Spring is one of the four temperate seasons. Spring marks the transition from winter into summer.
Spring begins in the northern hemisphere around the months of March, April, May, or as reckoned astronomically extending from the vernal equinox. As in summer, the axis of the Earth is tilted toward the Sun, and the length of daylight rapidly increases. 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 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.
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.