Rain is in the forecast for the next two days, which feels like the perfect time to share a new study from our neighbors at The University of Texas at Austin.
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Most of us are familiar with the “urban heat island effect,” where cities are warmer than surrounding rural areas due to more concrete and less greenery. However, this study takes it a step further, showing major cities also get significantly more rain than their rural counterparts, a phenomenon you might call the “urban rain effect.”
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And guess what? Houston tops the list, receiving more rain than any other U.S. city by comparison in the world! Yes, you read that right.
UT researchers analyzed rainfall data from 1,056 cities worldwide between 2001 and 2020, discovering that 60% of these cities receive more precipitation than their surrounding rural areas. Houston stood out in a big way, on average, we get nearly 5 inches more rain per year than nearby cities.
The real concern?
As Houstonians know all too well, this extra rain makes flash flooding an even bigger problem for our city.
What makes this study unique though, is its global scale.
“If the local climate is hotter and wetter, it may have a larger rainfall anomaly compared to cities in cooler, drier places,” Xinxin Sui, a doctoral student at UT’s Cockrell School of Engineering explained.
And as we all know, Houston is both hot and wet.
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Below is a list of the cities with the largest precipitation anomalies. In this study, an “anomaly” refers to a deviation in rainfall from what’s expected.
Typically, an entire area is assumed to receive roughly the same amount of rain on average. This study shows that’s not the case.
The main reason cities receive more rainfall than their rural neighbors, according to the study, is tall buildings. These skyscrapers slow down or block wind, causing air to converge above the city center.
“Slowing the winds results in a stronger upward motion of air,” Liang Yang, a professor at UT’s Jackson School of Geosciences, who co-authored the study explained. “This upward motion promotes the condensation of water vapor and cloud formation, which are critical conditions for producing rainfall and precipitation.”
In other words, our towering skyline is contributing to Houston’s extra rain.
Dev Niyogi, a professor at UT’s Jackson School of Geosciences and Cockrell School of Engineering, who also was the study’s main author, used a sponge analogy to explain the findings.
Imagine a sponge in the sky over southeast Texas. The part that gets squeezed or pinched the most is over Houston.
In a way, it’s almost like Houston is pulling rain away from the surrounding cities, concentrating it right over the city.
The study uncovered another fascinating correlation: not all urban areas experience more rain.
Large cities in valleys and lowlands, where precipitation is influenced by nearby mountains, actually receive less rain. This includes cities we’d normally associate with rain like: Seattle, Washington; Kyoto, Japan; and Jakarta, Indonesia.
These findings were published in the Proceedings of the National Academy of Sciences. It’s a dense scientific read, but if you’re up for the challenge, you can check out the full study here.
