Spring season
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Spring is one of the four seasons of temperate zones. Astronomically, it begins with the spring equinox (around March 21 in the Northern Hemisphere, and September 23 in the Southern Hemisphere), and ends with the summer solstice (around June 21 in the Northern Hemisphere and December 21 in the Southern Hemisphere). In meteorology, it is by convention instead counted as the whole months of March, April, and May in the Northern Hemisphere and September, October, and November in the Southern Hemisphere. However, in the Irish Calendar it is counted as the whole months of February, March and April.
As in summer, the axial tilt of the Earth is toward the Sun, and daylight hours are greater than or equal to 12 hours. The hemisphere begins to warm significantly, causing new plant growth to spring forth, giving the season its name. Snow (if any) begins to melt, and rivers 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 see "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 due to 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 airmasses from spreading westward and instead force them directly at each other. Besides tornados, supercell thunderstorms can also produce dangerously large hail and very high winds, for which a severe thunderstorm warning or even tornado warning is usually issued. Often, spring storms trigger dozens of warnings, one right after the other, often simultaneously along a line hundreds of miles or kilometers long. Even more so than winter, the jet streams play an important role in severe weather in the springtime.
Some of the worst blizzards have occurred in the spring. including the Great Blizzard of 1993, which brought hurricane conditions and then light snow to northern Florida on March 13, and deposited up to five feet (1.5 meters) of snow in parts of the Appalachian Mountains. A massive springtime "upslope" winter storm in 2003 brought up to eleven feet or 3.3 meters of snow to parts of Colorado and three feet or 90cm to Denver, which gets more snow in March and April (and again in October and November) than during the entire winter (December to February).
Hurricane season also begins in late spring, on May 15 in the northeastern Pacific and June 1 in the northern Atlantic. Prior to these dates, hurricanes are almost unheard of and even tropical storms are exceedingly rare, one of the earliest ever being Tropical Storm Ana in mid-April 2003.