A couple of cool days, it seems, is all it takes to forget we're in the midst of Sydney's hottest years on record.

Who remembers, for instance, that last month included the earliest trio of consecutive days of 32 degrees or warmer weather ever recorded in the city?

Or that average overnight temperatures exceeded any previous October by a full 1 degree, according to the Bureau of Meteorology.

Unusual heat can be harder still to recall when it lands during cooler months, as in the 12-day heatwave in May 2014, the longest of any recorded.

While unseasonably warm weather can be blissful, it can throw agriculture and nature more generally out of kilter, Sarah Perkins-Kirkpatrick, a heatwave expert at the University of NSW, said. During warmer months, the impact on health can be severe.

"Sometimes even short, sharp heat spikes can have an impact," Dr Perkins-Kirkpatrick said.

Keeping track of heatwaves in Sydney, or at dozens of sites across Australia, has now been made a lot easier thanks to a revamp of UNSW's Scorcher website.

"People might think 'It's really hot today', but what did this date look like a year or 10 years ago?", she said.

Abnormal warmth in Sydney - based on two or more consecutive days in the top 10 per cent of records for the specific date - can be identified in any year going back to 1910.

Here's how the May 2014 event appears with the solid red line spiking above the long-term trend:

The use of historic data also helps to highlight how heatwaves have become more common, as background warming from climate change makes such events much more likely.

New research by Dr Perkins-Kirkpatrick published on Friday by the American Meteorological Society found that the May 2014 event - which lasted 19 days at a national level - was 23 times more likely because of human activity such as burning fossil fuels.

"Not every year is going to be hotter than every previous year," she said. But for most Australian sites - particularly in the east and south, heatwaves are becoming more intense, more frequent and last longer."

That's something worth keeping in mind on the next chilly day.

Dr Sarah Perkins-Kirkpatrick is a senior researcher in the Climate Change Research Centre at UNSW, and will speak at Climate for Change, presented by UNSW and The Sydney Morning Herald, at UNSW on Tuesday, November 10.