Summer

Summer or summertime is the hottest and brightest of the four temperate seasons, occurring after spring and before autumn. At or centred on the summer solstice, daylight hours are the longest and darkness hours are the shortest, with day length decreasing as the season progresses after the solstice. The earliest sunrises and latest sunsets also occur near the date of the solstice. The date of the beginning of summer varies according to definition, climate, tradition, and culture. When it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere, and vice versa.

The modern English summer derives from the Middle English somer, via the Old English sumor.

The meteorological convention defines 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 countries including Australia, New Zealand, Austria, Denmark, Russia and Japan. It is also used by many people in the United Kingdom and Canada. In Ireland, the summer months according to the national meteorological service, Met Éireann, are June, July and August.

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. Reckoning by hours of daylight alone, summer solstice marks the midpoint, not the beginning, of the seasons. Midsummer takes place over the shortest night of the year, which is the summer solstice, or on a nearby date that varies with tradition.

A variable seasonal lag means that the meteorological midpoint of the season, which is based on average temperature patterns, occurs several weeks after the time of maximal insolation.

In the Julian calendar used in the ancient Roman world, summer began on 9 May, its midpoint was the summer solstice on 24 June, and summer ended on 10 August.

Likewise, in Anglo-Saxon England, summer began on 9 May and its midpoint was the summer solstice or Midsummer, on 24 June. In England, 24 June continued to be called Midsummer Day and was one of the quarter days of the English calendar. Elsewhere in northern Europe, midsummer and the solstice were traditionally reckoned as the night of 23–24 June.