Tasmania’s hot weather and heatwaves explained
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The Bureau of Meteorology works closely with Tasmania’s emergency services to forecast conditions and model fire behaviour. (Supplied: The Mercury)
We are very fortunate in Tasmania to be surrounded by water.
On our hottest days, Bass Strait acts like a buffer, decreasing the temperature about the north coast and usually, sea breezes don’t have too much trouble reaching most Tasmanians to cool them down in the afternoon.
A typical set-up for a hot day in Tasmania is courtesy of the mainland states.
Building heat over the mainland can be brought over Tasmania by strong northerly winds ahead of approaching frontal systems.
Think of the air as a three-dimensional blob that gets hot while sitting in the harsh summer sun over Victoria.
As that blob gets pushed south by northerly winds, it chills slightly as it travels over the cooler waters of Bass Strait, then warms up again the further it travels over Tasmania.
On days with extreme heat, Cape Bruny in the south can experience temperatures more than 40 degrees, while somewhere along the north coast like Devonport could struggle to get to 25.
Luckily for those in Tasmania, these blobs of hot air are usually pushed away to the east quickly by passing cold fronts, and it’s quite difficult to get the right weather pattern set up for having more than two or so really hot days.
Places like Adelaide can get trapped under a hot blob, with no system to move it away for days.
This can sometimes last for weeks about central Australia.
The recent heatwave in northern Tasmania around the New Year period was a little unusual.
Hot and humid air built over the north for a period of a few days without any significant wind changes to move the blob around, and the south (including Hobart) remained relatively cool due to sea breezes.
This event was caused by a strong ridge of high pressure sitting to the south, prohibiting any significant cold fronts from coming over the state and flushing out the heat and humidity.
Forecasting the temperature in Tasmania can be quite tricky, with complex coastline on all sides and varying strengths and timing of sea breezes.
A good rule of thumb for picking the hottest places in Tasmania on any given day is to draw a line that the wind would follow, and the further over land you travel the hotter it gets – until you hit a cool sea breeze, that is.
Where are the hottest places in Tasmania?
Typically, the hottest places are in the east and south-east.
The highest recorded temperature in Tasmania is 42.2 degrees Celsius at Scamander on January 30, 2009.
Hobart has been up to 41.8 on January 4, 2013, and the hottest day in Launceston was January 30, 2009 when the thermometer rose to 39.0.
What about the ‘feels like’ temperature?
The “feels like” temperature is a very basic way to estimate how a temperature might feel to us by taking into account humidity and wind chill.
In warmer months in Tasmania, more humid conditions tend to make the temperature feel more uncomfortable and will increase the feels like temperature value.
Conversely, stronger winds make it feel cooler because the passing wind has a chilling effect on the body, even if it is hot wind.
The key thing that the feels like temperature doesn’t account for though is the feeling of the sun shining on your bare skin.
Some studies have shown that it can increase what the temperature feels like by up to 8 degrees.
What is a heatwave?
A heatwave occurs when maximum and minimum temperatures both remain unusually hot over a three-day period.
Forecast maximum and minimum temperatures are compared to long-term history for that location and are also compared to the temperatures over the previous 30 days.
Put simply, it is a measure of the forecast compared to what is considered hot for that location, and how much of a shock to the body the heat will be compared to recent temperatures.
By factoring in the overnight minimum temperature, heatwave assessments also consider how difficult it could be to cool down at night after a hot day.
Not having the ability to cool down overnight makes it really hard on our bodies going into another hot day.
During hot weather conditions we usually also experience elevated fire danger.
The bureau works closely with fire agencies in Tasmania to assist with the modelling of fire behaviour on these significant days, and more generally over the fire season to assist with planning.
There is more information about hot weather on the Bureau of Meteorology website, including understanding heatwaves on the National Weather Services page.
Luke Johnston is a senior meteorologist with the Bureau of Meteorology in Tasmania
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