Yesterday was the vernal equinox, or as Phil Plait calls it, the “March and not Vernal because of the Southern Hemisphere not quite equinox so let’s say it’s the quasinox.” And about damn time, too.
In the older post he links to, he discusses the fact that we (officially) use the solstices and equinoxes to mark the boundaries between the seasons, and that in some ways it makes more sense to do what’s more common in some countries and traditions, to regard the solstices and equinoxes as the midpoints of the seasons. So, for example, June 21 (or 20) would be the middle of summer (i.e. Midsummer; here and elsewhere, I’m speaking from a Northern Hemisphere-centric point of view; adjust as needed for the Southern Hemisphere), not the start of it: summer would run from about six weeks before that — early May — to six weeks after, early August. Likewise December 21 (or 22) would be the middle of winter.
That’s even in our own cultural heritage. If the vernal equinox were the central day of spring, then spring would begin about six weeks earlier — around February 2. And in our folklore, it does… if the groundhog doesn’t see his shadow.
From a weather point of view (and Plait emphasizes he’s not considering weather, but then he says to him it makes more sense for the Sun to hit its peak at summer’s midpoint, but why that makes more sense if not for weather reasons, he doesn’t make clear) that seems, on the face of it, much more sensible. You think of summer as the hottest time of the year, and the reason it’s hot is because as the Sun’s path moves northwards, the angle of incidence gets smaller and smaller, and the amount of solar radiation per second striking a plane tangent to the Earth’s surface gets larger and larger. The angle of incidence is smallest, and the amount of solar radiation is the largest, at the summer solstice.
But, continuing from a weather point of view, the summer solstice is not the hottest day of the year (on average). Just as it takes a turkey several hours to cook after you start heating it, it takes time for the atmosphere, the water bodies, and the land to warm up. How long that takes, and therefore when peak temperature occurs, I would guess depends on a number of factors including latitude, altitude, and proximity to large water bodies and to water currents (e.g. the Gulf Stream). Just looking at a number of U.S. cities I find Phoenix, AZ seems to reach peak temperature about July 7, while Los Angeles apparently doesn’t start cooling off until around August 20. (But Los Angeles’s minimum temperature seems to be in mid-December, before the solstice! Same for Denver. Very weird, I don’t understand that.) Mid to late July, say around July 21 — a month after the summer solstice — seems to be fairly typical, though, and early to mid January for the coldest date. Center the seasons around those, rounded to a convenient nearby date, and you’d have spring from about March 1 to June 1, summer from June 1 to September 1, fall from September 1 to December 1, and winter from December 1 to March 1. Which in fact is the definition meteorologists use.