Steamy streets explained after Christchurch wild weather triggered the phenomenon

Tuesday, 19 November 2019

EXPLAINER: Christchurch had an action-packed day on Monday with heavy rain, hail, thunderstorms, wild winds, lightning and even a tornado that tore off roofing and injured two people.

When the tornado touched down, most Cantabrians were looking up at the sky, getting their phones out or running for cover.

But another interesting phenomenon was also happening in the streets below.

Some drivers watched as the roads around them turned white with heavy hail or slick with rain, before thick steam rose up from the asphalt.

**READ MORE:

* Two people injured, buildings damaged, asbestos warning after tornado hits Christchurch

* No tornadoes for Tuesday, but heavy rain and thunderstorms forecast for south

* Live: Tornado hits Christchurch, causing injuries and damaging property

* Melted roads caused by hot weather pose a potential safety hazard this summer**

So what caused this steamy, street affair? Here's what the experts say:

STEAMY STREETS

Before the storm hit, it had been a sunny day in Canterbury, with temperatures coming close to 25 degrees Celsius, Niwa meteorologist Ben Noll said.

That meant the black asphalt on the roads had a chance to absorb heat and become warmer than the air above them.

So when the storm rolled down the hills and across the city, bringing heavy hail and rain, the cool liquids met a warm ground below.

'As the cool rain droplets hit the hot pavement, they immediately evaporate, or changed from a liquid to a gas. This is what creates the steam. We then often see the steam rising up above the road, since it is warmer than the surrounding, now rain-cooled, air,' Noll explained.

Although the steam was something Christchurch residents noticed on Monday, it's not an unusual thing to see after a thunderstorm.

MetService meteorologist Andrew James said it's quite common for there to be warm temperatures and sunshine ahead of thunderstorms.

Thunderstorms can often form in warm, moist air and unstable conditions in the atmosphere.

MIRAGE/ WAVY LINES

So what are some of the other things Kiwi drivers may notice during stormy or hot weather?

On scorching hot days during the summer months, motorists may notice a mirage-like effect on the roads.

These wavy lines appear to hover over the asphalt and might be something you notice on a sunny, 30C day driving on the Desert Road.

Noll said the mirage-effect is something you might notice if you were driving through the desert in particular, like in Australia or the southwest United States.

'It occurs because the pavement is much hotter than the air above it,' he said.

'The interface of the very hot air near the ground and the (relatively) cooler air above it acts as a boundary that reflects light. As you are driving along, this may produce a mirror-like effect, producing reflections of the sky onto the road that fade away as you get closer to them.'

'SUMMER ICE'

In the hot months, 'summer ice' is something drivers can also come across.

While motorists are used to warnings of black ice during the winter months, 'summer ice' is a road condition where dust, dirt, and oil build up on the road during warmer months.

In January this year, the NZTA warned motorists about it, explaining that when it rains, the road became greasy, making it slippery, and the greatest risk was shortly after rain started.

At the time an agency spokesperson said the slippery roads were due to accumulated dirt and oil on the road after summer rain, and that it was an issue in New Zealand and overseas.