According to the astronomical definition, spring begins on March 21 and lasts until June 20, the vernal equinox in the northern hemisphere. According to this definition, therefore, the traditional mid-summer'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. 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'). Calendars typically give the first, but the second and third are more in line with common use.
According to solar term, spring begins on February 4 and ends on May 4.
In the southern hemisphere, spring is generally accepted to begin on September 1 and last until November 30.
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 snow less areas "spring" may begin as early as February during warmer years, with subtropical areas having very subtle differences, and tropical ones none at all. Sub arctic 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.
The hurricane season officially begins in late spring, on May 15 in the northeastern Pacific and June 1 in the northern Atlantic. Before these dates, hurricanes are almost unheard of and even tropical storms are rare, one of the earliest ever being Tropical Storm Ana in mid-April 2003.
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 Prague Spring.
The first day of spring is the beginning of the new year, Nowruz, in the Iranian calendar. Nowruz (also Norooz, Newroz, Navroj, and many other variants) marks an important traditional holiday festival celebrated in Iran as well as in many other countries with a significant population from one of various Iranian peoples, such as Azerbaijan, Afghanistan and Tajikistan, and by Kurdish communities in Turkey and Iraq and elsewhere. Several Turkic peoples also celebrate Nowruz.