News

How hot was it last Friday? It might have depended on where you stood, or what stood over you. We mean trees, of course.

When the weather is reported, the forecast versus what it actually feels like can be dramatically different. Let’s take for example the rain forecast. The weather report may say there’s 40% of rain, and that could be right even though it’s dry outside where you are. Anyone who’s driven through a thunderstorm south of I-20 and then exited a few miles north into dry sunshine knows that the forecast is technically “right.”

This variability is also demonstrated in summertime heat differences in one neighborhood to another or even from one street to another.

We posted this example on Instagram recently to show how the temperature one feels can be greatly affected by the canopy cooling effect of trees:

Today was HOT! Around mid-afternoon, the official local temperature in Atlanta was 91 degrees. The report estimated that the air “feels like 100 degrees.” We took the surface temperatures of a city street in Reynoldstown. Take a look at the differences in measured surface temps for: a grass lawn with no tree cover (99 degrees), sidewalk in full sun (127 degrees), center of street (138 degrees), and sidewalk shaded fully by a mature street tree (90 degrees). All 4 spots are within 100 feet and taken within minutes of the others. This isn’t a scientific study, but it shows how much heat various surfaces can absorb… and radiate. Tree canopy can provide incredible relief (or is that re-LEAF) from intense sunrays. Impervious surfaces can hold onto a lot of heat and warms the air. A city has lots of buildings, streets, and other impervious surfaces that all get hot and create the “urban heat island effect” of raising air temperatures. This small example shows how trees can made a big difference, it’s called the “canopy cooling effect.” More trees = cooler temperatures. #moretreesplease

Although the built environment of streets, buildings, sidewalks, and driveways soak up and radiate a lot of heat, even yards that are lawns of only turf grass are hot, too, if they don’t include some trees. Small yards can use midstory (smaller and medium sized) trees, shrubs, or other plant materials that all have a better cooling effect than turf grass.

Images 5 and 6 (left to right, below) show the temperatures on the same day at the same time for a yard planted with shrubs and a small tree (92 degrees) as well as another full-sun garden planted with small perennials and plants (95 degrees). Both are cooler than ground covered in only turf grass (99 degrees). None of these are as cool as even a concrete sidewalk that’s kept shaded by a mature canopy tree (90 degrees).

In the heat of summer, how much cooler one feels can be greatly affected by how much tree canopy surrounds them. Imagine a summer day hotter than 91 degrees. Where would you rather walk: under the shade of a street tree, in a forested area, or through a parking lot surrounded by acres of asphalt? We’ll report back in a future post with more temperature readings; meanwhile, we’d love to see your temperature checks. Post them to social media and be sure to tag us @treesatlanta #canopycoolingeffect .

Before we let you go, keep in mind that the cooling effect of trees is more than just shade. Here’s some “cool facts” about evapotranspiration, trees’ natural air conditioning system, as well as the many benefits of trees.

Additional resources: 

• Using Trees and Vegetation to Reduce Heat Islands
• Tree Equity in Atlanta (CBS46 video)
• Feel the heat: Students track impact of extreme heat in Atlanta neighborhoods (11alive video)

Posted: 8/2/21