the winter solstice, being the day of the year which has fewest hours of daylight, ought to be the middle of the season, but seasonal lag means that the coldest period normally follows the solstice by a few weeks. In the USA (and sometimes in Britain) the season is regarded as beginning at the solstice and ending on the following equinox â€” in the Northern Hemisphere, depending on the year, this corresponds to the period between 21 or 22 December and 20 or 21 March. In the UK, meteorologists consider winter to be the three coldest months of December, January and February. In Scandinavia, winter traditionally begins on 14 October and ends on the last day of February. 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 through July in the Southern Hemisphere.)
Also, many mainland European countries tend 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.
In Chinese astronomy and other East Asian calendars, winter is taken to commence on or around 7 November, with the JiÃ©qÃ¬ (known as 立冬 lÃ¬ dōngâ€”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 late February or early March. This "thermological winter" is earlier than the solstice delimited definition, but later than the daylight (Celtic) definition. Depending on seasonal lag, this period will vary between climatic regions.
Cultural influences such as Christmas creep may have led to the winter season being perceived as beginning earlier in recent years, although high latitude countries like Canada and Russia are usually well into their real winters before the December solstice.
See also: Effect of sun angle on climate
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 23.44Â° 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. Compared with these effects, the changes in the distance of the earth from the sun are negligible.