Weather News

Records are all about timing

Ben Domensino, Wednesday January 18, 2017 - 12:29 AEDT

Last night was warmer than any official minimum temperature previously recorded in Sydney, but it wasn't a new record.

Confused? Don't worry, this long-standing weather head-scratcher isn't as illogical as it sounds.

A meteorological day runs from 9am to 9am, not midnight to midnight. This detail dates back to the early days of weather observing, when it made more sense to take manual readings at 9am rather than midnight.

Meteorologists and climatologists like consistency in their historical records, so 9am remains the official daily reporting time in the present-day around the world.

Today, air temperature is recorded at around 800 weather stations across Australia, about 500 of which are Automatic Weather Stations (AWS). Observations are made using either a mercury-in-glass thermometer, alcohol-in-glass thermometer or an electronic sensor called a Resistance Temperature Detector (RDT). The latter is able to measure ambient temperature every second.

At 9am each day, the lowest and highest temperatures observed during the previous 24 hours are recorded as the official minimum and maximum.

As this time period crosses into two days, things can get a bit confusing. Last night in Sydney was one of those cases.

Sydney's lowest temperature last night was 28 degrees, although the official minimum was 25.3 degrees, which occurred at 9am yesterday. Therefore, the city's minimum temperature record still stands at 27.6 degrees from February 2011.

The same issue can arise when a cool day is followed by a warmer night, causing the official maximum temperature to occur at night or even the following morning.

- Weatherzone

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