Some cities repel rain, while others attract it, study finds

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AUSTIN (KXAN) — If you’ve ever seen rain on the radar moving toward a city suddenly split in two as though something was blocking it from reaching the city, you may have jokingly blamed a force field.

That joke may actually be true. A recent study done by researchers from the University of Texas found cities are capable of repelling rain.

“I’ve been seeing some of these images where thunderstorms come in, and they go around the city,” said Dev Niyogi, a professor of geological sciences and environmental engineering at the University of Texas. “And this is something we need to study… it does seem like there is an effect of Austin on the rainfall around it.”

Niyogi said scientists are aware of some factors that play a role in how weather moves in and around cities like Austin, including:

For a city to influence the weather, Niyogi says, it must have a diameter of 25 kilometers, about 15 miles across. He said about half of the cities in the United States meet these criteria.

While those factors are understood, Niyogi and a team of researchers wanted to look closer at another, less researched aspect: the shape of the city itself.

How a city’s shape influences the rain

Similar research into the effect a city’s shape can have on weather has been done in the past, but they looked more at the wind. Niyogi wanted to look closer at the impact the city’s shape had on rainfall.

The team categorized cities into three different shapes:

  • Circular: Think Paris or Indianapolis.
  • Triangular: Niyogi referenced coastal cities where people build along the beach and then taper off further inland.
  • Rectangular/elongated cities that follow a road or river.

Austin is a rectangular-shaped city, according to Niyogi. He said that despite its “awesome shape” and sprawl, most of the city and its suburbs follow I-35.

This photo taken on Jan. 25, 2018 in Paris shows an illuminated Eiffel Tower, next to the Seine River, with water levels raised to the height of the Zouave statue of the Pont d’Alma bridge. According to Dev Niyogi, Paris’ circular shape makes rain more likely to form than in a city like Austin. (Photo by GEOFFROY VAN DER HASSELT/AFP)

The team then ran several computer models based on the different city shapes. Using the Weather Research and Forecasting (WRF) model, a weather prediction system designed by the National Center for Atmospheric Research, they tested how rain developed in each of the city shapes.

They also placed the shaped cities in different locations: inland or on the coast.

“Whether they are square, whether they are round, whether they elongate it does have an impact in the manner in which thunderstorms get formed, where they form, whether they come over the city,” Niyogi said.

Niyogi’s team found circular cities attracted rain, seeing 22% greater rainfall daily accumulation than the other models. Triangular cities had the least amount of rain accumulation.

They noticed the greatest impact was in coastal cities.

“The circular one clearly has the largest impact, because that’s where the storms can come in, they can have an interaction.”

The computer models showed that in the rectangular and triangular-shaped cities, the rain had a harder time developing.

Designing a city to affect the weather

According to Niyogi, the shape of the city is just another factor influencing the weather, and understanding that impact can help us direct the weather where we want it.

“Maybe there is a way that we can design Austin, or any city we live in, in a way that it can make it possible that there is a slightly more chance that it rains where we want it to rain,” Niyogi said.

This could be an aquifer or lake. Niyogi said in areas like the southwest, where water is becoming more scarce, this method could be essential.

Niyogi pointed out that we now have enough knowledge for this, saying his team works with city planners to do this around the world.


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