Inversion brings a chill in July

MetService has issued a weather warning for the south.
Cromwell spent nearly a week of last month under low cloud and Dunedin was the least sunny of the country’s main centres.

Niwa’s monthly climate summary for July said frequent southwesterly winds were partly responsible for high rainfall in parts of Otago and Southland.

However, it was a relatively mild July overall, despite some hoar frosts in the south.

Mid-month, an inversion, which acts like a "lid" in the atmosphere, trapping moisture and pollutants, developed over many areas in the South Island.

Normally, air temperature decreases with an increase in altitude, but during an inversion warmer air is held above cooler air.

The report said the inversion was formed due to the South Island having a spell of calm weather in mid-July, and it brought with it persistent cold temperatures and hoar frosts.

Cromwell was particularly affected, with an average daily temperature of -0.6degC and receiving only 1.2 hours of sunshine for six consecutive days between July 14 and July 19.

The maximum temperature at Tiwai Point, near Invercargill, on July 17 was just 2.3degC, which made it the coldest July day there since records began in 1972.

In Lauder, also on July 17, the temperature stayed below freezing, reaching a maximum of just -2.8degC, the report said.

Meanwhile, temperatures across coastal Otago and Southland were near average, and nationally, an average temperature of 8.6degC made this July the country’s 11th-warmest since Niwa’s seven-station temperature series began in 1909.

As for rainfall, western Otago and inland parts of Southland experienced normal or well-above-normal rain.

Heavy rain on July 21 caused surface flooding on some Queenstown roads and rockfalls on the Queenstown-Glenorchy road, the Crown Range Rd and State Highway6 between Frankton and Kingston.

The rest of the South had below-normal or well-below-normal rainfall, including in North Otago and eastern parts of Central Otago, Niwa reported.


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