Spring, the season of the year occurring between winter and summer. Astronomically, spring in the Northern Hemisphere begins about March 21 and ends about June 21, when the northern summer starts. In the Southern Hemisphere, spring lasts from about September 23 to December 22. The season's exact start in the Northern Hemisphere occurs at the vernal, or spring, equinox, when the sun crosses the Equator from south to north. In the Southern Hemisphere spring begins at the autumnal, or fall, equinox.
Beyond the tropics, in regions having four distinct seasons, spring marks the start of warmer weather, renewed plant growth, and increased animal activity. Because the entire hemisphere does not become warmer at the same time, these changes may start either before or after the equinox, depending to a great extent on distance from the Equator. The atmosphere is often somewhat unstable in spring, making stormy and changeable weather common.