Today is June 21, the southern hemisphere's winter solstice, which means it's the shortest day of the year across Australia. How short exactly?

• Sydney has about 9 hours and 54 minutes between sunrise and sunset today, with sunrise at 7:00 am and sunset at 4:54 pm.
• Melbourne will have 9 hours and 33 minutes, with sunrise at 7:35 am and sunset at 5:08 pm.
• Hobart, Australia’s southernmost city, will have the shortest day, with just 9 hours and one minute between sunrise at 7:42 am and sunset at 4:43 pm.
• At the other end of the country, poor old Darwin will have to cope with a mere 11 hours and 24 minutes between sunrise at 7:06 am and sunset at 6:30 pm.

The question is whether today should actually be recognised as the start of winter in Australia?

Image: It's clear as ice crystal that winter started on June 1. Source: Myriams-Fotos on Pixabay.

In many countries, seasons start on the solstices and the equinoxes (the spring and autumn equinox is when the sun sits directly over the equator, and like the solstices, they also occur around the 21st of the month). The USA is one such country. Summer in America has just officially started.

But here in Australia, our Bureau of Meteorology officially defines the seasons by months. The Bureau explains why here. It says:

"These definitions reflect the lag in heating and cooling as the sun appears to move southward and northward across the equator. They are also useful for compiling and presenting climate-based statistics on time scales such as months and seasons."

So in short, it's easier to package up the stats neatly if we define seasons by months. But is this meteorologically accurate? Or in other words, are the three calendar months of June, July and August really the coldest quarter-year period in Australia?

The short answer is that they are, or close enough anyway.

Look at the average maximums for Sydney:

• May: 19.5°C
• June: 17.0°C
• July: 16.4°C
• August: 17.9°C
• September: 20.1°C

And for Melbourne:

• May: 16.7°C
• June: 14.1°C
• July: 13.5°C
• August: 15.0°C
• September: 17.3°C

As you can clearly see, June, July and August have the coldest average max temps in Australia’s largest cities, to use just two examples.

And things tend to warm up pretty quickly in September, so if you ran a three-month average from June 21st to September 21st, any relatively mild temps in the first three weeks of June would be cancelled out by the warmer weather in the first three weeks of September.

Bottom line: it's both convenient and accurate to start the seasons on the 1st of the month, rather than the day of the equinox or solstice.

So now that we're officially in winter no matter how you measure it, we'll all be reminded of it, with the chilly, wet weather currently affecting southern WA set to head eastwards and affect pretty much all of southern Australia by the end of the week.