Stop Shivering -- Today Is Not Actually The First Day Of Winter

Astronomical cycles do not lie.

It’s finally gotten cold in the southern half of Australia, and the calendar says it’s June 1. So it must be the start of winter this chilly Friday, right?

Well, right and wrong. Allow me to explain.

Astronomically speaking, seasons actually start around the 21st of the month, give or take a day.

It’s all to do with the 23.5 degree tilt of the earth and the fact that the sun shines more on the northern or southern hemisphere at different times of the year -- plus some other stuff you learnt in high school geography but have long since forgotten.

On June 21 (give or take), the sun is directly over the Tropic of Cancer in the northern hemisphere. That means less sunlight for us down here in Australia and cooler weather for the next three months. Simple.

When you think about it, a winter that runs from June 21 to September 21 rather than June 1 to September 1 makes intuitive sense. There’s rarely any decent snow down in our ski resorts till late June and it always sticks around till late September.

Conversely, summer in most parts of Australia never really hots up until just before Christmas and stays that way most years until late March.

"It’s all to do with the 23.5 degree tilt of the earth," Image: Getty.

Astronomical cycles do not lie. The real seasons do not start and finish on the first of the month.

In America, where they love their old-school metrics like the imperial system and the Fahrenheit temperature scale, they measure seasons from the 21st. So that’s one thing they actually get technically right.

But the Australian Bureau of Meteorology -- along with many other similar weather bodies around the world like the UK’s Met Office --measure the seasons from the first of the month. Winter is June 1 to August 31 and so on.

They do this for convenience. Or as a spokesperson for the Bureau once told me, “Because calendar months work nicely for everybody.”

Breaking the seasons into convenient three-month blocks does indeed work nicely. It helps the Bureau deliver seasonal data and predictions in a time-frame we can all relate to.

Speaking of seasonal predictions, the Bureau released its monthly and seasonal outlooks yesterday, and it’s good news for those who hate the cold. The Bureau predicted that:

  • The southeast mainland of Australia is likely to be drier than average across the whole of winter
  • June has high chances of being drier than average in the southeast mainland, and is also likely to be drier across much of central and western Australia, except the southwest.
  • Winter days and nights are likely to be warmer than average for most of Australia, except parts of the far north.

According to the Bureau, below-average pressure over the Tasman Sea is likely to weaken the westerlies that bring rain to southern Australia.

This means fewer outbreaks of that awful gusty cold showery weather which most of us call “a typical day in Melbourne”, and which snow-lovers rely on for decent falls.

This is a broad seasonal outlook and should not be relied on. Weather has a way of making fools of meteorologists the way the economy laughs at economists.

But overall, the Bureau has identified several strong underlying pointers to a dryer and warmer winter than usual.

Climate change also inevitably plays a role in the likelihood of a warmer winter.

The graph below uses data from over 100 Bureau of Meteorology weather stations, many of which have been collecting data all the way back to 1910. It clearly shows an overall warming trend in line with the predictions of climate science.

In a warming world, an average or cooler than average winter becomes less likely each year.

Image: Australian Bureau of Meteorology.

Meanwhile, it is now just eight days until the 2018 Australian ski season starts on the Saturday of the Queen’s Birthday long weekend.

While a very light dusting of snow has fallen this week, the ski slopes are still mostly grass, bushes and rock. However, all resorts should be able to open beginner slopes on the holiday weekend thanks to snowmaking.

While dry weather is obviously not conducive for natural snowfalls, it creates cold nights ideal for pumping out the mixture of compressed air and water that’s almost indistinguishable from “real” snow.

When the real snow comes is anyone’s guess. Season 2017 was a bumper season in NSW and Victoria, but such seasons become a little less likely each year.