Why thunderstorms seem to 'split' around cities

You might have encountered this scenario before:

You look beyond the city where you live and see towering thunderstorm clouds in the distance. They're moving right toward you and you think your garden will finally get the moisture it needs.

But just as the storm gets close, it dissipates or seems to split in two — avoiding the city altogether. 

It may seem like junk science to say your city has some sort of "force field" that dissipates storms.

But Dev Niyogi, a professor in the department of geological sciences and the department of civil, architecture and environmental engineering at the University of Texas at Austin, has done research showing the phenomenon is real. 

Niyogi's 2011 research, which focused on the Indianapolis, Ind., metropolitan area, analyzed radar imagery over a 10-year period, and found storms split closer to the city, then merge again downwind. More than 60 per cent of the storms analyzed also changed structure over the Indianapolis area, compared with just 25 per cent over rural regions.

Niyogi says the effect happens in cities all over the world, but metropolitan areas about 25 kilometres in width are the right size for this phenomenon to occur.

Cities act as "islands" of heat, with concrete, asphalt and air pollution trapping the warm air, compared with cooler rural areas, where infranstructure is less dense. The distinct temperature differences create a different air pressure.

"As the storms are approaching, they see these two different air masses," he said. "And the two sort of collide with each other and they create a front, almost like … a lake breeze front."

That front can force storms around a city, Niyogi says.

Cities can create storms, too

Niyogi's research team also used model simulations that included and excluded the city of Indianapolis. They found storms only developed when the city was present in the model.

Natalie Hasell, a meteorologist with Environment and Climate Change Canada in Winnipeg, says the heat cities give off can affect storm production.

"If you add heat to the [storm] system, that should increase or destabilize the atmosphere, so [that] could lead to greater storms forming, especially downstream of a city," she said.

Hasell says air pollution could also play a role in this.

"There are more particles in the air in the city that may or may not play a role when it comes to thunderstorm formation," she said.

Niyogi says some scientists believe emissions can interact with clouds and create thunder, but produce no rain at all.

All cloud, but no rain

Cities may not just be causing storms to split. A phenomenon called virga — where rain falls from clouds, but evaporates before hitting the ground — could be affected by urban areas, too.

Douw Steyn, a professor emeritus of atmospheric science at the University of British Columbia, says the rain evaporates because the conditions near the surface are warmer and drier.

"Both of those are urban characteristics," Steyn said. "You're more likely to see the rain evaporating over the city than over ... surrounding countryside because the city is warmer and its air drier."

Not all thunderstorms will react to urban areas, and meteorologists note that where a thunderstorm hits is mostly based on luck and location.

But those frustrated with a lack of water for their grass may have some research on their side if they think their city is getting the short end of the stick when it comes to rainfall.