Summer is the warmest season of the year, between spring and autumn. In the Northern Hemisphere, it is usually defined as the period between the summer solstice (year’s longest day), June 21 or 22, and the autumnal equinox (day and night equal in length), September 22 or 23; and in the Southern Hemisphere, as the period between December 22 or 23 and March 20 or 21. The temperature contrast between summer and the other seasons exists only in middle and high latitudes; temperatures in the equatorial regions generally vary little from month to month.