Spring is one of the four temperate seasons, the transition time between winter and summer. Spring and 'springtime' refer to the season, and also to ideas of rebirth, renewal and regrowth. The specific definition of the exact timing of 'spring' varies according to local climate, cultures and customs. At the spring equinox, days are close to 12 hours long with day length increasing as the season progresses. 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.