Summer is the warmest of the four temperate seasons, between spring and autumn. It is marked by the longest days and shortest nights. The seasons start on different dates in different cultures based on astronomy and regional meteorology. However, when it is summer in the southern hemisphere it is winter in the northern hemisphere, and vice versa. In areas of the tropics and subtropics, the wet season occurs during the summer. Tropical cyclones develop and roam the tropical and subtropical oceans during the summer. In the interior of continents, thunderstorms are most likely to produce hail during the afternoon and evening. Schools have a summer break to take advantage of the warmer weather and longer days.
From an astronomical view, the equinoxes and solstices would be the middle of the respective seasons, but a variable seasonal lag means that the meteorological start of the season, which is based on average temperature patterns, occurs several weeks later than the start of the astronomical season. According to meteorologists, summer extends for the whole months of June, July and August in the northern hemisphere and the whole months of December, January and February in the southern hemisphere. This meteorological definition of summer also aligns with the commonly viewed notion of summer as the season with the longest (and warmest) days of the year, in which daylight predominates. From the astronomical perspective, days continue to lengthen from equinox to solstice and summer days progressively shorten after the solstice, so meteorological summer encompasses the build-up to the longest day and a diminishing thereafter, with summer having many more hours of daylight than spring.
The meteorological reckoning of seasons is used in Austria, Denmark and the former USSR; it is also used by many in the United Kingdom, where summer is thought of as extending from mid-May to mid-August. The definition based on equinox to solstice is more frequently used in the United States where a temperature lag of up to half a season is common.
Elsewhere, however, Solstices and equinoxes are taken to mark the mid-points, not the beginnings, of the seasons. In Chinese astronomy, for example, summer starts on or around 5 May, with the jiéqì (solar term) known as lìxià (立夏), i.e. "establishment of summer", and it ends on or around 6 August. An example of Western usage would be William Shakespeare's A Midsummer Night's Dream, where the play takes place over the shortest night of the year, which is the summer solstice.
In Ireland, the summer months according to the national meteorological service, Met Eireann, are June, July and August. However, according to the Irish Calendar summer begins 1 May and ends 1 August. School textbooks in Ireland follow the cultural norm of summer commencing on 1 May rather than the meteorological definition of 1 June.
In southern and southeast Asia, where the monsoon occurs, summer is more generally defined as lasting from March to May/early June, their warmest time of the year, ending with the onset of the monsoon rains.
In some areas of the United States, summer season begins on the Memorial Day weekend (the last Monday in May) and ends at the Labor Day weekend (the first Monday in September). Likewise, another set of pop-cultural reference points for summer is the time when elementary and secondary schools close down for the "summer vacation". This period usually lasts from around early to mid June until around late August to early September, depending on where the school is located. In the United States, summer is often fixed as the period from the summer solstice to the fall equinox.