Thunderstorms are common over the summer months, but there are ways of spotting when one is going to strike in Rotorua.
Rotorua is an inland area where heating is strongest over summer, therefore prone to thunderstorms over the warmer season.
Rotorua weatherman Brian Holden said that in the city there were signs to spot when a thunderstorm was heading this way: "We don't get them terribly often but when we do get them, it's usually after a long, hot day. You can see the big fluffy cumulonimbus clouds, in Rotorua you usually see them building up around the hills."
Mr Holden said that on a hot day, the clouds built up and as they did, they picked up more and more moisture and energy.
"There's huge updraughts which suck the water up so high that they start to freeze and you get hail. When you get all that energy happening, they build up a static charge in various parts of the cloud ... The discharge of the energy gives the boom."
The clouds built up around hills because the hills trapped rising air, Mr Holden said.
Niwa scientist Dr Mike Revell advised that if menacing, cauliflower-like clouds were seen billowing high, put the washing on hold and bring the dog inside - a thunderstorm could be on the way.
"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. In New Zealand, the conditions that favour summer thunderstorm formation occur most often inland, where heating is strongest, and over high ground."
Dr Revell said thunderstorms caused by heating would often begin over the ranges or higher ground because the degree of heating was the same as at lower elevations, but the hills were "poking their noses up" into colder air, so the heating was higher up, therefore instability was greater.
Pinpointing exactly where thunderstorms would erupt, in a timeframe that was helpful to people likely to be affected, remained a long-term goal for scientists at Niwa, Dr Revell said.
"At the moment, our computer models are very effective at forecasting the conditions under which thunderstorms are likely to develop over an area of several hundred square kilometres, but we remain some way off being able to forecast exactly when and where, within the several hundred square kilometre area, those individual cells will develop, to forewarn people nearby of the potential impacts."
In the meantime, Dr Revell said, look to the summer skies for the tell-tale signs of thunderstorm development.
"It's not often you get to watch the complete cycle of wild weather evolving from beginning to end. A summer thunderstorm is natural theatre on a grand and spectacular scale."
