Apparently professional meterologists begin the seasons on the first day of the month. They have followed this convention for around 120 years, or as long as there have been professional meterologists. Australia was founded after the Enlightenment; therefore it has been pre-disposed to scientific explanations of most things. In contrast, Europe and Asia have very long traditions of astronomy/astrology that tradionally rely on the sun to mark the change of seasons.

In Australia the seasons are taken to start at the beginning of the month so that winter begins on 1 June and spans June, July and August. Supposedly, this is because in the early days of the Colony the NSW (New South Wales Corps) Corps changed from their summer to their winter uniforms at the beginning of the month. It makes sense though as experience suggests that the coldest days are in the middle of July and hence in the middle of the winter season.

There is often some confusion as there is also an astronomical winter that is from the winter solstice, 21 June until the spring equinox, 23 September. This astronomical definition comes about as the winter solstice is the day when the Sun is at its furthest north, it is at its lowest in the sky and the length of daylight is the shortest in the year.

Source:

http://www.sydneyobservatory.com.au/2010/the-start-of-winter-and-the-shortest-day-winter-solstice-2010-from-australia/

In Northern Europe at least the Nordic countries solstices mark the midpoints of each season. Midwinter is the end of December, Midsummer the end of June etc. For some reason spring equinox is sometimes seen as the beginning of spring, but I never heard anyone consider the autumn equinox the beginning of autumn. Autumn just starts whenever the leaves start falling sometime in September or october depending on your location. So basicaly when seasons begin and end depends on local cultural conventions.

It's also related to the fact that the Northern Hemisphere Temperate seasons don't really fit here. There is no "Winter" in the terms most North Americans would think of and native plants as not deciduous so the splashes of colour in Autumn are limited to areas with a high concentration of imported trees. Most of our holidays are imported from the Northern Hemisphere as well, we celebrate Halloween with Autumn themes when the temperate could easily hit 40 degrees C. We celebrate Christmas with snowy themes and then go to the beach. It adds to the disconnect between calendar and weather.

Aborigines typically saw 3 or 6 seasons, which better match Australia's climate. I find three works well for me in Sydney.

"In the northern hemisphere, it is commonly understood that the first day of Spring, Summer, Autumn and Winter occur with the relevant equinox (for Spring and Autumn) or solstice (for Summer and Winter). Every schoolchild knows this. "

This is just plain wrong.

I know it's the accepted story in North America, but picking the middle of summer (the solstice) as the start of summer makes absolutely no sense. They are using a scientifically significant occurrence (the longest day) for the wrong reason. If you want to measure seasons using daylight, then the solstice is the middle of summer, not the start - which would put the start of summer in early May (northern hemisphere)

Australia, as well as European countries just use the calendar months for an approximation of the seasons that happens to fit in pretty well when the warmest weather occurs (Summer).

Interesting to read how most of above contributors make a (failed) effort to bend Australian thinking into the universal laws of changes in the solar system.

21/6 and 21/12 are not in the 'middle of something', they mark a gentle but definite transition where the light starts to come back (for Australia 21/6) or the light starts to disappears (21/12).

But light appearing and disappearing isn't weather or climate. If that were true the coldest day of the year would be 21/6 in Australia, in reality it is closer to 15/7, the middle of Australian Winter. In that sense the Quarter Days make more sense than the Equinoxes and Solstices. By Lammas (1 August) the coldest days have finished and we are on the road to warmer weather. Maybe the Celts (and Indians and Chinese) are right after all and the seasons should be be marked on the Quarter Days.

At any rate weather seasons are always a bit arbitrary as opposed to Astronomical ones. The D'harawal people (modern Sydney area) saw 6 seasons:

Burran - Hot and Dry, January to March

Marrai'gang - Wet becoming cool, April to June

Burrugin - Cold and Frosty, June to July

Wiritjiribin - Cold and Windy, July to August

Ngoonungi - Cool becoming Warm, September to October

Parra'dowee - Warm and Wet, November to December

Months are approximate, but being a Sydneysider I can tell you this is a heck of a lot more accurate than the four season model, which ever dates you use.

Anyway, don't peddle your equinox/solstice seasonal theory in Russia, they know it's fake news. Summer began in Russia on 1 June.

When experienced temperatures or customs such as starting to use a winter uniform become the determinator for our seasons, all hell breaks loose.

With global warming these definitions will shift and change. It is all as if the opponents of Galilei are coming to life again.

The only reliable indicator is the distance and angle of the sun to the equator.

Between the Chinese and Indians you have more people using the Quarter days than the Solstice Equinox days as seasonal markers. We are talking two things here. The Astronomical seasons and the Climatological seasons. Perhaps we could add a third one, a cultural season. Does anyone argue with the Chinese over their Spring Festival/New Years? The February Quarter Day is a useful marker for them, as it was for the ancient Celts. And perhaps that is the definition of season for most people, a useful marker, something to help you plan ahead.

You want to dismiss all the other meanings of season in favour of the Astronomical meaning. That simply is neither possible nor practical.

The simple answer is that this is the convention. However, this convention is based on the observed changes in temperature (up or down) during the year. In Australia the hottest time of year is usually around the middle of January and the coldest about the middle of July. So these are considered the MIDDLE of summer and winter not the START. Obviously the northern hemisphere will be 6 months different.

It is crazy to use the solstices and equinoxes as the start of seasons as they are the middle of the solar seasons not the start. If you used the solar dates, seasons should actually start about 45.6 days before each solstice or equinox. Hence seasons should start on the 6th or 7th of November, February, May and August (winter, spring summer and autumn in the northern hemisphere and summer, autumn, winter and spring in the southern hemisphere). We know that it takes longer for temperatures to peak and trough than the peaks and troughs of the sun so it is much more convenient and sensible to use the 1st of June, September, December and March as the start of the seasons.