Wednesday, March 28, 2012
In spring, the axis of the Earth is increasing its tilt 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 "spring forth," giving the season its name.
The first climatological day of spring, as mandated by the World Meteorological Organization, is March 1 every year in the northern hemisphere and September 1 every year in the southern hemisphere.
Canada, the USA, and some European countries customarily observe the astronomical date (ca. March 20-21/ca. September 22-23) while Australia, New Zealand, and some other European countries follow the climatological date (March 1/September 1). Customary dates are often declared "official" by the mass media although no legal basis exists for this designation. Preferences for the astronomical or climatological dates vary in other countries. Lunar dates are used across much of east Asia to determine the beginning of spring.
The ecological beginning of spring (vernal season) has no fixed dates but is marked locally by the beginning of the growing season for most plants when the local mean daily temperature reaches 6 degrees C/42 degrees F. Many ecologists also recognize a pre-spring (prevernal) season that precedes spring. Pre-spring is a transitional time at the end of winter when only the hardiest plants like the crocus begin to bloom.
Tropical regions of the Earth do not have a spring season.