From Glossary of Meteorology


Astronomically, between the summer solstice and autumnal equinox in the Northern Hemisphere, and beteen the winter solstice and vernal equinox in the Southern Hemisphere; the warmest season of the year everywhere except in some tropical regions.

Popularly and for most meteorological purposes, summer is taken to include June, July, and August in the Northern Hemisphere, and December, January, and February in the Southern Hemisphere, the opposite of winter.
See Indian summer, Old wives' summer, St. Luke's summer, St. Martin's summer.