MISSOULA, Mont. — Every day, the sun’s heat and light reach urban, suburban and rural areas in the same way. Given identical weather conditions, the temperatures in these areas are often not the same. Why is this? In the video above, Meteorologist Mitchel Coombs discusses what this feature is and the factors that influence its development.
The difference in temperatures between urban and rural areas comes down to the surfaces in each environment and how well they absorb and emit heat. Urban areas are much more efficient at this, hotter than the surrounding areas. This is why they are known as urban heat islands.
Let’s use the Missoula Valley as an example. Temperatures around the valley are warming up to around 85 degrees. In a more rural part of the valley, that is where those temperatures will stay.
At a suburban community park, those temperatures increase 1 to 2 degrees, to around 86 to 87.
As we move more into Missoula and a commercial district with numerous businesses, temperatures will continue to warm by a few degrees. Temperatures are now in the upper 80s.
In the heart of downtown Missoula, the temperature has warmed to 92 degrees. The downtown area tends to warm up more quickly and significantly as compared to the rest of the valley.
There are multiple reasons these urban heat islands occur.
First, reduced natural landscapes in urban areas: Trees, grass, ponds and rivers tend to cool the air by providing shade, transpiring water from plant leaves and evaporating surface water.Second, urban material properties: Man-made materials like asphalt, concrete and roofing have a low albedo, reflecting less of the sun’s energy and absorbing and emitting more of the sun’s heat, as compared to natural surfaces like trees and grasses.Third, urban geometry: The size and spacing of buildings within a city influence wind flow and urban materials’ ability to absorb and release solar energy.Fourth, heat generated by human activity: Buildings, air conditioning units, vehicles and industrial facilities all emit heat into the urban environment.Fifth, weather and geography: Calm, clear weather conditions result in more severe heat islands by maximizing the amount of solar energy reaching urban surfaces and minimizing the amount of heat that is carried away. Conversely, strong winds and cloud cover suppress heat island formation. Geographic features can also impact the urban heat island effect. For example, western Montana’s mountains can block the wind from reaching the city but can also create flow patterns that push that wind through the city.
The larger the city, the stronger the urban heat island effect will be on that city. Portland, Oregon, and Salt Lake City, Utah, are much larger and will see a greater influence from the urban heat island effect as compared to our larger cities in western Montana, simply due to size.
That said, this does not mean the impacts in our area cannot be significant. If you are out enjoying a warm, sunny day, make sure to stay hydrated and take plenty of breaks in the shade to cool off.
Visit www.nbcmontana.com/weather for the latest temperature forecasts.