Astronomically, the vernal equinox (usually 20 March in the Northern Hemisphere, and 22 September in the Southern Hemisphere), should be the middle of spring (b ased on the angle of the sun and insolation) and the summer solstice (usually 21 June in the Northern Hemisphere and 21 December in the Southern Hemisphere) sho uld be the middle of summer (because the sun is at its highest), but daytime tem peratures lag behind insolation by several weeks because the earth and sea have thermal latency and take time to warm up.