Young Blonde Girl Outside In The Spring With White Flowering Trees

Meteorologists generally define four seasons in many climatic areas: spring, summer, autumn (or fall) and winter. 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 (currently usually 20 March in the Northern Hemisphere, and 22 or 23 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.
During the vernal equinox the sun's rays are directly above the equator, making day and night (but not daylight and darkness) nearly equal on all parts of the world.
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 Ireland spring traditionally started on 1 February.