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.
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. 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, like Auckland, local sea breeze convergence plays a key role.
NIWA's Dr Mike Revell has been studying the intricacies of the atmosphere for nearly four decades. He says that summer thunderstorms occur with deep cumulonimbus clouds that typically extend from about a kilometre above the surface, up through the troposphere (the lowest layer of the atmosphere) to a height of 10 kilometres or so.
A number of key ingredients are required for their development.
A boost from below
Cumulonimbus clouds grow in parcels of air rising from the Earth's surface, and an initial trigger to lift the air off the ground is essential for their development. "Often the sun's heating of the land is enough," Dr Revell says. "The warmed land heats the air immediately in contact with it and, as we learnt at school, warm air rises. This is known as convection.
"There are other common lifting mechanisms too, like breezes converging from different directions. When that happens, there's nowhere for the air to go but up. An advancing cold front, or a breeze blowing over a hill, might also start the process."
Summer thunderstorms in New Zealand are commonly triggered by a combination of these mechanisms.
Let there be clouds
A good supply of moisture at low levels is also essential. As the parcel of 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 says.
Higher still and higher
Continued growth of those clouds into active thunderstorms relies on 'instability' in the troposphere. That means the parcel of lifted, saturated air remains warmer (and hence less dense) than its surroundings, so is able to keep on rising. This process is helped by latent heat released as water droplets condense out of the rising parcel of saturated air.
"In 'stable' conditions, the rising air will quickly run into a layer that is of equal or higher temperature – known as an 'inversion layer' – putting a lid on the upwards motion," Dr Revell explains. "But if the air in the middle and upper levels of the troposphere is unusually cold, the parcel is more likely to remain warmer in comparison, and keep rising higher and higher. That's what we mean by instability.
"Thunderstorms caused by heating will often begin over the ranges or higher ground," he adds, "because the degree of heating is the same as at lower elevations, but the hills are poking their noses up into that colder air, so the heating is higher up and instability is correspondingly greater."
Interestingly, some of the most vigorous thunderstorm growth can occur when a thin inversion layer lies underneath a deep layer of unstable air. Convection is initially supressed by the low level inversion, allowing the parcels of air below to become considerably warmer than the air above the inversion. Eventually daytime heating will make the convection strong enough for these parcels to burst through the stable layer and then really strong updrafts will occur.
Upwards growth is eventually suppressed by the tropopause, an ever-present inversion layer marking the top of the troposphere, at a height of nine to twelve kilometres above New Zealand. Here the top of a mature cumulonimbus cloud is forced to spread laterally, forming its classic anvil shape.
Reaching flashpoint
"The convection process begins when the sun rises and starts to heat the land surface," says Dr Revell. "Then, depending on atmospheric conditions, the thunderstorms are ready to break by about mid-afternoon. They can last well into the evening – normally as a result of new cells continually replacing decaying cells, rather than a single, long-lived storm."
Lightning is an electrical discharge that occurs in a thunderstorm. Lightning occurs 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 the freezing level. 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.
Hail and tornadoes
Hail and tornadoes are the spectacular – but often destructive – accompaniments of very active thunderstorms.
For hail to form, the cumulonimbus cloud must tower well above the freezing level and contain an updraft strong enough to fling water droplets quickly and repeatedly up into the frigid air, adding a new layer of ice with every visit. Eventually the hailstones become heavy enough to counteract the updraft (which may be as strong as 100km/h) and fall to Earth.
Tornado formation is complex and still only partially understood. A very active thunderstorm with a powerful updraft is required. Strong upward motion in a confined area creates a vortex (in the same way that bathwater pouring down a narrow plughole circulates rapidly) which, under favourable conditions, narrows and accelerates into the characteristic funnel of intensely circulating wind.
The 'Holy Grail' of forecasting
Pinpointing exactly where thunderstorms will erupt, in a timeframe that is helpful to people likely to be affected, remains a long-term goal for the scientists at NIWA who study the weather.
"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," says Dr Revell. "They can also model with considerable accuracy the processes taking place within a single cumulonimbus cell once the growth phase has begun. 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. Our modelling resolution and capability are improving all the time – and that remains the 'Holy Grail'."
In the meantime, Dr Revell says, 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."
