Warm and bubbly reason for dark and stormy clouds

By ANNE BESTON, Environment Reporter

That stormy weather overhead might sound threatening but really it's just about warm bubbles.

The thundery weather that caused storms over the central North Island is expected to move south today, but don't rule out another bout of thunder frightening the horses again next week.

On Wednesday, the MetService issued a special advisory for Taupo north, warning that "the atmosphere is in the right mood for oversized cloud buildups", and telling residents to watch out for "an extra hefty thunderstorm cloud or two".

MetService lead forecaster John Couch said spring was the traditional season of thunderstorms because of the onset of warmer weather while the remnants of winter's cool weather were still with us.

The clash of cold and warm air causes pockets of moist, warm air near the ground, heated by the sun through breaks in the cloud cover, to rise and expand into cooler parts of the atmosphere above.

The warm bubbles of air, what we see as "cauliflower" clouds but which are called towering cumulus, rise on an updraft of air until they become thunderclouds, or cumulonimbus.

The warm air gains speed as it rises and, on the way up, the electric charge within the cloud separates, causing thunder and lightning.

As the warm air bubble goes ever-higher, the air cools and the cloud vapour condenses into torrential rain. If the air bubble goes high enough, the vapour freezes and forms hailstones.

Most thunderstorms in New Zealand happen at 10,700-12,200m. In the tropics they reach much higher.

There are about 2000 thunderstorms in the world at any time.

Mr Couch said the MetService tracked thunderstorms from the Waikato to Wanganui yesterday, but forecast they would move further to the Wellington region and upper South Island today.

Although there had been a spate of them over the past week, there had not been an unusually high number this spring.