Here in Southeast Louisiana, we are no stranger to hot temperatures during the summer months, but have you ever wondered why it feels much hotter in the cities than it does in surrounding areas? This is due to the urban heat island effect. This describes a metro area that is noticeably warmer compared to surrounding areas.

Baton Rouge would be an example of an urban heat island surrounded by more rural areas. Notice how Baton Rouge has more grey/beige colors than the more rural areas, which are more green, in the image below.

Satellite view of Baton Rouge. || Credit: Google

Urban heat islands are created by the effects of city development, including streets, buildings, parking lots, etc. Materials such as asphalt and concrete absorb much more heat from solar radiation than vegetation. This heat builds up in confined areas like cities and creates a warming effect. Rural areas have more greenery and less development; therefore, solar radiation can reflect up more easily. Vegetation also has a cooling effect due to evapotranspiration, moisture evaporation from plants and soils.

Sometimes you may see a haze near the horizon due to air pollutants such as vehicle exhausts and chemical plants nearby. These pollutants can create a greenhouse layer that traps heat beneath it, further enhancing the warming at the surface when heat cannot escape. Light, calm winds can also make the effect worse by preventing the mixing of the atmosphere, which in turn, keeps the heat trapped at the surface.

Daytime urban heat island effects

Heading toward sunset, the effects of the urban heat island are more pronounced. As the city holds so much heat, it is much slower at cooling off when solar radiation shuts off. Buildings and streets retain more heat than rural areas. For example, the temperatures in the city may be near 90°F in the early night hours while rural areas cool off near 82°F.

This creates the problem that there is not much relief during the overnight hours from the heat during the day. Heat stress has cumulative effects; the longer you are exposed to it, the more it will affect you.