Spring is one of the four temperate seasons, the transition period between winter and summer. Spring and "springtime" refer to the season, and broadly to ideas of rebirth, renewal and regrowth. The specific definition of "spring" as a season differs, however, among scientific disciplines such as astronomy and meteorology, and in cultural and human terms. The exact definitions are explored below.
Meteorologists generally define four seasons in many climatic areas, winter, spring, summer and autumn (or fall). These are demarcated by the values of their average temperatures on a monthly basis, with each season lasting three months. The three warmest months are by definition summer, the three coldest months are winter, and the intervening gaps are spring and autumn. Spring, when defined in this manner, can start on different dates in different regions. In terms of complete months, in most North Temperate Zone locations, spring months are March, April and May (Summer is June, July, August; autumn is September, October, November; winter is December, January, February). The vast majority of South Temperate Zone locations will have opposing seasons with spring in September, October and November.
Astronomically, the vernal equinox (usually 20 March in the Northern Hemisphere, and 22 September in the Southern Hemisphere), should be the middle of spring (based on the angle of the sun and insolation) and the summer solstice (usually 21 June in the Northern Hemisphere and 21 December in the Southern Hemisphere) should be the middle of summer (because the sun is at its highest), but daytime temperatures lag behind insolation by several weeks because the earth and sea have thermal latency and take time to warm up.
Some cultures, such as those that devised the Celtic and East Asian calendars, call the spring equinox "mid-spring", but others (especially in the USA and sometimes in England) regard it as the "first day of spring". For most temperate regions, signs of spring appear long before the middle of March, but the folklore of 21 March being the "first day of spring" persists, and 21 June as the "first day of summer" is common in the USA. In South Africa, Australia and New Zealand, spring begins on 1 September, and has no relation to the vernal equinox.
In East Asian Solar term, spring begins on 4 February and ends on 5 May. Similarly, according to the Celtic tradition, which is based solely on daylight and the strength of the noon sun, spring begins in early February (near Imbolc or Candlemas) and continues until early May (Beltane).
The 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. It therefore varies according to the climate and according to the specific weather of a particular year.