For meteorological purposes, summer is taken to include June, July, and August in the Northern Hemisphere, and December, January, and February in the Southern Hemisphere.
(summer; is taken; to include June, July, and August in the Northern Hemisphere, and December, January, and February in the Southern Hemisphere)
summer
is taken
to include June, July, and August in the Northern Hemisphere, and December, January, and February in the Southern Hemisphere