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Almost Melbourne's coldest June day in 25 years

Ben Domensino, Friday June 29, 2018 - 11:21 EST

Melbourne almost had its coldest June day since 1993 on Thursday, until Friday morning became too warm.

For the purpose of being able compare weather observations to historical records, the procedure for identifying daily minimum and maximum temperatures at individual locations in Australia has remained unchanged for well over a century.

At 9am each day, the lowest and highest temperatures measured during the previous 24 hours are officially recorded as that day's minimum and the previous day's maximum.

On most occasions, the maximum temperature during the 24 hours to 9am occurred on the previous day - often in the afternoon - while the minimum temperature occurred on that morning, typically around sunrise. However, this isn't always the case.

The highest temperature measured in Melbourne on Thursday was 9.8 degrees. This maximum temperature would have made it the city's coldest June day in 25 years. However, the mercury climbed to 10.6 degrees at 9am on Friday morning, which then became the Thursday's official maximum temperature. This is now only the lowest June maximum in two years.

While this method of observing daily temperatures seems odd, it's a legacy from a time when measuring the temperature was a manual task and without this consistency over time, it would be impossible to compare modern weather observations to long-term historical records.

During most of the 20th century, air temperature was measured using mercury in glass thermometers. These thermometers had a constricted neck that prevented the mercury from dropping back down the glass tube after it had risen, allowed the highest temperature of the previous 24 hours to be checked at 9am each day. A special index marker on the thermometer's neck also allowed the observer to check the lowest temperature of the previous 24 hours at 9am.

Today, air temperature at most weather stations is measured automatically using electronic sensors called resistance temperature detectors, which sample the air temperature each second.

Despite being able to monitor the temperature throughout the day and night, daily maximums and minimums are still recorded during the 24 hours ending at 9am each day.

So when comparing daily highs and lows to historical records, we have to wait until 9am to know just how they ranked.

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