Spectacular summer storms hard to predict

A summer thunderstorm is natural theatre on a grand scale. Photo / APN

A pocket of thunderstorm warnings, marked "low risk", hovered over Gisborne's New Year's Eve celebrations last night, but for weather scientists it's almost impossible to be any more precise.

The 30,000 revellers at the annual Rhythm and Vines concerts there may have wanted more, but pinpointing exactly where thunderstorms will erupt, in a timeframe that is helpful to people likely to be affected, remains an unattained "holy grail" for scientists, according to Dr Mike Revell, principal scientist at the National Institute of Water and Atmospheric Research (Niwa).

"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," he said.

"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."

Dr Revell said it was worth looking out for the distinct, menacing, cauliflower-like clouds of a developing thunderstorm. "The convection process begins when the sun rises and starts to heat the land surface. Then, depending on atmospheric conditions, the thunderstorms are ready to break by about mid-afternoon."

To become active thunderstorms, parcels of lifted, saturated air must keep rising - spurred by the latent heat released by condensation.

Eventually the warm air hits the ever-present inversion layer at the top of the troposphere, 9km to 12km above land. The top of the cloud is then forced to spread laterally.

Lightning erupts in the freezing heights of the troposphere as super-cooled water droplets collide with ice crystals, building up static electricity.

A narrow channel of air is superheated as lightning passes through, expanding and rumbling as thunder.

"It's not often you get to watch the complete cycle of wild weather evolving from beginning to end," Dr Revell said. "A summer thunderstorm is natural theatre on a grand and spectacular scale."
Forecast

Auckland
Today: Long fine spells. Northwest breezes developing.
Tomorrow: Fine and cloudy periods. Northwesterlies.
Thursday: Rain developing, some heavy falls. Northwesterlies.

Hamilton
Today: Long fine spells. Northwest breezes developing.
Tomorrow: Cloudy periods. Northwesterlies.
Thursday: Rain developing, some heavy falls. Northwesterlies.

Wellington
Today: Becoming cloudy, some drizzle. Gale northerly.
Tomorrow: Drizzle turns to rain later. Gale northerly.
Thursday: Rain, sometimes heavy, eases to showers. Northerly eases.