Queensland and NSW floods: some key numbers

The floods in Queensland and New South Wales are continuing to disrupt people’s lives, and the rain hasn’t quite finished yet.

What sorts of measurements have been taken? Cosmos investigates.

Start simple: how is rainfall measured?

While volume might intuitively make more sense for measuring rainfall, it’s actually recorded in height. Rainfall is measured in a tube with units of length (normally millimetres) noted on the side. Because you’re just looking for the height of rainfall, it doesn’t matter what size the tube is (although the Bureau of Meteorology has standard instruments, of course) – roughly the same height of rainfall should be falling over a square metre as would over a square centimetre.

Have records tumbled?

The sheer amount of rain that’s fallen has been extraordinary.

At Brisbane Airport alone, for instance, 232.0 mm of rain fell on 26 February, and a record 234.2 mm fell the next day. For context, this weather station records a monthly average for February of 162.6 mm. In total this year, February saw 803.8 mm fall.

Dozens of stations have recorded new local records like this. But so far, no state or national records have been broken for daily rainfall. The highest ever daily rainfall recorded in Australia was 907 mm at Crohamhurst, on Queensland’s Sunshine Coast, in 1893. New South Wales’ record is 809 mm in a day, at Dorrigo in the Northern Tablelands in 1954.

Before 9am on 28 February, Rosebank in NSW saw 701.8 mm in 24 hours, which is the highest total since that 1954 record. A non-BOM gauge nearby recorded 775mm.

But nationwide, rainfall in February was actually 24% below average. Incredibly, the Queensland state total was 18% below average.

While there has been intense rain in south-east Queensland and northern NSW, it’s been much drier than usual in other parts of the country – like northern Queensland, Victoria and Tasmania, and parts of South and Western Australia.

Welcome to a La Niña year, folks: the weather gets strange.

How does this compare to the world’s heaviest rainfall?

Because heavy rainfall is a function of time, there’s not a single answer to this. The World Meteorological Organisation keeps eight world records based on different units of time.

In Unionville, Maryland, US, on 4 July 1956, 31.2 mm of rain fell in a minute. That’s a tremendous amount to appear on the ground in a minute, but it didn’t stay that intense for long.

The heaviest recorded rainfall over the course of an hour also happened in the US: 305mm, on 22 June 1947 in Holt, Missouri, US. That’s nearly 10 times the amount of the minute record – but if we were assuming that 31mm of rain fell every minute, we’d be expecting 60 times.

The highest amount of rainfall that the WMO ever clocked in 24 hours was on 7–8 January 1966, in Foc-Foc, La Reunion. The Indian Ocean island was in the path of a tropical cyclone, and this recording station collected 1825 mm rain.

Why are all these records measured to 9am and not midnight?

Because people are involved. Historically, all of the BOM’s rain gauge stations were manual, with a person (often a volunteer) checking and emptying the rain gauge every 24 hours and sending the amount of rain recorded to the BOM. It’s easier to both visit and read a gauge at 9am than at midnight.

While there are plenty of automated stations now, using a tipping-bucket system, they still record daily rainfall totals to 9am to stay consistent with both historical observations and manual gauges.

Because rainfall is recorded to 9am, there’s an odd recording quirk that happens here: rainfall that’s recorded as falling on 10March actually fell from 9am on the 9^th until 8:59am on the 10^th. So if you notice a lot of rainfall on a Wednesday afternoon, you may be a little confused to check the records and see it’s recorded in Thursday’s total.