Today, 20 June 2012, is called the Summer Solstice and marks the longest day and shortest night of the year. This day also signifies the astronomical (or official) beginning of summer in the Northern Hemisphere.
During the Summer Solstice, the Sun is directly above the Tropic of Cancer (23.5° north) – here it comes to the highest point it can ever reach in the sky. This is the reason why the days before and after the solstice have the most daylight of any days in the year.
Meteorologists consider summer to be the hottest three months (June, July and August) of the year, with a realistic temperature peak from mid-June to mid-September.
Soon after the Summer Solstice, the length of daylight will grow shorter each day until the Winter Solstice occurs on December 21st – when the shortest amount of daylight occurs.
Also note that on the Summer Solstice, there are 24 hours of daylight north of the Arctic Circle (66.5° north of the equator) and 24 hours of darkness south of the Antarctic Circle (66.5° south of the equator).
Between the Tropic of Cancer and the Tropic of Capricorn (23.5° latitude south) – which is the region commonly known as “the tropics” – there really are no seasons as the sun is never very low in the sky so it stays warm and humid (“tropical”) all year-round.
Only those people in the upper latitudes north and south of the tropics experience seasons.