Is Hobart the sunniest capital in Australia? Yes… and no

Updated October 15, 2018 17:13:03

Does Hobart get the most sunlight hours of any state capital?

ABC Hobart has been tasked with investigating this question, sent in by Alex Newman.

Mr Newman said he was told this was true when looking into getting solar panels installed.

“I’m an architect and interested to know if its true or just salesperson spin,” he said.

Hobart is Australia’s most southern capital, sitting at 42 degrees south.

That means two things: it’s a long way from the equator which means long summer days, but it’s also affected by weather systems that bring clouds.

The answer is vastly different when the question is looked at from a weather point of view compared to an astronomical point of view.

Hobart can claim the most light in one day, but it won’t have the title of the sunshine city any time soon.

To get to the bottom of this question ABC Hobart has sought advice from an astronomy expert, a weather forecaster and an energy consultant.

Home of the longest day

We’ll put weather aside for a moment and look at Hobart’s sunlight levels in an astronomical way.

Astronomy expert Martin George says it is true to say that Hobart does get more sunlight than any other capital.

“Certainly in the summer months we get a lot sunlight in Hobart,” Mr George said.

“In fact, between the periods of the September equinox and the March equinox when the sun is in the southern half of the sky, Hobart does get more sunlight than any other state capital.

“That’s not allowing for bad weather of course, but we’re talking about the duration between sunrise and sunset.”

On December 22 this year, the longest day of the year and summer solstice, the total time between sunrise and sunset in Hobart is 15 hours and 21 minutes.

That’s compared with Sydney’s longest day of 14 hours and 25 minutes.

And in Brisbane the longest day is 13 hours and 53 minutes.

“As you go farther north in the summer months, the days are not so long,” Mr George said.

Mr George said unfortunately for Hobart, it’s the opposite situation in winter, where the city gets the shortest day.

Hobart’s twilight zone

Hobart is also home to the longest period of twilight over summer.

“If you include the time between the very first little glow in the morning and then very last glow of twilight at night, when we have our longest day that period runs for 19 hours and 53 minutes,” Mr George said.

“We only get a few hours of complete darkness at the latitude of Hobart.

“As you go farther south the twilight period gets longer and longer.”

Down at a latitude of 49 degrees south there’s no complete darkness at all, because the sun isn’t far enough below the horizon at any time.

In Antarctica there’s perpetual daylight, because the sun doesn’t go below the horizon at all.

After the longest day, it’s all down hill.

“When we reach the point of the solstice, the difference between one day and another is very, very small — typically a second or two,” Mr George said.

“By the time we get through to the equinox, the difference is much larger, we’re talking about two or three minutes.”

It’s a different story, weatherwise

When this question was put to the Bureau of Meteorology, it was a straight out “no”.

Forecaster Lizzie Donovan said Hobart gets the least amount of average sunshine hours of any capital city.

“How many daylight hours we get is a different question to how many sunshine hours we get, because we get cloud a lot in Tasmania,” she said.

Ms Donovan said to get sunshine, there needs to be minimal cloud coverage which means less moisture in the atmosphere.

“There’s a couple of different systems that drive how much moisture and cloud cover we have in the atmosphere at any one time,” she said.

“The most important one for this particular question is what’s known as the subtropical ridge.”

The subtropical ridge is a band of high-pressure systems.

“What happens with the subtropical ridge throughout the year is during our summer months, which is the northern half of the country’s wet season, the ridge moves down over the south of the country,” she said.

“It essentially blocks any frontal systems from being able to cross the southern half.

“While this is happening, the north of the country is under the influence of the monsoon, so they have increased cloudiness and less sunshine.”

In turn, the south gets less clouds and more sunshine.

But, in winter months it’s reversed.

“We’re at the mercy of frontal systems which is what we see through winter,” she said.

“With a frontal system we get increased moisture, a big cloud band and we get rainfall.”

The middle of the country is where the most sunshine hours happen.

Places like Alice Springs and Tennant Creek get the most amount of sunlight hours.

The sunniest capital is Darwin, with nine sunlight hours.

That’s compared with Perth and Brisbane which get about eight hours.

Hobart sits at the bottom with just an average of five sunny hours.

The BOM regards “sunshine hours” as clear skies.

It uses the Campbell-Stokes sunshine recorder to measure sunlight hours.

“It’s a glass sphere and focuses the sun’s rays onto a calibrated paper card,” she said.

“When the sky is clear and the focussed rays burn a trace on the card, then we can determine the daily length of bright sunshine.

What does this all mean for solar panels?

So, with long summer days but the lowest amount of actual sunshine, is it worth installing solar panels if you live in Hobart?

Goanna Energy’s Marc White says it can be hard to get bang for buck.

“It is harder compared to installing the same solar system in one of the other capital cities,” he said.

“We might get quite a good amount of sunlight, but what we don’t get is the irradiance or energy in the sunlight — and that’s obviously what solar panels use to generate the power.

“Hobart has got one of the lowest solar irradiance levels of any of the capital cities.”

Mr White said across the year, the average energy output from solar panels is the equivalent of about 2.5 hours per day, or around 15 per cent.

Those living in the more northern capitals are getting up to six hours out of their panels.

“If you put a wind turbine up in Tasmania you can get energy out of that 40-50 per cent of the time,” Mr White said.

But, even still Tasmanians really like solar panels.

There’s 32,000 solar systems in the state.

“Tasmanians have invested over $200 million in solar systems, but in terms out output it’s only 1-2 per cent of the state’s needs,” he said.

“For the amount of money, it’s a relatively small amount of energy output.”

Mr White said there was still a business case for solar systems in Hobart, as long as they are appropriately sized.

“Solar sellers want to sell bigger systems, but solar buyers can often minimise their payback period by investing in smaller systems.”

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Topics: earth-sciences, science, weather, astronomy-space, solar-energy, hobart-7000, tas

First posted October 02, 2018 06:32:36