the spring equinox in March, when day and night are almost equal.
Expected climate impacts
The winter solstice doesn't necessarily mean it will be the coldest day of the year, but rather the beginning of a period when temperatures drop noticeably. This is due to a phenomenon known as "thermal lag" in the atmosphere and oceans, where the Earth takes time to lose the heat it previously absorbed. Therefore, the weeks following the winter solstice (January and February) are often the coldest, requiring preparation for the cold spells, frost, and rain that characterize this season, as well as the need to pay attention to general health and take precautions against seasonal winter illnesses.
