<P> In the Northern Hemisphere, some authorities define the period of winter based on astronomical fixed points (i.e. based solely on the position of the Earth in its orbit around the sun), regardless of weather conditions . In one version of this definition, winter begins at the winter solstice and ends at the vernal equinox . These dates are somewhat later than those used to define the beginning and end of the meteorological winter--usually considered to span the entirety of December, January, and February in the Northern Hemisphere and June, July, and August in the Southern . </P> <P> Astronomically, the winter solstice, being the day of the year which has fewest hours of daylight, ought to be in the middle of the season, but seasonal lag means that the coldest period normally follows the solstice by a few weeks . In some cultures, the season is regarded as beginning at the solstice and ending on the following equinox--in the Northern Hemisphere, depending on the year, this corresponds to the period between 21 or 22 December and 19, 20 or 21 March . </P> <P> In the UK, meteorologists consider winter to be the three coldest months of December, January and February . In Scandinavia, winter in one tradition begins on 14 October and ends on the last day of February . In Russia, currently calendar winter starts on 1 December and lasts through to the end of February, though traditionally it was reckoned from the Christmas (25 December in Julian calendar, or 7 January in Gregorian) until the Annunciation (25 March in Julian). In many countries in the Southern Hemisphere, including Australia, New Zealand and South Africa, winter begins on 1 June and ends on 31 August . In Celtic nations such as Ireland (using the Irish calendar) and in Scandinavia, the winter solstice is traditionally considered as midwinter, with the winter season beginning 1 November, on All Hallows, or Samhain . Winter ends and spring begins on Imbolc, or Candlemas, which is 1 or 2 February . 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, June and July in the Southern Hemisphere .) </P> <P> Also, many mainland European countries tended to recognize Martinmas or St. Martin's Day (11 November), as the first calendar day of winter . The day falls at midpoint between the old Julian equinox and solstice dates . Also, Valentine's Day (14 February) is recognized by some countries as heralding the first rites of spring, such as flowers blooming . </P>

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