Behind the Forecast: Urban Rain

Science Behind the Forecast: Urban Rain

Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m.

LOUISVILLE, KY (WAVE) - On Sunday, August 19 we saw almost 4 inches of rain in around 75 minutes in central Jefferson County as a storm formed, hovered and died right over the city. Meteorologist Ryan Hoke noticed that the origin point/updraft of this stationary downpour in Louisville was right near Audubon Park on I-65. Which is close to the max urban heat island of the city.

Scientists have long suspected that cities were causing more rainfall but exactly how is still unknown.

In the summer, the weather is mainly influenced by local processes such as moist, cooler air flowing off a lake, colliding with hot air over land and forming clouds. Other times of the year these local processes are overshadowed by large frontal systems traveling across the country.

For a shower or thunderstorm to form there are a few necessary ingredients:

  • Unstable air: Air that is warmer than its surroundings. Once this is lifted it will continue rising like a hot air balloon
  • A source of lift: Cold front, mountain or even city buildings
  • Moisture: If there is enough moisture then the moisture the initially warmer air, which is cooling as it rises, will condense into clouds and rain. Warmer air has the capacity to hold more water vapor.

A NASA study tracked rainfall anomalies around and downwind of cities during summer months. There are three main theories as to why cities and areas nearby see more rainfall.

  • The first is the urban heat island effect: Cities absorb immense amounts of heat on their own but other machinery (cars, industrial plants) coupled with a lack of vegetation, sends temperatures soaring. A city’s average temperature can be 6 to 8 degrees warmer than surrounding landscapes. The extra heat from a city may be a source of unstable air thus, in the right situation, may help trigger cloud and rain formation soaking areas downwind.
  • The second theory is that cities disrupt airflow across the Earth’s surface: Buildings provide lift which pushes warm, moist air into the cooler air aloft forming rain clouds.
  • The third theory is that instead of air building up over a city that it actually is divided because the way the buildings are situated or the urban heat island. When the air collides downwind of the city, the air is forced up forming rain clouds.

The study looked at cities in the south-central and southeastern U.S. that were not near major geographic landforms such as rivers, oceans or mountains since they affect rainfall amounts. The study found that the rainfall per hour was 20 percent greater in areas downwind of cities than upwind.

Something else that may factor into the rainfall over a city is the pollution. The increased amounts of pollutants in an urban area, in turn, leads to an increase in condensation nuclei which are necessary for the formation of clouds.

The study also found that city-induced rain may shift as a city grows and may become more pronounced.

Understanding this will help forecasters warn an area of potential flooding situations like the one we saw on Sunday.

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