Spring is one of the four conventional temperate seasons, following winter and preceding summer. There are various technical definitions of spring, but local usage of the term varies according to local climate, cultures and customs. When it is spring in the Northern Hemisphere, it will be autumn in the Southern Hemisphere and vice versa. At the spring equinox, days are approximately 12 hours long with day length increasing as the season progresses.
Spring and "springtime" refer to the season, and also to ideas of rebirth, rejuvenation, renewal, resurrection and regrowth. Subtropical and tropical areas have climates better described in terms of other seasons, e.g. dry or wet, monsoonal or cyclonic. Often, cultures have locally defined names for seasons which have little equivalence to the terms originating in Europe. Spring is the time when many plants begin to grow and flower.
Meteorologists generally define four seasons in many climatic areas: spring, summer, autumn (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 most Northern Hemisphere temperate locations, spring months are March, April and May, although differences exist from country to country. (Summer is June, July, August; autumn is September, October, November; winter is December, January, February.) Most Southern Hemisphere temperate locations have opposing seasons with spring in September, October and November.
Swedish meteorologists define the beginning of spring as the first occasion on which the average daytime temperature exceeds zero degrees Celsius for seven consecutive days, thus the date varies with latitude and elevation. In Australia and New Zealand, spring conventionally begins on 1 September and ends 30 November.
During early spring, the axis of the Earth is increasing its tilt relative to 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.[citation needed]
Any snow begins to melt, swelling streams with runoff and any frosts become less severe. In climates that have no snow, and rare frosts, air and ground temperatures increase more rapidly.
Many flowering plants bloom at this time of year, in a long succession, sometimes beginning when snow is still on the ground and continuing into early summer.[citation needed] In normally snowless areas, "spring" may begin as early as February (Northern Hemisphere), heralded by the blooming of deciduous magnolias, cherries, and quince, or August (Southern Hemisphere) in the same way.[citation needed] Many temperate areas have a dry spring, and wet autumn (fall), which brings about flowering in this season, more consistent with the need for water, as well as warmth. Subarctic areas may not experience "spring" at all until May.
While spring is a result of the warmth caused by the changing orientation of the Earth's axis relative to the Sun, the weather in many parts of the world is affected by other, less predictable events. The rainfall in spring (or any season) follows trends more related to longer cycles—such as the solar cycle—or events created by ocean currents and ocean temperatures—for example, the El Niño effect and the Southern Oscillation Index.