Originally published here by USA Today. Written by Doyle Rice.

Cities make it rain, study says

Sure, we’ve all heard of the urban heat island effect, where cities are hotter than surrounding rural areas. Now scientists have discovered a similar urban “rain island effect,” where cities appear to get more rainfall then the hinterlands around them.

In the first study of its kind, researchers looked at rainfall patterns in more than one thousand cities around the world and found that over 60% of those cities receive more precipitation than their surrounding rural areas.

In some cases, the difference can be noteworthy, according to the new study, which was published Monday in the peer-reviewed journal Proceedings of the National Academy of Sciences. For instance, researchers found that notorious flood spot Houston, on average, picks up nearly five inches more rain per year than its surrounding rural areas.

The study findings have serious implications for urban areas, the most serious of which is worsened flash flooding in densely built parts of town.

Why do cities get more rain?

Why would cities receive more rainfall than the nearby area? Study co-author Zong-Liang Yang, professor at the Jackson School of Geosciences at the University of Texas at Austin, said one key factor is the presence of tall buildings, which block or slow down wind speeds. This leads to a convergence of air toward the city center.

“Cities tend to have buildings, particularly tall buildings, that cause the air to converge, or pile up,” said meteorologist Marshall Shepherd, formerly of NASA and now with the University of Georgia, and a co-author of the study. “Think of two trains colliding; their front ends go up. That’s convergence.”

So the city’s buildings provide a source of lift to push warm, moist, surface air into the cooler air above it, where it can develop into rain clouds.

Another factor is the urban heat island itself: According to NASA, this increased temperature may provide a source of unstable air. If air over a city is warmer than the air surrounding it, it wants to rise. As the city-warmed air rises, it cools and forms rain-producing clouds that soak the area downwind.

Other cities also make it rain

In addition to Houston, the Miami-Fort Lauderdale-West Palm Beach metropolitan area is the other U.S. city that has a pronounced difference between urban and rural precipitation, the new study said. Worldwide, cities such as Ho Chi Minh, Vietnam; Kuala Lumpur, Malaysia; and Lagos, Nigeria, all stood out.

Overall, cities with greater populations, aerosol emissions, and urban heat island effects were more likely to have increased annual precipitation than others.

How did they do the study?

Researchers used high-resolution satellite data to study daily precipitation data for 1,056 cities around the world from 2001 to 2020. The authors compared annual precipitation amounts, extreme precipitation events, and long-term changes in precipitation for each city with those of surrounding rural areas. 

The analysis showed that 63% of cities received more annual precipitation within and downwind of their urban areas, relative to nearby rural areas.

Check out the full paper here.