What is an urban heat island?

(WHSV) - Did you know that in cities it gets warmer than in the surrounding rural areas? It’s known as the urban heat island effect.

Different surfaces impact temperature with their albedo. A surface’s albedo is a measure of the fraction of sunlight that reflects off the surface. Surfaces that have a high albedo have a lot of sunlight reflected off of them, so not as much heat is retained. The opposite is true for surfaces that have a low albedo.

Many dark surfaces have a low albedo, like asphalt. Of course, there are more roads, driveways, parking lots, etc. to trap more of that heat in urban areas.

We also see this effect work when there is snow on the ground. Snow has a high albedo, so the surface does not absorb as much sunlight. This means areas with snow cover are harder to warm. The urban heat island effect shows its greatest when there is a light wind and has more of an effect at night. Days when we see a lot of sunshine and have greater solar radiation also enhances this effect.

You may notice with many of our forecasts, Harrisonburg is forecasted with some of the warmest temperatures at night in the area. Much of this has to do with this effect, as heat is held in longer than surrounding areas. These differences can feel significant, Downtown Harrisonburg at times can be 5-10 degrees warmer than rural areas of Rockingham County at night!

The Dale Enterprise station where we get most of our records will show a lot of differences at times compared to our station’s weather information in Downtown Harrisonburg.

If you have a thermometer in your car, you’ll also notice this if you drive from a rural area to an urban area.

The urban heat island effect can create a significant problem during heat waves since temperatures are even more intense in densely populated areas.

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