Summer is the hottest of the four temperate seasons, falling after spring and before autumn. At the summer solstice, the days are longest and the nights are shortest, with day length decreasing as the season progresses after the solstice. The date of the beginning of summer varies according to climate, tradition, and culture. When it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere, and vice versa.
From an astronomical view, the equinoxes and solstices would be the middle of the respective seasons, but sometimes astronomical summer is defined as starting at the solstice, the time of maximal insolation, often identified with the 21st day of June or December. A variable seasonal lag means that the meteorological center of the season, which is based on average temperature patterns, occurs several weeks after the time of maximal insolation. The meteorological convention is to define summer as comprising the months of June, July, and August in the northern hemisphere and the months of December, January, and February in the southern hemisphere. Under meteorological definitions, all seasons are arbitrarily set to start at the beginning of a calendar month and end at the end of a month. 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. The meteorological reckoning of seasons is used in Australia, Austria, Denmark, Russia and Japan. It is also used by many in the United Kingdom. In Ireland, the summer months according to the national meteorological service, Met Éireann, are June, July and August. However, according to the Irish Calendar, summer begins on 1 May and ends on 1 August. School textbooks in Ireland follow the cultural norm of summer commencing on 1 May rather than the meteorological definition of 1 June.
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.
Where a seasonal lag of half a season or more is common, reckoning based on astronomical markers is shifted half a season. By this method, in North America, summer is the period from the summer solstice (usually 20 or 21 June in the Northern Hemisphere) to the autumn equinox.
Reckoning by cultural festivals, the summer season in the United States is traditionally regarded as beginning on Memorial Day weekend (the last Weekend in May) and ending on Labor Day (the first Monday in September), more closely in line with the meteorological definition for the parts of the country that have four-season weather. The similar Canadian tradition starts summer on Victoria Day one week prior (although summer conditions vary widely across Canada's expansive territory) and ends, as in the United States, on Labour Day.
In southern and southeast Asia, where the monsoon occurs, summer is more generally defined as lasting from March, April, May and June, the warmest time of the year, ending with the onset of the monsoon rains.
Because the temperature lag is shorter in the oceanic temperate southern hemisphere, most countries in this region use the meteorological definition with summer starting on 1 December and ending on the last day of February.
Summer is traditionally associated with hot or warm weather.
In the northern Atlantic Ocean, a distinct tropical cyclone season occurs from 1 June to 30 November. 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 season runs from 1 November until the end of April with peaks in mid-February to early March.
Schools and universities typically have a summer break to take advantage of the warmer weather and longer days.
In the Southern Hemisphere, school summer holiday dates include the major holidays of Christmas and New Year's Day. School summer holidays in Australia, New Zealand and South Africa begin in early December and end in early February, with dates varying between states. In South Africa, the new school-year usually starts during the 2nd week of January, thus aligning the academic year with the Calendar year. In India, school ends in late April and resumes in early or mid June. In Cameroon and Nigeria, schools usually finish for summer vacation in mid-July, and resume in the later weeks of September or the first week of October.
A wide range of public holidays fall during summer, including:
- Memorial Day (United States) or Victoria Day (Canada) through Labor Day
- Independence Day (Jordan) (25 May)
- Bank holidays in the United Kingdom and Ireland
- Festa della Repubblica, Italian national day and republic day (2 June)
- National Day of Sweden (6 June) and Midsummer, sometimes referred to as the "alternative National Day"
- Canada Day (1 July)
- Independence Day (United States) (4 July)
- Bastille Day, National Day of France (14 July)
- Belgian National Day (21 July)
- Ólavsøka, Faroe Islands (29 July)
- Swiss National Day (1 August)
- Independence Day (Pakistan) (14 August)
- Independence Day (India) (15 August)
- Australia Day (26 January)
People generally take advantage of the high temperatures by spending more time outdoors during summer.
Summer is normally a low point in television viewing, and television schedules generally reflect this by not scheduling new episodes of their most popular shows between the end of May sweeps and the beginning of the television season in September, instead scheduling low-cost reality television shows and burning off commitments to already-cancelled series. There is an exception to this with children's television. Many television shows made for children and are popular with children are released during the summer months, especially on children's cable channels such as the Disney Channel in the United States, as children are off school. Disney Channel, for example, ends its preschool programming earlier in the day for older school age children in the summer months while it reverts to the original scheduling as the new school year begins. Conversely, the music and film industries generally experience higher returns during the summer than other times of the year and market their summer hits accordingly. Summer is most popular for animated movies to be released theatrically in movie theaters.
With most school-age children and college students (except those attending summer school and summer camp) on summer vacation during the summer months, especially in the United States, travel and vacationing traditionally peaks during the summer, with the volume of travel in a typical summer weekend rivaled only by Thanksgiving. Teenagers and college students often take summer jobs in industries that cater to recreation. Business activity for the recreation, tourism, restaurant, and retail industries peak during the summer months as well as the holiday season.
- Children cycling during summer
- Barefoot skiing
- Fig trees bear fruit when summer is near
- Using a personal cooling device in summer