So, a bit of sun comes out this week and we all scream “heatwave!”. It has been difficult to escape the heat on at least one of the last seven days — or the news and social media reporting on it (guilty as charged).

Heat records have tumbled daily. To name a few: last Friday, Hobart recorded its hottest ever day, reaching 42 degrees; on Monday the nation sweltered through its hottest day on record with a maximum average temperature of 40.33 degrees; and the outback, Australia’s heat engine, has baked in an unprecedented run of days over 42 degrees.

What does it all mean? Is it a heatwave, climate change, El Nino … or just summer?

Let’s go with one of the most talked-about weather topics this week: Sydney’s 42.3 degree scorcher on Tuesday. Temperatures over 40 degrees are rare on Sydney’s coast due to the cooling effect of the afternoon seabreeze. On this day, hot and dry north-westerly winds were strong enough to inhibit the development of the seabreeze, which enabled the temperature to be higher in the CBD than the western suburbs.

Was this a heatwave? I’m sorry to spoil a good headline, but no. One hot day does not constitute a heatwave. Different regions around Australia and the world have their own set of criteria for a heatwave, for example, for Perth it’s three consecutive days where temperatures of 35 degrees or higher.

The most widely accepted definition (courtesy of the World Meteorological Organisation) is five consecutive days where the maximum temperature is at least 5 degrees above the average. Sydney’s January average maximum temperature is 26 degrees so 42.3 degrees was staggering, but the next day the temperature was back to 25 degrees.

Is it climate change? No, one individual hot day in one city cannot be directly attributed to climate change. Or even a hot week. Equally, a record cold day isn’t evidence that climate change or global warming isn’t real. While the two are related, there is a big difference between weather and climate. I like the Bureau of Meteorology’s definition: Climate is what you expect; weather is what you get.

Basically, weather is the state of the atmosphere at a given place and time, which we measure as wind, temperature, pressure etc. Climate is an average of this weather data over a long period (generally at least three decades). If you think about millions of bits of weather data collected over decades being plugged into a computer to determine trends, statistics and averages, one day or week of extreme weather may be interesting, but too brief or isolated to impact a climate average based on decades of weather for that area.

But before we ditch climate change altogether, I’d like to note that climate change is increasing the likelihood of such hot days occurring.

What about El Nino? Kind of. El Nino is that climate pattern that typically results in hotter and drier conditions over southern and eastern Australia (opposite of the rainy La Nina we had during the last two summers). Climate indicators, such as sea surface temperatures in the Pacific and the Southern Oscillation Index (atmospheric pressure difference between Tahiti and Darwin), were at El Nino levels for a little while last year but not maintained for long enough for an El Nino to be declared. Those “El Nino-type” conditions lead to a drier late winter/spring period resulting in reduced cloud cover and increased solar radiation reaching the ground due to clear skies.

This is important because the hot air mass currently engulfing central Australia started to build months ago. The Bureau of Meteorology’s most recent Climate Statement notes that the September to December 2012 period was abnormally hot across Australia with the national average maximum temperature during this time the highest on record.

Another factor at play is the Australian monsoon. Northern Australia essentially has two seasons, dry and wet. The latter is onset by the monsoon trough when it migrates south over Australia’s tropics during summer. But this year things haven’t gone to plan. The monsoon has stood us up and stayed north. This lack of cloud cover over northern Australia has enabled more solar radiation to reach the ground due to clear skies, further adding to the build up of heat over central Australia from the last four months.

If it’s not a heatwave or climate change, and kind of El Nino, was this Sydney scorcher even significant? Yes. It was the city’s fifth hottest day in over 150 years of records and the ghastly 35 degree temperature at midnight made it Sydney’s hottest night ever recorded.

But more importantly, no longer is Sydney’s scorcher just a random hot day. Rather, it’s the byproduct of a much more widespread, persistent heat event affecting Australia. Four months of record hot conditions have created this large, stagnant pool of hot air over the outback. This hot air mass is sitting there, ready to be picked up by moving weather systems and carried to other parts of country. Indeed, winds have been dumping this hot arid air over towns and cities from west to east since late December.

So while we can’t call Sydney’s 42.3 degree scorcher a heatwave in itself, or directly attribute it to climate change or El Nino, we can certainly say that it’s a symptom of a very significant, record-breaking nationwide heat event.

That’s the meteorology part. Now I’ll put my media hat on: this significant nationwide heat event needs a name. How about Mega Inland Australia Heatwave (MIAH)? It’s not an official term but let’s go with it for now.

MIAH took a long time to build and it will take a long time to disperse. And it doesn’t look like the monsoon is retreating south over northern Australia for at least another week. As a result we can continually expect to see high temperatures across most of Australia for the next few weeks. It’s a big deal.

Thanks to MIAH, it’s going to be a long, hot summer.

Peter Fray

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