Sydney, Brisbane and everywhere in between have just endured another hot and humid day as the January sweats continue to drip up and down Australia’s eastern seaboard.

Shortly before 2pm local time on Thursday, the temperature in Brisbane was feeling like 38ºC, close to 6ºC warmer than the actual air temperature at the time. This large difference between the actual temperature, and how hot it felt, was caused by an abundance of moisture in the atmosphere combined with light winds.

When the air is laden with moisture, it gets harder for our bodies to lose heat through the evaporation of sweat. Lower wind speeds make it even harder for our sweat to evaporate, which can lead to oppressive mugginess when we encounter moisture-laden air and low wind speeds on a hot day.

Meteorologists track how much moisture is in the air by monitoring the dew point temperature, which refers to the temperature air needs to cool to for condensation to occur. Higher dew points mean more moisture is in the air, which makes it harder for our bodies to cool by evaporating sweat.

Brisbane’s uncomfortable feels like temperature of 38.2ºC at 1:50pm AEST on Thursday coincided with a dew point of 26ºC and a wind speed of 6 km/h. This was a recipe for sweaty armpits.

Further south, Sydney was feeling close to 35ºC at times on Thursday as the dew point reached close to 25ºC. This wasn’t far off the 25.9ºC dew point that was observed in Sydney on Thursday last week, setting a new hourly dew point record for the city.

Image: Modelled dew point temperatures at 8am AEDT on Thursday, January 18, according to the ECMWF-HRES model.

It was no coincidence that Brisbane and Sydney were experiencing such sweaty weather at the same time on Thursday. Large areas of eastern Australia have been gripped by unusually humid weather like this over the last few weeks.

The average relative humidity in Sydney and Brisbane has been running more than 10 percentage points higher than usual during both mornings and afternoons since the start of January.

This recent spate of sweaty weather in eastern Australia has been caused by two main things:

1. Abnormally warm sea surface temperatures over the Tasman and Coral Seas, with waters measuring about 1 to 3ºC warmer than average for this time of year.
2. Frequent easterly winds under the influence of a positive Southern Annular Mode (SAM), which has helped to transport the moisture-laden maritime air onto the land.

Image: Sea surface temperature anomalies on January 16, 2024. Source: NOAA

Fortunately for Sydneysiders that are sick of sweating sweaty clothes, a drier southwesterly change will sweep through the city late on Thursday. This drier air was already affecting some of Sydney’s western suburbs on Thursday afternoon, with 3pm dew points of 23.5ºC in the city and less than 15ºC at Camden, Holsworthy and Badgerys Creek.

Thursday’s dry change will bring some welcome relief from the mugginess in Sydney for the next few days, although it will still remain warm into the weekend. Higher temperatures and higher dew points will then return next week in the leadup to Australia Day.

Unfortunately for Brisbane, there won’t be much relief from the hot and humid weather during the next week. Temperatures in the city and other coastal areas of southeast Qld could feel about 4 to 8ºC warmer than the air temperature suggests for at least another seven days.