Today is September 1, which means it's the first day of spring.

Or does it?

In many countries, the seasons are based on the position of the sun relative to the earth. This is called the astronomical calendar, and in countries that use it, the seasons start on around the 20th or 21st of the month.

The USA is one such country. In America, seasons start on equinoxes (days of nearly equal sunlight around the world) and solstices (the longest or shortest day of the year, when Earth's poles reach their most extreme tilt towards or away from the sun).

Here in Australia, as with most of Europe, we start the seasons on the first day of the month in which the equinox or solstice falls. So instead of starting spring on September 20 or 21, we just draw a line and start it when the month starts.

So in Australia, yes, today is indeed officially the first day of Spring.

The Bureau of Meteorology is on record as stating that commencing seasons on the first day of the month is much less confusing to the Australian public than using the astronomical calendar. It certainly makes record-keeping simpler for everybody.

Defining seasons by the months might be convenient, but is it meteorologically accurate?

For example, when you define winter as June 1 to August 31, is that really the coldest quarter of the year - or would temperatures be slightly cooler from June 21 to September 20?

Weatherzone meteorologist Ben Domensino crunched the numbers for our largest city Sydney, and found that, on average, the coldest 91-day period of the year (roughly a quarter of the year) runs from May 29 to August 26.

Winter starts on June 1 in Australia, so in the case of Sydney, it seems our meteorological calendar is a truer reflection of winter than the astronomical calendar.

Or in other words, we're doing it right. The fact that winter seems to knock off in the last week of August will certainly ring true with anyone who experienced the harbour city's glorious run of mid-20s days last weekend.

So now that we know that it's both convenient and meteorologically accurate to start seasons on the first of the month, one important question remains: what can we expect from the spring of 2020?

"This spring, with the developing La Niña in the Pacific Ocean, all signs point to a wetter than average season, in particular in northern, central and eastern parts of the country," Ben Domensino says.

"Last spring we saw a lot of hot dry days. With potentially more cloudy conditions this spring, there is a chance some areas won't be seeing daytime maximum temperatures as high as they were last spring.

"We've already had a much slower start to the bushfire season this year and that trend should continue as we head into summer. However there may be an increased a flood risk in some areas with dams brimming, including Sydney where the city's main water storage Warragamba Dam is full."

Bottom line: while winter is officially over, best not to throw the heavy jumpers up in the attic for a few weeks yet, as we all know how variable spring weather can be. And definitely keep your raincoats on their hooks for the next three months.