Trees are COOL (Enough to Combat a Heat Island!)

Trees are our first line of defense when it comes to combatting the urban phenomenon of the Heat Island. No, Heat Island isn’t a new reality show. Although it is a reality.  

What is a Heat Island? 

The easy explanation by NASA’s Climate Kids is that an urban heat island occurs when a city experiences much warmer temperatures than nearby rural areas. The difference in temperature between urban and less-developed rural areas has to do with how well the surfaces in each environment absorb and hold heat. Cities tend to have more impervious surfaces, such as buildings, sidewalks, roads and patios, which hold more heat.  

Cities throughout the world are experiencing urban heat islands, and Denver is no different. Take a look at Denver’s Heat Island map:

This map uses impervious surfaces as a proxy for heat. In this map, the darker colors represent hotter areas 

…and now take a look at the map of the city’s trees:  

In this map, the darker the color, the higher percentage of tree canopy cover there is. 

See how they are essentially opposites of each other? Meaning, the places with lower heat vulnerability are also the places with the highest tree canopy coverage. It’s clear to see that trees make a difference.  

Now that we know it’s this easy, what can we do? You guessed it.  

Plant. More. Trees.   

Now you may be asking, what is it about trees? It can’t just be the shade they offer. What else is taking place? It’s a great question with only a halfway-surprising answer. Because a lot of it does have to do with the shade they provide and the sunlight they deflect. But there is also something else happening that helps trees cool their surrounding area… 

Evapotranspiration.  

Over at USGS.gov (United States Geological Survey), a science bureau within the United States Department of the Interior, there is a simple illustration that says it all: Evapotranspiration simply describes how water working through the tree ends up cooling the air around it. Water (precipitation) comes in through the roots and is released as vapor (transpiration). And, as an added bonus, the soil releases even more water to cool the air down low (evaporation).

So, one of the best ways we can combat heat islands is by planting more trees. Two ways you can do that: apply for a free tree in your public right-of-way and plant more trees downtown.  

All of the above combined results in a drop in temps and a happier, healthier – and much cooler –community. If you’ve read all the way through and you still want to learn more, check out this PBS interview from 2021, featuring Denver’s own City Forester Mike Swanson talking about Heat Islands.