Spring (season)
Spring is one of the four temperate seasons. Spring marks the transition from winter into summer.
According to an astronomical definition, spring begins on the vernal equinox (usually March 20 in the northern hemisphere) and lasts until the summer solstice (usually June 21 the northern hemisphere). According to this definition, therefore, the traditional midsummer's day is the first day of summer. The meteorological definition (used in order to express whether 'spring' has been hot or cold or wet, for example) has spring starting on March 1, since this is more in line with weather conditions thought to be typical of spring. According to solar term, spring begins on February 4 and ends on May 4. Calendars typically give the first, but the second and third are more in line with common use.
In the southern hemisphere, the meteorological definition is from September 1 through November 30, corresponding to the fall months in the north.
The phenological definition of spring relates to the blossoming of a range of plant species. It therefore varies according to the climate (as in 'Spring comes late to the north-east').
In recent decades season creep has been observed, which means that many phenological signs of spring are occurring earlier by a couple days per decade.
As in summer, the axis of the Earth is tilted toward the Sun and the length of daylight rapidly increases in the northern hemisphere. The northern hemisphere begins to warm significantly causing new plant growth to "spring forth," giving the season its name. Snow begins to melt and streams swell with runoff and spring rains. Most flowering plants bloom this time of year, in a long succession sometimes beginning even when snow is still on the ground, and continuing into early summer. In normally snowless areas "spring" may begin as early as February during warmer years, with subtropical areas having very subtle differences, and tropical ones none at all. Subarctic areas may not experience "spring" at all until May or even June, or December in the outer Antarctic.
Severe weather most often occurs during the spring, when warm air begins to invade from lower latitudes while cold air is still pushing from the Polar Regions. Flooding is also most common in and near mountainous areas during this time of year because of snowmelt, many times accelerated by warm rains. In the United States, Tornado Alley is most active by far this time of year, especially since the Rocky Mountains prevent the surging hot and cold air masses from spreading eastward and instead force them directly at each other. 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 winter, the jet streams play an important role in severe weather in the springtime.
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