Spring is one of the four temperate seasons. Spring marks the transition from winter into summer.
Winter, Spring, Summer and Autumn (or Fall) are marked by the values of their average temperatures on a monthly basis, with each season lasting 3 months. Spring can vary per region. In the vast majority of northerly hemisphere locations Spring occurs during the months of March, April and May. (Summer is June, July, August, Autumn is September, October, November, and Winter is December, January, February.) Most Southern Hemisphere locations will have an opposite year. According to the astronomical definition, spring begins on the Vernal Equinox (usually March 20 in the Northern Hemisphere, and September 22 in theSouthern Hemisphere), and lasts until the summer solstice (usually June 21 in the Northern Hemisphere and December 21 in the Southern Hemisphere). In 2009, spring fell on March 20 in the Northern Hemisphere. According to this definition, therefore, the day called Midsummerâ€™s Day in some traditions is the first day of Summer.
The natural (phenological) definition of spring relates to 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. The first swallow to arrive for the flowering of lilac may be the indicator of spring. It therefore varies according to the climate and according to the specific weather of a particular year
In recent decades season creep has been observed, which means that many natural signs of spring are occurring earlier in many regions by a couple of days per decade.
In spring, the axis of the Earth is tilted toward the Sun and the length of daylight rapidly increases for the relevant hemisphere. The hemisphere begins to warm significantly causing new plant growth to â€œspring forth,â€ giving the season its name. Many flowering plants bloom this time of year, in a long succession sometimes beginning even if snow is still on the ground, continuing into early summer.
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, many times accelerated by warm rains. In the US, 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 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 unstable and severe weather in the springtime in the Northern Hemisphere.
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.
Spring is the end of winter, and the culmination of lengthening days.