Meteorology
Animation of snowcover changing with the seasons.
Meteorological winter is the season having the shortest days and the lowest temperatures. Night-time predominates the winter season, and in some regions it has the highest rate of precipitation as well as prolonged dampness because of permanent snow cover or high precipitation rates coupled with low temperatures, precluding evaporation. 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 [1].
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.
Animation of snowcover changing with the seasons.
Meteorological winter is the season having the shortest days and the lowest temperatures. Night-time predominates the winter season, and in some regions it has the highest rate of precipitation as well as prolonged dampness because of permanent snow cover or high precipitation rates coupled with low temperatures, precluding evaporation. 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 [1].
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 some countries as heralding the first rites of Spring (season), 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 and lasts through early March. 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 Irish definition of November 1 (May 1 in the Southern Hemisphere) is usually too early to fit this standard. (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 on average colder than Imbolc). No matter the reckoning, winter is the only season that spans two calendar years in the northern hemisphere.
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 some countries as heralding the first rites of Spring (season), 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 and lasts through early March. 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 Irish definition of November 1 (May 1 in the Southern Hemisphere) is usually too early to fit this standard. (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 on average colder than Imbolc). No matter the reckoning, winter is the only season that spans two calendar years in the northern hemisphere.
Causes
See also: Effect of sun angle on climate
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 23.44° 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 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. This effect is compounded by the larger distance that the light must travel through the atmosphere, allowing the atmosphere to dissipate more heat.
See also: Effect of sun angle on climate
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 23.44° 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 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. This effect is compounded by the larger distance that the light must travel through the atmosphere, allowing the atmosphere to dissipate more heat.
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