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How can Tasmania have four seasons in one day?

By meteorologist Tristan Oakley, Tuesday August 3, 2021 - 03:54 EST
ABC image
Most people who have been to Tasmania know how fickle the weather can be. - ABC

In 1992, the band Crowded House made "four seasons in one day" famous after Neil and Tim Finn decided Melbourne's weather was worth writing a song about.


With only Bass Strait separating Victoria from Tasmania, Tasmanians can empathise with what the Finn brothers were singing about.


When an interstate friend or family member comes to visit and asks what clothes to bring, we often recite it. 


But what causes the four seasons in one day experience?


The answer is not entirely straightforward.


Obviously, it would not be possible to literally fit all four seasons into 24 hours.


So, what are we referring to when it feels like four seasons in one day?


Firstly, let's define our seasons and how weather elements of rainfall, temperature and wind play their parts.


In Tasmania, summer is warm to hot with minimal rainfall. 


Autumn is usually windy, can be wet, and days start to get colder. 


Winter is cold and generally the wettest season. Spring is usually windy, can be wet, and days start to get warmer.


For the optimal four seasons in one day experience, we want weather situations or phenomena that can create a day that is dry, wet, hot, cold and windy.


A cold front is the obvious weather feature to produce this.


A cold front is a synoptic scale (large scale) feature that marks the boundary between a warm and cold air mass.


A cold front crossing Tasmania is often preceded by warm, dry north-westerly winds, and followed by colder south-westerly winds and, often, rainfall.


There you have it, the cold front has ticked all the boxes: warm, dry, cold, wet and windy. It's worth noting, timing is important with cold fronts.


If a cold front crosses Tasmania overnight, it would only result in 'four seasons in one night',


An example of a cold front-induced four seasons in one day occurred in Hobart on May 10, 2021.


The observed conditions at 9am: 15.9 degrees Celsius with 19kph north-westerly winds.


The maximum temperature (17.5C) occurred during the morning and by 12:30pm the temperature had dropped below 10C and didn't rise for the rest of the day.


By 3pm, the observed conditions were: 9.5C with 15kph southerly winds.


Additionally, 3.2mm of rain fell between 9am on May 10 and 9am on May 11 and the strongest recorded wind gust was 46kph.


Sometimes the change when a cold front crosses is even more pronounced.


At midday on November 21, 2019, the temperature was 36.1C in Hobart and plunged to 26C in just 10 minutes. By 4pm, the temperature was 17.9C.


Another cause of four seasons in one day is a sea breeze combined with a shower.


This is more localised and is a mesoscale (small scale) weather feature.


When the land is noticeably warmer than the water (usually during warmer months) a sea breeze can form, bringing increases in wind speed and pushing cooler air from over water onto land.


The boundary between the cool air displacing the warm air is called a sea breeze front.


Initially, the weather is dry but add enough moisture and atmospheric instability and a shower, or even a thunderstorm, can form on the sea breeze front.


Once again, we have all ingredients for four seasons in one day: warm, dry, cold, wet and windy.


A frosty winter morning in northern Tasmania followed by increasing northerly winds and developing rain is another situation for four seasons in one day.


The day starts cold and dry, the afternoon is warmer and windier, and then wet when the rain starts.


This is a more subtle four seasons in one day experience because there is no abrupt temperature change like with fronts.


Of course, there are other weather situations and phenomena that can produce four seasons in one day in Tasmania.


There is also a cheat way to get the experience by driving between two locations with different microclimates, such as Hobart and kunanyi/Mount Wellington.


Westerly winds can produce showers and windy conditions on the mountain, but not in Hobart.


The temperature will also be significantly colder at the summit, than in Hobart.


So, in Hobart we get dry and warm, and on kunanyi/Mount Wellington we get cold, wet and windy.


The varying weather conditions in Tasmania mean it is always worth paying close attention to the forecast because "even when you're feeling warm, the temperature could drop away, like four seasons in one day".


Tristan Oakley is a meteorologist with the Bureau of Meteorology







- ABC

© ABC 2021

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