MILDURA this week went close to recording its hottest day since the airport weather station opened in 1946.

The Bureau of Meteorology was forecasting a maximum of 47 degrees on Friday, which would have surpassed the previous record of 46.9 set in January 1990.

Ultimately, it fell just short of that mark at 46.8 degrees.

But what exactly does it mean when you hear about a day or a streak being the hottest "on record"?

We dug through the Bureau of Meteorology data, and a bit of history, to find out.

It starts with a key distinction ...

Why do records only go back to 1946?

Before the Mildura Airport weather station, there was the old Mildura Post Office station in the city centre on Deakin Avenue.

Records from the current and former sites are not directly comparable.

Less sophisticated equipment was used at the post office, especially between 1889 and 1906 when measurements were not made inside what’s called a Stevenson screen.

Over time, the urban heat island effect – where concrete, asphalt and heat-retaining masonry buildings can amplify temperatures – would have also had an inflationary impact on records. Today, if you put the same equipment at both locations, they would most likely return different readings.

So if you see a report that says a day or month was “Mildura’s hottest since…” and the year is prior to 1946, it’s worth noting the earlier figure may not be as reliable.

When were Mildura's hottest days?

The hottest day recorded at the Mildura Airport weather station was January 3, 1990. That day, the mercury hit 46.9 degrees.

It's one of now nine days to have hit 46 or higher.

Use the tabs in the chart below to see every day above 45 degrees since 1946.

According to the bureau's data, the highest temperature ever recorded at the old Mildura Post Office was 50.7 degrees, but this was on January 7, 1906, during the particularly unreliable period of the former station.

When was Mildura's longest streak of 45 degrees or more?

Readers had plenty of questions in January when Sunraysia Daily reported Mildura was poised to record 45 degrees temperatures on three consecutive days for the first time since before airport records started.

Some recalled previous heatwaves they believed had met such a mark.

But while that particular run of hot weather didn't ultimately make it to a third 45-degree day, it did nonetheless equal the airport's record.

This week also joined the club.

There have now been four occasions of two consecutive days at 40 or above at the airport: December 2019, January 2019, February 2017 and February 2004.

At the former station, four consecutive days in 1939 were recorded hitting the mark, culminating on January 13, the day Victoria was hit by the Black Friday bushfires.

Streaks of four days were also recorded in January 1906, January 1905 and February 1892.

When was Mildura's longest streak of 40 degrees or more?

The streak many residents recall is the one that started January 27, 2009 and culminated on February 7, the day Victoria was hit by the Black Saturday bushfires.

Mildura went 12 consecutive days reaching 40 degrees or higher in January-February 2009.

The next closest to that record was a mere six days in January 1979.

The record at the post office was nine days, which happened twice in 1939, first culminating on January 14 and then February 15.

When was Mildura's longest streak of 35 degrees or more?

The longest run of days 35 degrees or above at the airport was 14 days, which happened twice: in January 2018 and March 2008.

Perhaps surprisingly, given the run of extremely hot days, the streak of early 2009 referenced earlier didn't quite get there: it had only 13 days hitting 35.

Prior to 1946, the longest streak on record of 35-plus days was 17, which ended on February 13, 1906.

When was Mildura's hottest month?

The hot weather in January 2019 did break one key record – it was the hottest month ever recorded at the airport, with a mean maximum temperature of 37.8 degrees.

That was well above the prior record, 37.1 degrees in January 2001.

At the previous station, a mean maximum of 39C was recorded in January 1906.

Want to explore this data for yourself?

To verify the numbers in this story, you can export data from the Bureau of Meterology website, in the Climate Data Online section.

For Mildura Airport, access the daily maximum temperature page or the monthly mean maximum temperature page and select "All years of data" to download the raw data.