WeatherWhys for June 20,2014 from accuweather.com

When summer begins, the sun stands directly over the Tropic of Cancer. In the weeks that follow, the direct rays of the sun will appear to migrate southward, reaching the equator on the first day of autumn. The north-south migration of the sun in the sky is caused by the tilt of the Earth's axis and the movement of the Earth around the sun. Temperature lag causes the warmest part of the summer to come four to five weeks beyond the solstice.