Why cities have been getting much more rain lately than rural areas

By Stephen Beech via SWNS

Most cities get more rain than surrounding rural areas, according to a new global study.

American researchers who analyzed precipitation levels in more than 1,000 cities worldwide found that more than 60% receive more rain than the surrounding countryside.

The difference in some cases was "significant" with, for example, Houston in Texas, on average, receiving almost five inches (12.7 cm) more rain per year than its surrounding rural areas.

The research team says their findings, published in the journal Proceedings of the National Academy of Sciences (PNAS), could have wide-ranging implications- the most serious of which is worse flash flooding in densely built urban areas.

Previous studies have shown that cities and towns are often much warmer than their surrounding rural areas, known as the urban "heat island" effect.

However, researchers say that the urban heat island has a lesser-known twin counterpart - called the urban precipitation anomaly - where the presence of urban development measurably affects the amount of rain.

Previous studies only looked at certain cities and storm cases.

But study author Xinxin Sui, a doctoral student at The University of Texas in Austin, and other researchers analyzed precipitation data from satellites and radar systems, examining daily rain anomalies for an "unprecedented" 1,056 cities from 2001 to 2020.

She said: “In general, we found that over 60% of these global cities have more rainfall than the surrounding countryside.

"Then we compared with different climate zones and found that if the local climate is hotter, if it’s wetter, then it may have a larger rainfall anomaly compared to the cities in cooler and dryer places."

As well as Houston, large cities with the biggest precipitation anomalies include Ho Chi Minh, Vietnam; Kuala Lumpur, Malaysia; Lagos, Nigeria; and the Miami-Fort Lauderdale-West Palm Beach metropolitan area.

Study author Professor Dev Niyogi explained that urban areas tend to take rain from one location and concentrate it in another, much like a sponge that is being squeezed.

He said: “If you were to pinch one part of the sponge, you would have water coming down more forcefully from one side.

“The amount of water you have in the sponge is the same, but because now you have that dynamic sort of squeezing the atmosphere, you have more ability to take the water out from that location.”

Although less common, the study also found that some urban areas actually receive less rain than their surrounding rural counterparts.

That phenomenon was usually found in cities situated in valleys and lowlands, where precipitation patterns are controlled by nearby mountains.

The cities where that was most pronounced included Seattle in the US, Kyoto in Japan and Jakarta, Indonesia.

Co-author Professor Liang Yang said one key factor why most cities receive more rainfall than their rural neighbors is the presence of tall buildings which block or slow down wind speeds, leading to a convergence of air toward the city center.

Yang said: “The buildings further enhance this convergence by slowing the winds, resulting in a stronger upward motion of air.

"This upward motion promotes the condensation of water vapor and cloud formation, which are critical conditions for producing rainfall and precipitation."

The research team found that population has the largest correlation with urban precipitation anomalies compared to other environmental and urbanization factors.

Niyogi says that is because larger populations typically create denser and taller urban areas, along with more greenhouse gas emissions, and therefore more pronounced heat.

Yang said the increased chances of rainfall in cities combined with the impervious surfaces that make up their urban environments can be a recipe for flash flooding.

He added: “Combining these two factors means we must develop innovative ways to prepare for flash flooding."

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