A fellow Australian I follow on Twitter just said (September 1st there) it was the first day of spring. Growing up, the start of December, March, June and September were the start of summer, autumn, winter and spring respectively (or winter, spring, summer and autumn/fall for those of you in the northern hemisphere).
When I moved to the US, I noticed a lot of people talk as if the solstices and equinoxes were the seasonal boundaries.
This confused me because the solstices and equinoxes are technically the middle of the seasons, at least astronomically.
But thinking about that more, doesn't that mean that spring in the southern hemisphere (or fall in the northern hemisphere) really starts around August 8th? Not September 1st, not US Labor Day and not the equinox around September 22nd?
Well, it turns out if you measure seasons meteorologically rather than astronomically, there's a lag. So the start-of-the-month convention is a reasonable approximation of meteorological season boundaries in many places.
I say many places because the seasonal lag depends on location: both latitude and the presence of large bodies of water.
The lag isn't symmetrical either, places with large summer lag can (will?) have short winter lag and vice versa.
I haven't done this yet, but a fun project might be to extract average temperature data for a place from Wikipedia and calculate the effective lag between astronomical and meteorological seasons.
I wonder how meteorologists calculate the date of peak temperature. Is it the middle of the three month window with the highest average temperate? I guess that's a decent measure one could use for seasonal lag, even if not what is "officially" used.
This file from NOAA looks like it has the necessary data for Boston. I haven't parsed it yet to do a "windowing" based calculation of peak date but given it's averaged over 30 years (I assume from the URL) it's pretty smooth.
Visually inspecting it, it appears the peak average temperature is around July 22nd, a full month after the solstice. That would put the meteorological start of summer in Boston around June 7th.
If the meteorological midsummer is the average temperature high point and the meteorological midwinter is the average temperature low point, what is the middle of spring or autumn meteorologically speaking? Is it just the midpoint of midsummer and midwinter on either side?
Boston newsreader said today (September 22nd) was the start of fall.
Is the meteorological lag really that long?
Of course they can't be using the true meteorological lag because then the start of fall would vary from place to place (and I presume would never be that long anywhere).
I wonder what the origins of people calling the equinox the start of the season are.
The NOAA data linked above gives highest average temperature as around July 22nd and the lowest average temperature as around January 17/18th.
So the midpoint between meteorological midsummer and meteorological midwinter in Boston is around October 20th. That would put the start of fall at around September 5th (give or take a few days as midsummer and midwinter aren't exactly six months apart so we can't really assume each season is 90 days long meteorologically).
US Labor Day or even September 1st seem much better dates for the "start of fall" than September 22nd.
The only way I think one could justify a "start of fall" this late is if meteorologically the fall lag was much longer than the summer or winter lags (which might be justified depending on the meteorological definition of fall).
But the fact people view the equinox as the fixed start of the season means they can't be thinking meteorologically (as then it would vary by latitude and existence of water bodies) OR astronomically (where the equinox is mid-season by definition).
Joel Dueck points out the agricultural notion of seasons. It's been remiss of me not to mention that as presumably the agricultural definition is historically the most important.
But I'd still imagine the practical agricultural definition (i.e. what our agrarian ancestors would have based their life on) would be a function of meteorology (and hence localized).