Winter

Winter is one of the four seasons of the year. In the Northern Hemisphere, it begins around December and ends around March; in the Southern Hemisphere, the seasons occur at the opposite time of year. Winter is not caused by Earth being farther from the Sun. Its main cause is the tilt of Earth’s axis, which is about 23.4 degrees.

When a hemisphere is tilted away from the Sun, sunlight reaches the ground at a lower angle and the days are shorter. As a result, that hemisphere receives less solar energy and temperatures often fall, although local climate, altitude, oceans, and winds also influence winter conditions.

Around the winter solstice, the hemisphere experiencing winter has its shortest day and longest night of the year. After the solstice, daylight gradually increases, although temperatures may remain low for some time because land and water cool and warm slowly.

Snow forms when moist air contains enough water vapor and temperatures in the cloud and near the ground allow ice crystals to develop and reach the surface without melting. Winter also affects living things: many plants slow their growth, some animals migrate or become less active, and others grow thicker fur or feathers. Winter varies greatly from place to place, so a winter season can be cold and snowy in one region while remaining mild in another.