More direct rays strike the earth in middle latitudes north than middle latitudes south.
Also, the days are longer in north than in south. Angle of sun striking the earth, length of day both caused by the tilt of the earth relative to the plane of its orbit around the sun. During the Northern Hemisphere's summer, the day-light hours in northern latitudes are longer than in middle latitudes. Explain why northern latitudes are not warmer. The angle of the sun is lower in the northern latitudes so the amount of energy per square area is less than in the middle latitudes.