Auckland: Four seasons in one day

As Auckland merges to create a supercity, the Herald looks back at how Auckland has changed over the years. Click here to view the full series.

Not far north of New Zealand, not quite as far as Norfolk Island, a subtropical high pressure point generates Auckland's worst weather. Lucky Auckland.

New Zealand, as early settlers quickly discovered, is not a tropical paradise. Most of the country lies in the path of the westerly trade winds and the sea currents that whip around the Southern Ocean and sweep into the Tasman bringing sometimes icy air, heavy rainfall and frequent frosts.

Auckland is sheltered from the worst of it. By the time the southwesterly has hit the Southern Alps and crossed the North Island it reaches the Auckland region dry and comparatively mild.

Auckland's wet weather is brought mainly by its next most frequent wind, the north-easterly, and the rain that it brings from the subtropical high pressure belt is relatively warm.

The downpour can be torrential and brief, a relief from the clammy humidity that commonly precedes it. The dark passing stormcloud is likely to quickly give way to another burst of sunshine, the fabled "four seasons in one day".

Auckland, lying 13 degrees south of the Tropic of Capricorn and sheltered by the land to its south, could be unpleasantly hot were it not for the proximity of two oceans.

The city occupies a narrow strip of land, the narrowest region of New Zealand, lying fairly low between the stormy Tasman and the vast Pacific.

Oceans moderate climate. Sea is slower than the air above to warm up and slower to cool down. On hot days a cooling breeze comes off the sea, and when the air cools, particularly at night, the sea breeze is warm.

Thanks mainly to the oceans, Auckland is never extremely hot or cold. The temperature in most of the region does not rise above 32C or fall below zero.

In summer it averages 20C, in Winter 11C.

An Auckland year seems to have just two seasons. The first half of the year is typically warm and dry, the second half is cooler, and decidedly wet.

Spring can be Auckland's wettest season, dominated by southerlies and sou'westerlies. Summer seems to arrive late and stay late. The reason, again, is the sea.

Weather scientist Jim Salinger explains that the oceans retain heat from the summer until well into the winter. It is late August before the sea temperature has dropped to its lowest point - and late February before the water is at its warmest.

An Auckland summer can continue through March and the weather is still mildly warm in April and early May. But winter squalls can persist through September, October and well into November.

Auckland has more wind than its residents may be aware. It is windier than the Bay of Plenty, Nelson or Central Otago. It simply lacks a fiercely cold wind like a Wellington southerly, a Christchurch easterly or a Dunedin sou'wester.

Auckland's southerlies produce clear, sunny skies and cooler temperatures. It is the warmer northeasterlies that are liable to bring cloud.

On days that neither of those prevailing winds are blowing, the city is likely to receive light onshore sea breezes both from the west and east.
When this happens the breezes create a "convergence zone" somewhere over the region that can produce cloud and wind, even rain, over a small area. That is how one area of Auckland can get a shower while those not far away continue to enjoy a sunny day.

It happens most often in summer when the air temperature is hottest and prevailing winds are weakest. And it commonly happens in the afternoon when the morning's light onshore airflows from Auckland's varied bays are overtaken by stronger breezes flowing from the west and east coasts.

If the breeze is stronger from the west the convergence zone of cloud and possibly showers will be over the East Coast or the Gulf. If the stronger breeze is from the east, the unsettled weather will be over western parts of the city.

Rainfall in the Waitakeres and Hunua hills is twice that of lower parts of the region, especially in winter westerlies.

Overnight frost and morning fog are rare events in Auckland. Air temperatures recorded at Whenuapai drop below zero an average of seven times a year but Albert Park has recorded just one air frost in 70 years.

Ground temperatures fall below zero 32 times a year at Whenuapai and four times a year in Albert Park.

Fog in Auckland is usually caused by warm, moist air moving over colder ground and is most common in late summer and autumn when the nor-easterly is bearing the remains of tropical storms.

The fog can stay on high ground for one to three days and they occur several times year. A different sort of fog occurs on lower ground, usually on calm mornings in winter, and sometimes appears on the harbours.

The Waitemata fog has usually flowed down from the Albany or Henderson basins on still, cloudless nights and soon drifts out of the harbour entrance - but not before it has given morning commuters an entrancing vision of the Skytower floating on a cloud.

Microclimates

On the afternoon of February 14, 1988, a "cloud street" formed in a prevailing weak nor'easterly windflow from Warkworth to the Waitakeres.

It produced several large thunderstorms that dropped 148mm of rain during a period of three and a half hours. The drenching was confined to the area around Whenuapai. Just 8km away, no rain fell at all.

Is it sunny?

The Auckland region sees the sun on average for about half the hours of daylight in a year. Waiheke Island has more than the average.

The region is cloudier during winter than summer. More cloud accumulates in the wst than the east, particularly around the Waitakere Ranges.

Humid?

Auckland has a similar relative humidity (water in air) to other parts of New Zealand but because its mean temperature is higher than places to the south the "vapour pressure" is higher.

This is what leads people to think Auckland's humidit is higher than other centres.

Auckland's wettest month: April 1956.
Auckland's driest month: February 1909 (the only zero ever recorded)
Auckland's driest year: 1859 (Feb to Jan, 1860, 649mm)
Auckland's wettest year: Oct-Sept 1916-17 (2242mm at Albert Park)

- NZ Herald

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