Telltale signs of thunderstorm

Warm, humid conditions provide the perfect conditions for summer thunderstorms such as those experienced in Wanganui and many other parts of the country recently.

Thunder and lightning have been frequent visitors to Wanganui during the past month. The most recent thunderstorms were on Friday evening, and were experienced in Wanganui, Waikato, Waitomo, Taumarunui, Taupo, Hawke's Bay, Taranaki, Taihape, Manawatu, and Tararua.

Mike Revell from Niwa has been studying the intricacies of the atmosphere for nearly four decades and said thunderstorms can occur anywhere in New Zealand at any time of the year.

"Arguably the most impressive, however, are those that develop on a warm, humid summer's day, when heating of the land begins a process that can transform a serene morning sky into a spectacular tumult by mid-afternoon," Dr Revell said.

"So if you're gazing skywards this summer and notice menacing, cauliflower-like clouds billowing high, put the washing on hold and bring the dog inside - a thunderstorm might be on the way."

Dr Revell said a number of key ingredients were required for a thunderstorm.

The first is air lifting off the ground, and an initial trigger is needed to start the process.

"Often the sun's heating of the land is enough. The warmed land heats the air immediately in contact with it and, as we learned at school, warm air rises. This is known as convection," Dr Revell said.

A good supply of moisture at low levels is also essential. As the air rises, it cools and becomes less able to hold any moisture it contains.

"If it is sufficiently humid, the rising air will eventually cool to saturation point and clouds will form," Dr Revell said.

Instability in the lower levels of the atmosphere, or troposphere, will help the clouds develop into active thunderstorms, Dr Revell said.

"That means the parcel of lifted, saturated air remains warmer, and hence less dense, than its surroundings, so is able to keep on rising," he said.

The convection process begins when the sun rises and starts to heat the land. Thunderstorms are usually ready to break by about mid-afternoon.

Lightning and thunder are the defining characteristics of a thunderstorm. Lightning begins when an electrical charge is built up within a cloud, due to static electricity generated by super-cooled water droplets colliding with ice crystals near freezing levels.

"When a large enough charge is built up, a large discharge will occur and can be seen as lightning. Thunder is the shockwave caused by the sudden expansion of a narrow channel of air, as it is superheated by the lightning passing through it," Dr Revell said.

The conditions that favour summer thunderstorm formation happen most often inland, where heating is strongest, and over high ground. The North Island's Central Plateau and ranges, the upper Canterbury plains and the South Island high country are prime breeding grounds. At other locations, such as Auckland, local sea breeze convergence plays a key role.