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 and universities 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.
Summer in Fethiye, Turkey.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 the United States, summer is often fixed as the period from the summer solstice (June 20 or 21, depending on the year) to the fall equinox (September 22 or 23, again depending on the year). Unofficially, the U.S. summer season is commonly regarded as beginning on Memorial Day weekend and ending on Labor Day weekend. Likewise, another set of pop-cultural reference points for summer in the USA 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.
Wet season thunderstorm at night in Darwin, Australia.See also: Hail, Tropical cyclone, and Wet season
Summer is traditionally associated with hot dry weather, but this does not occur in all regions. The wet season occurs during summer across many parts of the tropics and subtropics. Where the wet season is associated with a seasonal shift in the prevailing winds, it is known as a monsoon. The wet season is the main period of vegetation growth within the savanna climate regime. However, this also means that wet season is a time for food shortages before crops reach their full maturity. This causes seasonal weight changes for people in developing countries, with a drop occurring during the wet season until the time of the first harvest, when weights rebound. Malaria incidence increases during periods of high temperature and heavy rainfall.
The onset of the rainy season signals the departure of the Monarch butterfly from Mexico. Tropical species of butterflies show larger dot markings on their wings to fend off possible predators and are more active during the wet season than the dry season. Within the tropics and warmer areas of the subtropics, decreased salinity of near shore wetlands due to the rains causes an increase in crocodile nesting. Other species, such as the arroyo toad, spawn within the couple months after the seasonal rains. Armadillos and rattlesnakes seek higher ground.
Image of Hurricane Lester from late August 1992.In the Northern Atlantic Ocean, a distinct tropical cyclone season occurs from 1 June to 30 November, sharply peaking from late August through September. The statistical peak of the Atlantic hurricane season is 10 September. The Northeast Pacific Ocean has a broader period of activity, but in a similar time frame to the Atlantic. The Northwest Pacific sees tropical cyclones year-round, with a minimum in February and March and a peak in early September. In the North Indian basin, storms are most common from April to December, with peaks in May and November. In the Southern Hemisphere, the tropical cyclone year begins on 1 July and runs all year round and encompasses the tropical cyclone seasons which run from 1 November until the end of April with peaks in mid-February to early March.
Across interior North America, cumulonimbus clouds produce hail between the months of March and October during the afternoon and evening hours, with the bulk of the occurrences from May through September. Cheyenne, Wyoming is North America's most hail-prone city with an average of nine to ten hailstorms per season.
In higher latitude locations, summer is the time for road resurfacing, as winter ice and snow leaves potholes behind in the pavement due to the expansion and contraction of ice and snow during the winter months. Construction jobs tend to have minimum temperature requirements in order for work to be accomplished, such as the laying of concrete. This is because materials like concrete take increasingly longer to dry within cold temperature regimes. Also, working within warmer weather regimes is done to prevent expansion of ice within the new material, which decreases its potential strength and integrity.
 School break
In most countries children are out of school during this time of year for summer holidays, although dates vary. In the Northern hemisphere, some begin as early as mid-May, although in England and Wales, school ends in mid- to late July. In the Southern hemisphere, school holiday dates include the major holidays of Christmas and New Year's Day. Summer school holidays in Australia begin a few weeks before Christmas and end in late January to mid-February, with the dates varying in different states.