Spring is one of the four conventional temperate seasons, following winter and preceding summer. There are various technical definitions of spring, but local usage of the
term varies according to local climate, cultures and customs. When it is spring in the Northern Hemisphere, it will be autumn in the Southern Hemisphere. At the spring
equinox, days are close to 12 hours long with day length increasing as the season progresses. Spring and "springtime" refer to the season, and also to ideas of rebirth,
rejuvenation, renewal, resurrection and regrowth. Subtropical and tropical areas have climates better described in terms of other seasons, e.g. dry or wet, monsoonal or
cyclonic. Often the cultures have locally defined names for seasons which have little equivalence to the terms originating in Europe. Meteorologists generally define four
seasons in many climatic areas: spring, summer, autumn (fall) and winter.
These are demarcated by the values of their average temperatures on a monthly basis, with each
season lasting three months. The three warmest months are by definition summer, the three coldest months are winter and the intervening gaps are spring and autumn.
Spring, when defined in this manner, can start on different dates in different regions. In terms of complete months, in most north temperate zone locations, spring months
are March, April and May, although differences exist from country to country. (Summer is June, July, August; autumn is September, October, November; winter is
December, January, February). Most south temperate zone locations have opposing seasons with spring in September, October and November. Swedish meteorologists
define the beginning of spring as the first occasion on which the average daytime temperature exceeds zero degrees Celsius for seven consecutive days, thus the date
varies with latitude and elevation. In Australia and New Zealand, spring conventionally begins on 1 September and ends 30 November. The beginning of spring is not always
determined by fixed calendar dates. The phenological or ecological definition of spring relates to biological indicators; the blossoming of a range of plant species, and the
activities of animals, or the special smell of soil that has reached the temperature for micro flora to flourish. It therefore varies according to the climate and according to the
specific weather of a particular year. Most ecologists divide the year into six seasons that have no fixed dates. In addition to spring, ecological reckoning identifies an earlier
separate prevernal (early or pre-spring) season between the hibernal (winter) and vernal (spring) seasons. This is a time when only the hardiest flowers like the crocus are in
bloom, sometimes while there is still some snowcover on the ground. While spring is a result of the warmth caused by the changing orientation of the Earth's axis relative to
the Sun, the weather in many parts of the world is overlain by events which appear very erratic taken on a year-to-year basis. The rainfall in spring (or any season) follows
trends more related to longer cycles or events created by ocean currents and ocean temperatures. Good and well-researched examples are the El Niño effect and the
Southern Oscillation Index. Unstable weather may more often occur during spring, when warm air begins on occasions to invade from lower latitudes, while cold air is still
pushing on occasions from the Polar regions. Flooding is also most common in and near mountainous areas during this time of year because of snowmelt, accelerated by
warm rains. In the United States, Tornado Alley is most active this time of year, especially since the Rocky Mountains prevent the surging hot and cold air masses from
spreading eastward and instead force them into direct conflict.
Besides tornadoes, supercell thunderstorms can also produce dangerously large hail and very high winds, for
which a severe thunderstorm warning or tornado warning is usually issued. Even more so than in winter, the jet streams play an important role in unstable and severe
weather in the springtime in the Northern Hemisphere. In recent decades season creep has been observed, which means that many phenological signs of spring are
occurring earlier in many regions by a couple of days per decade. Spring is seen as a time of growth, renewal, of new life (both plant and animal) being born. The term is
also used more generally as a metaphor for the start of better times, as in the Prague Spring. Spring in the Southern Hemisphere is different in several significant ways to
that of the Northern Hemisphere. This is because: there is no land bridge between Southern Hemisphere countries and the Antarctic zone capable of bringing in cold air
without the temperature-mitigating effects of extensive tracts of water; the vastly greater amount of ocean in the Southern Hemisphere at all latitudes; at this time in Earth's
geologic history the Earth has an orbit which brings it in closer to the Southern Hemisphere for its warmer seasons; there is a circumpolar flow of air (the roaring 40s and
50s) uninterrupted by large land masses; no equivalent jet streams; and the peculiarities of the reversing ocean currents in the Pacific.