Spring Landscape. Spring is one of the four 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 is autumn in the Southern Hemisphere and vice versa. At the spring equinox, days and nights are approximately twelve hours long, with day length increasing and night length decreasing 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, dry or wet, monsoonal or cyclonic. Cultures may have local names for seasons which have little equivalence to the terms originating in Europe. Meteorologists generally define four seasons in many climatic areas: spring, summer, autumn 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. Thus, in the US and UK, spring months are March, April and May, while in New Zealand and Australia, spring conventionally begins on September 1 and ends November 30. 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 some cultures in the Northern Hemisphere, the astronomical vernal equinox is taken to mark the first day of spring, and the summer solstice is taken as the first day of summer. In Persian culture the first day of spring is the first day of the first month which begins on 20 or 21 March. In other traditions, the equinox is taken as mid-spring. In the traditional Chinese calendar, the spring season consists of the days between Lichun, taking Chunfen as its midpoint, then ending at Lixia. Similarly, according to the Celtic tradition, which is based solely on daylight and the strength of the noon sun, spring begins in early February and continues until early May. The beginning of spring is not always determined by fixed calendar dates. The phenological or ecological definition of spring relates to biological indicators, such as the blossoming of a range of plant species, the activities of animals, and the special smell of soil that has reached the temperature for micro flora to flourish. These indicators, along with the beginning of spring, vary according to the local climate and according to the specific weather of a particular year. Some ecologists divide the year into six seasons. In addition to spring, ecological reckoning identifies an earlier separate prevernal season between the hibernal and vernal seasons. This is a time when only the hardiest flowers like the crocus are in bloom, sometimes while there is still some snowcover on the ground. 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. 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. In normally snowless areas, spring may begin as early as February or August, heralded by the blooming of deciduous magnolias, cherries, and quince. Many temperate areas have a dry spring, and wet autumn, 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 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 Nino effect and the Southern Oscillation Index.