Winter

Winter is the coldest and darkest season of the year in temperate and polar climates. It occurs after autumn and before spring, when the hemisphere is oriented away from the Sun as a result of the tilt of Earth's axis. Different cultures define different dates as the start of winter, and some use a definition based on weather.

When it is winter in the Northern Hemisphere, it is summer in the Southern Hemisphere, and vice versa. Winter typically brings precipitation that, depending on a region's climate, is mainly rain or snow. The moment of winter solstice is when the Sun's elevation with respect to the North or South Pole is at its most negative value; that is, the Sun is at its farthest below the horizon as measured from the pole. The day on which this occurs has the shortest day and the longest night, with day length increasing and night length decreasing as the season progresses after the solstice.

The earliest sunset and latest sunrise dates outside the polar regions differ from the date of the winter solstice and depend on latitude; this is due to the variation in the solar day throughout the year caused by the Earth's elliptical orbit (see: earliest and latest sunrise and sunset).

Etymology

The English word winter comes from the Proto-Germanic noun *wintru-, whose origin is unclear. Several proposals exist, a commonly mentioned one connecting it to the Proto-Indo-European root *wed- 'water' or a nasal infix variant *wend-.

Cause

The tilt of the Earth's axis relative to its orbital plane plays a large role in the formation of weather. The Earth is tilted at an angle of 23.44° to the plane of its orbit, causing different latitudes to directly face the Sun as the Earth moves through its orbit. This variation brings about 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 causes the sunlight to hit the Earth at an oblique angle. Thus a lower amount of solar radiation strikes the Earth per unit of surface area. Furthermore, the light must travel a longer distance through the atmosphere, allowing the atmosphere to dissipate more heat. Compared with these effects, the effect of the changes in the distance of the Earth from the Sun (due to the Earth's elliptical orbit) is negligible.

The manifestation of meteorological winter in the northerly snow-prone latitudes is highly variable, depending on elevation, position versus marine winds, and the amount of precipitation. For instance, Winnipeg, a city in the landlocked Great Plains region within Canada, has a January high of −11.3 °C (11.7 °F) and a low of −21.4 °C (−6.5 °F).

In comparison, Vancouver, a city on Canada's west coast with a marine influence from moderating Pacific winds, has a January low of 1.4 °C (34.5 °F), with days well above freezing, at 6.9 °C (44.4 °F). Both cities are at 49°N latitude and in the same western half of the continent. A similar but less extreme effect is found in Europe: despite resting between 49°N to 61°N, the British Isles have no non-mountain weather stations with a below-freezing mean January temperature.

Timing

Meteorological reckoning

Meteorological reckoning is the method of measuring the winter season used by meteorologists based on "sensible weather patterns" for record keeping purposes, so the start of meteorological winter varies with latitude. Winter is often defined by meteorologists to be the three calendar months with the lowest average temperatures. This corresponds to the months of December, January and February in the Northern Hemisphere, and June, July and August in the Southern Hemisphere.

The coldest average temperatures of the season are typically experienced in January or February in the Northern Hemisphere and in June, July or August in the Southern Hemisphere. Nighttime predominates in the winter season, and in some regions, winter 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. Diamond dust, also known as ice needles or ice crystals, forms at temperatures approaching −40 °C (−40 °F) due to air with slightly higher moisture from above mixing with colder, surface-based air. They are made of simple hexagonal ice crystals.

The Swedish Meteorological Institute (SMHI) defines thermal winter as when the daily mean temperatures are below 0 °C (32 °F) for five consecutive days. According to the SMHI, winter in Scandinavia is more pronounced when Atlantic low-pressure systems take more southerly and northerly routes, leaving the path open for high-pressure systems to come in and cold temperatures to occur. As a result, the coldest January on record in Stockholm, in 1987, was also the sunniest.

Accumulations of snow and ice are commonly 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°S make the winters milder; thus, snow and ice are less common in inhabited regions of the Southern Hemisphere. In this region, snow occurs every year in elevated regions such as the Andes, the Great Dividing Range in Australia, the mountains of New Zealand, and in the southerly Patagonia region of South Argentina. Snow occurs year-round in Antarctica.

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 or Chinese) definition. Depending on seasonal lag, this period will vary between climatic regions.

Cultural reckoning

Ancient Indo-European cultures seem to have divided the year into two seasons: winter and summer. This idea survived in parts of Europe into the early middle ages, co-existing with the newer idea of four seasons.

In the Julian calendar used in the ancient Roman world, winter began on 10 November, its midpoint was the winter solstice on 25 December, and winter ended on 6 February.

Likewise, in Christian Anglo-Saxon calendars, winter began on 7 November, its midpoint was the solstice or Midwinter on 25 December, and winter ended on 6 February. Bede also mentions an older 'two-season' year, where summer ended and winter began at the full moon during the month of Winterfylleth, in October.

For many mainland European countries, especially Germanic regions, winter began at Martinmas (Saint Martin's Day) on 11 November. The day falls at the midpoint between the old Julian equinox and solstice dates. It was generally regarded as the beginning of winter in late medieval and early modern England.

In Celtic nations, winter began on 1 November and the time around the winter solstice was seen as the middle of winter. In Gaelic Ireland and Scotland, winter began with the festival of Samhain on 1 November and ended with Imbolc on 1 February. Likewise, in Wales, winter began with the festival of Calan Gaeaf ("calends of winter") on 1 November and ended on 1 February.

In Scandinavia, winter is deemed to begin earlier and end later than most of Europe. Among the Norsemen, it opened with the festival of Winter Nights (Vetrnætr), which began on the Thursday between 9 and 15 October. In an old Norwegian tradition, winter begins on 14 October and ends on the last day of February.

Valentine's Day (14 February) is recognized by some countries as heralding the end of winter and beginning of spring.

In Chinese astronomy and other East Asian calendars, winter is taken to commence on or around 7 November, on Lìdōng, and end with the arrival of spring on 3 or 4 February, on Lìchūn.

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 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 20, 21 or 22 December and 19, 20 or 21 March.

Since by almost all definitions valid for the Northern Hemisphere, winter spans 31 December and 1 January, meaning that the season takes up two calendar years, as summer does in the Southern Hemisphere. Each calendar year includes parts of two winters. This causes ambiguity in associating a winter with a particular year, e.g. "winter 2001." Solutions for this problem include naming both years, e.g. "winter 2001-02", or settling on the year the season starts in or on the year most of its days belong to, which is the later year for most definitions.