Winter is one of the four seasons of temperate zones. Calculated astronomically, it begins on the solstice and ends on the equinox. It is the season with the shortest days and the lowest average temperatures. It has colder weather and, especially in the higher latitudes or altitudes, snow and ice. The coldest average temperatures of the season are typically experienced in January in the Northern Hemisphere and in July in the Southern Hemisphere.

Aspects

Meteorology

Accumulations of snow and ice are mostly associated with winter in the Northern Hemisphere, due to the large land masses there. In the Southern Hemisphere, the more maritime climate and the relative lack of land south of 40 degrees South makes the winters more mild, and thus snow and ice are less common in inhabited regions of the Southern Hemisphere. In the Southern Hemisphere, snow occurs every year in elevated regions such as the Andes, the Great Dividing Range in Australia, and the mountains of New Zealand, and also occurs in the southerly Patagonia region of South America. Snow occurs year-round in Antarctica.

Period

Astronomically, winter starts with the winter solstice and ends with the vernal equinox. Others consider the seasons to be defined by weather, winter being approximately the whole months of June, July and August in the Southern Hemisphere and December, January and February in the Northern Hemisphere. By still another definition, the seasons are spoken of, not as quarters of the year, but elastic periods in a particular place determined by the weather, winter in that reckoning being when the weather turns decidedly cold.

In Celtic nations such as Ireland using the Irish calendar, the winter solstice is traditionally considered as midwinter, with 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 February 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 and May-July in the Southern Hemisphere.

Also many mainland European countries tend to recognize Martinmas, St. Martin's 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 smoe countries as heralding the first rites of Spring (season), such as flower blooming.