Sydney is on target to break its record for the longest run of warm nights, not good news for those who have difficulty sleeping in the humidity.

It has been two weeks since the temperature has dropped below 20 degrees in the city, the longest stint in 27 years, leading to an increasingly cranky Sydney population, says Brett Dutschke, meteorologist with The Weather Company.

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The record of 16 days, set in 1973, is almost certain to be broken as high humidity aids in keeping the temperature up.

The next few nights will fail to cool below about 23 degrees.