Behind the Forecast: Urban heat islands - how cities influence their weather

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Science Behind the Forecast: The cons and pros of urban heat islands
The Louisville skyline at night in March 2020.
The Louisville skyline at night in March 2020. (Source: John P. Wise)

LOUISVILLE, Ky. (WAVE) - Our cities are changing the weather around us. According to the Environmental Protection Agency (EPA), cities, on average, are up to 10°F warmer than their rural surroundings. This temperature variance is more pronounced at night compared to the day; it’s also larger during the winter than summer. The effects are best seen on clear, calm nights when rural areas can cool off significantly faster than cities.

The reason for the heat? Well, there are quite a few culprits. The EPA lists the following as reasons for heat island formation:

  • Reduced Natural Landscapes in Urban Areas
  • Urban Material Properties
  • Urban Geometry
  • Heat Generated from Human Activities
  • Weather And Geography

The removal of trees and other plants reduces the natural cooling effect that vegetation creates. Asphalt and concrete make up much of the urban landscape; they quickly trap heat and are stubborn to release it. Other materials used in cities, including our roofing, are also very good at absorbing solar radiation. The heat leftover from vehicles, plants and factories, and even air conditioners contribute to the hotter temperatures within cities. The waste heat created by these sources contributes to urban heat islands. Urban heat islands gradually absorb heat throughout the day, becoming more prominent after sunset and before slowly releasing heat at night.

The structure of the city can also influence how much it warms. Researchers have found that cities with straight and perpendicular streets, like those seen in North America, trap much more heat. They discovered that buildings exchange heat more depending on a city’s organization. Tall buildings clustered close together can influence how the wind flows through an area, impacting how it cools. Less “organized” cities, not formatted in a grid shape release, heat into the atmosphere effortlessly.

The weather also influences how severe a heat island becomes. On calm, clear days the amount of solar energy able to reach the ground is much higher, leading to more heat being trapped. Cloudy skies and strong winds can limit the formation and severity of a heat island.

2014 study found that Louisville was the fifth-worst heat island in the United States. Our average temperature difference hovers around 5°F.

While urban heat islands become a hotter topic during the summer months, their impact is also significant during the winter.

Researchers found that heat released from buildings created a more significant heat island effect during cold snaps. While a heat island during the summer can lead to skyrocketing utility bills, during the winter, it can help to decrease the amount spent on heating. Our environment absorbs a portion of the heat created by our furnaces and heaters during the fall and winter months. The additional heat actually makes spending time outside more tolerable and reduces the demand for additional heat.

Also, the warmer city temperatures during the winter can alter the type of precipitation seen. The average temperature difference of 5°F in Louisville can be the difference between rain inside the city while snow falls towards parts of Floyds Knobs and Bullitt County.

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