In the Texas A&M study, researchers say prior studies have reported inconsistent results, with some cities experiencing rainfall enhancement and others showing suppression.
The findings show thunderstorms are more likely to develop and strengthen over large urban areas.
Climatologist Dr. David Legates of the Cornwall Alliance for the Stewardship of Creation says a study done many years ago in St. Louis had similar findings.
"Their idea was that because you've got this big heat dome, it creates rising motions, and you've got surface roughness so that as the air comes in, you're getting convergence," he explains. "The air then forces to rise, and so that creates rising motion, which therefore gives you more clouds over the city and downwind, and more precipitation over the city and downwind."
In other words, cities can act like a trigger for clouds and rainfall. A heat dome makes the air hot enough to rise, and the rough surfaces of a city force more air upward. Rising air cools and forms clouds, which can lead to more precipitation over the city and nearby areas.
He says another issue is extra particulate matter in the air which helps in rain formation.
Another aspect of urbanization is the urban heat island effect, a phenomenon where cities experience warmer temperatures than the nearby countryside in part because cities replace natural land with materials like roads and dark roofs that absorb the sun's energy and trap and make heat.
Human activity like driving cars, running air conditioners, and operating factories also pumps extra heat into the air, and tall structures trap heat between them and block the wind from cooling the city down. A lack of plants also prevents trees and grass from cooling the air by releasing water.
"With humans changing the land surface, it changes the characteristic of what the atmosphere sees from above, what it's seeing below it," Legates summarizes. "As a result of having a big city there, you've got several factors," including heat, convergence of winds, pollutants, and convection.
During the day, cities can be 1°C to 3°C hotter, and the difference can jump up to 5°C hotter at night.
To help fight this, city planners use a few tricks like painting roofs white to reflect sunlight, covering flat roofs with plants and gardens, and planting trees to add shade over parking lots and sidewalks.
