From AMS Glossary
The season of the year composing the transition period from winter to summer; the vernal season, during which the sun is approaching the summer solstice.
In popular usage and for most meteorological purposes, spring is customarily taken to include the months of March, April, and May in the Northern Hemisphere, and September, October, and November in the Southern Hemisphere. Except in the Tropics, spring is a season of rising temperatures and decreasing cyclonic activity over continents. In much of the Tropics, neither spring nor fall is recognizable, and in polar regions, both are very short-lived.