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Winter is one of the four seasons of temperate zones. North American calendars go by astronomy and state that winter begins on the winter solstice, and ends on the vernal equinox. Calculated meteorologically, it begins and ends earlier (typically at the start of the month with the equinox or solstice) and is the season with the shortest days and the lowest temperatures. Either way, it generally has cold weather and, especially in the higher latitudes or altitudes, snow and ice.
Meteorology
Animation of snowcover changing with the seasons.
Animation of snowcover changing with the seasons.

Meteorological winter is the season having the shortest days and the lowest temperatures. Nighttime predominates the winter season, and in some regions it has the highest rate of precipitation as well as prolonged dampness due to permanent snow cover in such areas. During winter, Blizzards often develop and cause many transportation delays. A rare meteorological phenomenon encountered during winter is ice fog, which is composed of ice crystals suspended in the air and happening only at very low temperatures, below about −30 °C
Period

It is often said that, astronomically, winter starts with the winter solstice, and ends with the vernal equinox. In meteorology, it is by convention counted instead as the whole months of June, July and August in the Southern Hemisphere and December, January and February in the Northern Hemisphere. While in actuality, the most accurate start and end point is simply defined by when the first major wave of cold fronts and warm fronts hit a particular area, having no universally predetermined dates.

In Celtic countries such as Ireland using the Irish calendar, the winter solstice is traditionally considered as midwinter, the winter season beginning November 1 on All Hallows or Samhain. Winter ends and spring begins on Imbolc or Candlemas, which is February 1 or 2. This system of seasons is based on the length of days exclusively. The three-month period of the shortest days and weakest solar radiation occurs during November, December and January in the Northern Hemisphere (May-July in the Southern).

Also many mainland European countries tend to recognize Martinmas, St. Martins day (November 11) as the first calendar day of winter. The day falls at midpoint between the old Julian equinox and solstice dates. Also, Valentines Day (February 14) is recognized by some countries as heralding the first rites of spring, such as flower blooming.

In Chinese astronomy (and other East Asian calendars), winter is taken to commence on or around November 7, with the Jiéqì known as (立冬 lì dōng, literally "establishment of winter".)

The three-month period associated with the coldest average temperatures typically begins somewhere in late November or early December in the Northern Hemisphere. If "winter" is defined as the statistically coldest quarter of the year, then the astronomical definition is too late by almost all local climate standards, and the traditional English/Irish definition of November 1 (May 1 in the Southern Hemisphere) is almost always too early to fit this standard. No matter the reckoning, winter is the only season that spans two calendar years in the northern hemisphere. (In other words, there are very few temperate climates in which the vernal equinox is on average colder than the winter solstice, and very few temperate climates in which Samhain is colder than Imbolc).
Causes

The tilt of the earth's axis relative to its orbital plane has a dramatic effect on the weather. The Earth is tilted at an angle of 23°27' (23 degrees 27 minutes) 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 in winter 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 (see Effect of sun angle on climate). This effect is compounded by the larger distance that the light must travel through the atmosphere, allowing the atmosphere to dissipate more this already limited heat.
Symbolism

Winter is highly symbolic of many things to many people and has been used to represent various things by artists in all media. Some use winter to suggest death, as in Robert Frost's "Stopping by Woods on a Snowy Evening". Some use it to suggest the absence of hope, as in C. S. Lewis's The Lion, the Witch and the Wardrobe, where it was always winter but never Christmas. Winter is one concerto in Vivaldi's "The Four Seasons"; and there are many examples of four paintings, all showing the same scene in different seasons. Ursula K. LeGuin's novel The Left Hand of Darkness is set on a planet named Winter. In Alex Raymond's comic strip, Flash Gordon, there is a land called Frigia, where it is always winter. The land of Frigia is also featured in the serial Flash Gordon Conquers the Universe. Other uses of winter in the graphic arts occur in Winsor McCay's Little Nemo in Slumberland. There are many films in which a winter setting plays an important role, Fargo being an example. Novels such as Ethan Frome also use a winter setting to mirror the bleak, frozen feelings that the characters harbor. The film Requiem for a Dream concludes with Act III: Winter, in which the movie reaches its hellish and chilling climax.