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Sweltering Darwin heat the result of its highest April dew point in 60 years

By Jesse Thompson, Wednesday April 4, 2018 - 13:35 EST
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A high dew point has created hot and sticky conditions in Darwin this week. - ABC

Top End residents anticipating the onset of the long-awaited dry season woke to a rude surprise this week.

On Wednesday morning the dew point, a quantitative measure of how much moisture is in the air, was the highest recorded in April for 60 years.

Why this makes a number of people sweat comes down to a scale used to gauge discomfort, according to the Bureau of Meteorology's Peter Claassen.

"So we have this very scientific device that we use called the sweatometer, and that shows us different ranges in which it feels a bit sweaty," he told 's Kate O'Toole.

"A dew point of 19 to 22 is sweaty, 22 to 24 is very sweaty, and 24 plus is oppressive."

Measured in degrees Celsius, Wednesday morning's dew point peaked at 27.2 — a figure Mr Claassen described as "off the charts".

"It's the highest we've seen in April since 1958, and it actually led to a 'feels-like' temperature of 36 degrees at 8:30am, which is out of control."

In April, he said, the average dew points at 9:00am and 3:00pm were about 22C and 20C respectively.

Right. So what is dew point?

Many meteorologists prefer dew point readings to relative humidity when it comes to measuring the amount of moisture in the air.

"As the name implies, dew point is the temperature at which the air will need to cool to in order to have dew form," Mr Claassen said.

"It's essentially a direct measure of how much moisture is in the atmosphere."

Mr Claassen said it was direct because the relative humidity, a measure of how much moisture is in the air proportional to how much it can hold, was dependent on temperature.

Warmer air has a higher capacity for holding moisture, and this means the relative humidity will drop as a day warms up, even if the amount of moisture in the air stays the same.

By comparison, a higher dew point correlates to a higher amount of moisture in the air.

And because , a higher dew point also means a higher level of discomfort.

How much does it actually affect the temperature?

Many weather observers may have been puzzled by the disparity between the actual and apparent temperatures recorded by the BOM.

According to Mr Claassen, this is also related to the dew point.

"With every increase of that dew point, especially around that 20-degree range, we feel about a one or two-degree increase in the feels-like temperature as well," he said.

"So while the actual temperature was sitting at about 28 degrees [on Wednesday] morning at about 8:30, the feels-like temperature was eight degrees warmer than that at 36 degrees.

"It makes quite a big effect, actually, on how it's feeling outside."

As an example of just how drastic an effect the amount of moisture in the air can have, Mr Claassen recalled the highest dew point ever recorded in Saudi Arabia.

A dew point of 35 degrees and an air temperature of 42 degrees came together for an apparent temperature of 79C.

"You could cook things in that," he said.


© ABC 2018

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