Winter (/ˈwɪntər/) is the coldest season of the year in polar and temperate climates, between autumn and spring. Winter is caused by the axis of the Earth in that hemisphere being oriented away from the Sun.
Different cultures define different dates as the start of winter, and some use a definition based on weather. When it is winter in the Northern Hemisphere it is summer in the Southern Hemisphere, and vice versa. In many regions, winter is associated with snow and freezing temperatures.
The moment of winter solstice is when the sun’s elevation with respect to the North or South Pole is at its most negative value (that is, the sun is at its farthest below the horizon as measured from the pole).
The earliest sunset and latest sunrise dates outside the polar regions differ from the date of the winter solstice, however, and these depend on latitude, due to the variation in the solar day throughout the year caused by the Earth’s elliptical orbit (see earliest and latest sunrise and sunset).
The tilt of the Earth’s axis relative to its orbital plane plays a big role in the weather. The Earth is tilted at an angle of 23.44° to the plane of its orbit, and this causes different latitudes on the Earth to directly face the Sun as the Earth moves through its orbit. It is this variation that primarily brings about the seasons.
When it is winter in the Northern Hemisphere, the Southern Hemisphere faces the Sun more directly and thus experiences warmer temperatures than the Northern Hemisphere. Conversely, winter in the Southern Hemisphere occurs when the Northern Hemisphere is tilted more toward the Sun. From the perspective of an observer on the Earth, the winter Sun has a lower maximum altitude in the sky than the summer Sun.
During winter in either hemisphere, the lower altitude of the Sun causes the sunlight to hit that hemisphere at an oblique angle. In regions experiencing winter, the same amount of solar radiation is spread out over a larger area. This effect is compounded by the larger distance that the light must travel through the atmosphere, allowing the atmosphere to dissipate more heat. Compared with these effects, the changes in the distance of the earth from the sun are negligible.