Winter

Winter is the coldest and darkest season of the year in temperate and polar climates. It occurs between the seasons of autumn and spring. This period is defined by a significant decrease in solar energy and temperature. For many, winter is characterized by precipitation like rain or snow. It is also a time when nighttime lasts much longer than daylight.

The mechanics of winter are driven by the tilt of the Earth's axis. The Earth is tilted at an angle of 23.44° relative to its orbital plane. As the Earth moves in its elliptical orbit, this tilt causes different latitudes to face the Sun directly at different times. When a hemisphere is oriented away from the Sun, that region experiences winter. During this time, the Sun has a lower maximum altitude in the sky. This means the Sun appears lower on the horizon than it does in summer. Because of this low altitude, sunlight hits the Earth at an oblique angle. This angle causes a lower amount of solar radiation to strike the Earth per unit of surface area. Additionally, the light must travel a longer distance through the atmosphere. This allows the atmosphere to dissipate more heat before it reaches the surface.

There are several ways to define the timing of winter. Meteorological reckoning is one method used by scientists. Meteorologists define winter based on sensible weather patterns and average temperatures. In the Northern Hemisphere, this is typically December, January, and February. In the Southern Hemisphere, it covers June, July, and August. Another method is astronomical reckoning. This is based on fixed points in the Earth's orbit. One astronomical definition marks the winter solstice as the middle of winter. The solstice is the moment when the Sun's elevation is at its most negative value relative to the pole. This day features the shortest period of daylight and the longest night.

Historical records show that different cultures have viewed winter through various lenses. The English word winter likely comes from the Proto-Germanic noun *wintru-. Some scholars suggest it connects to a root meaning water. Ancient Indo-European cultures often recognized only two seasons: winter and summer. In the ancient Roman Julian calendar, winter began on 10 November and ended on 6 February. The midpoint was the winter solstice on 25 December. Other traditions had different markers. The Norsemen celebrated the festival of Winter Nights between 9 and 15 October. In China, the season begins around 7 November on the day of Lìdōng.

Winter weather varies greatly depending on geography and climate. In the Northern Hemisphere, large land masses often lead to more snow and ice. For example, Winnipeg, Canada, is a landlocked city in the Great Plains. It experiences very cold January temperatures. In contrast, Vancouver, Canada, is on the west coast. It experiences milder temperatures due to moderating Pacific winds. The Southern Hemisphere often has milder winters because of its maritime climate. There is less land south of 40°S, which keeps temperatures more stable. However, snow still occurs in elevated regions like the Andes or the mountains of New Zealand.

Biological life must adapt to survive the hibernal season. Ecologists use the term "hibernal" to describe this period of biological dormancy. Many animals use migration to move to warmer areas. Others use hibernation, which is a state of reduced metabolic activity. This is different from sleep because metabolic functions are markedly reduced. Animals like bears, bats, and frogs become dormant to survive. Some animals, such as squirrels and raccoons, do not hibernate but instead store food. Other animals use resistance to endure the cold. For instance, the Arctic fox and the snowshoe hare change their fur to white. This provides cryptic coloration, which helps them camouflage against the snow.

Winter also produces unique atmospheric phenomena. In very cold air, diamond dust can form. This consists of simple hexagonal ice crystals. It happens when moisture mixes with extremely cold, surface-based air. In some regions, winter brings heavy precipitation and prolonged dampness. This can be caused by permanent snow cover or low temperatures that prevent evaporation. In Scandinavia, the intensity of winter can depend on pressure systems. If Atlantic low-pressure systems move in specific routes, high-pressure systems can bring even colder temperatures. This demonstrates how complex global weather systems dictate the experience of the season.