Humidity has been at unbearable levels across eastern Australia so far this year, not only fuelling rain and thunderstorms but also making it feel significantly warmer than the temperature suggests.

Most suburbs of Sydney were forecast to reach from 29C to 31C on Thursday, however, humidity levels around 70 to 80 per cent at midday made it feel as warm as 36C, similar to the level of discomfort typically observed in Darwin.

Brisbane was also suffering through a humid Thursday — the city was a relatively cool 27C at midday but felt five degrees warmer.

The table below shows all capitals this January have been more humid than normal apart from Perth and Darwin, with Melbourne in front at 23 percentage points above normal.

The higher-than-normal atmospheric moisture has also spread well inland — revealed by the map below of water vapour pressure anomaly, which is a measure of how much moisture is in the air compared to average.

The amount of water vapour in the air has been well above average across Australia so far this year

Why humidity makes it feel warmer

So why does an increase in moisture make it feel warmer?

A human's natural cooling mechanism is through sweat evaporating off the skin, a process feasible due to an energy transfer during the phase change of sweat from a liquid to a gas.

But this cooling becomes far less efficient during high humidity as the sweat evaporates at a much slower rate since the air is already saturated with moisture.

It can therefore feel up to around seven degrees warmer during high humidity, expressed by the apparent temperature (AT) which is based on a mathematical model of an adult, walking outdoors, in the shade.

The AT is equivalent to the temperature that would bring the same amount of discomfort during normal humidity as that experienced under the current ambient temperature and humidity.

Warm seas fuelling muggy summer

The weather pattern so far this summer has been conducive for high humidity – a predominate wind flow feeding in moisture from both tropical waters north of Australia along with the Coral and Tasman seas.

"High pressure systems have been tracking well south of the country over the last few weeks, directing a prolonged period of very moist east to north-easterly winds not only on to the east coast of Australia but extending well inland," said a Bureau of Meteorology spokesperson.

While the wind direction is the major driver of the high humidity, warmer than normal water temperatures off Australia's eastern and northern coastline are also playing a role.

"This combined with above average sea surface temperatures in the Coral Sea and Tasman Sea is leading to the prolonged period of high humidity across eastern Australia," the spokesperson said.

The water temperature is currently warmer than normal off most of Australia's coastline

Right now, ocean temperatures are as much as 2.5C above normal off the Top End coast and 3C above normal off the east coast of Tasmania and Victoria.

The warm waters this summer have increased evaporation, adding extra moisture to maritime air, which is then blowing over the eastern mainland due to prevailing onshore winds.

Stifling humidity and storms ahead

Humid tropical air will again engulf eastern Australia on Friday, although it will stretch further south right down to Melbourne and inland to the central outback.

This will not only lead to another uncomfortably muggy day but also fuel a pocket of thunderstorms across western NSW and showers along parts of the east coast.

The moist pattern will then persist through the weekend, leading to further showers up and down the eastern coastline ahead of a major rain and storm outbreak from Monday from Hobart to southern Queensland.

This could deliver up to 100mm of rain on the NSW coast along with pockets of heavy rain west of the ranges.

The next eight days promise to deliver heavy rain across much of northern and eastern Australia

In the meantime, a monsoon will soak northern Australia with hundreds of millimetres of rain during the coming week, threatening to generate areas of seasonal flooding.

So is there any relief ahead? Current modelling is indicating a drier air mass from the cooler Southern Ocean may flush out the humidity from south east Australia around the middle of next week.

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