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SUMMER IN THE CITY: Raging Urban Heat Islands vs. Trees Hoping to Shade the Day

The forecast is for sweltering heat and poor air quality. Just another typical summer’s day in New York City and many other cities across the globe. But what can urban dwellers do to suffer less??

The following post describes notorious ‘urban heat islands’ and how incredible shade trees are to combat them. It also describes how you can steward and help them through basic, common sense tree care measures.

NYC’s ‘heat island’ (MVR’s) is the context for the prior “Tales of Islands” post, and those to come.

Beginning in the early 1900’s scientists observed that cities and their suburbs have warmer surface and air temperatures than nearby rural surroundings. Since then warmer city regions have grown exponentially and are commonly referred to now as “urban heat islands”.

Ecologists and researchers have come to understand that “urban heat islands” are generated by the replacement of green rural lands with buildings and pavement. They now regularly track the temperature increases in rapidly expanding cities.

Atlanta, Georgia is a good example. From 1973 to 1997 as Atlanta expanded and replaced 65% of its forest cover with roads and buildings, average summer temperatures rose about 6° F (American Forests, 2001).


Continued urban expansion and development has led to intense city heat islands that slow-cook huge land areas. The hottest heat island temps are found in the densest city sections with the least tree cover and vegetation.

Buildings and paving contribute to heat islands in two ways:

  • Low moisture content Asphalt and concrete are dry and watertight replacements for naturally moist soils. No moisture is available in these materials to dissipate the sun’s heat through evaporation as from soil surfaces or through the leaves of trees.

  • Excess heat accumulation Dark surfaces like asphalt paving and black roofing absorb 90-95% of sunlight, while reflecting only 5 to 10%. Lighter color roofing and pavements can reflect 25% to 40%. Vegetation can reflect as much as 45%.

  • Unshaded asphalt during peak summer hours in hotter climates can reach an unimaginable 160°F! UGH!

Complete cover of buildings and paving in some city sections can elevate air temperatures by as much as 20° to 40°F (NASA 2003). Buildings and paving also act as “heat batteries” by accumulating heat during the day and then radiating it back out at night, contributing to sweltering city summer nights.

In the worst case scenario, warm city

surface air can be trapped by a “lid” of warmer air aloft. This pollution-trapping effect is known as an “inversion”.

... when it's best just to stay indoors with the AC on, which most of us do.



Heat Makes Air Quality Terrible

Strong sunlight and heated air “bake” gasoline and vehicle exhaust emissions into lung-searing ozone, a main ingredient of urban smog. Elevated ozone reduces lung function, especially for those with asthma, bronchitis, or any lung disorder. It also increases sensitivity to allergens, irritates the eyes, and causes dizziness and nausea (EPA 2003).

Through-the-Roof Energy Costs

Hot unshaded paving has been scientifically linked to higher air-conditioning costs of nearby buildings. Peaking electricity demand during periods of extreme temperatures stresses the electrical grid and leads to inefficient operation. But how do heat islands affect the pocketbooks of city dwellers?

In a recent NASA study, researchers found when impervious surfaces surpass 35% of the total area, temperatures go haywire. At 65% urbanization, cities are almost 3°F warmer than surrounding area. That might not sound like much, but it has been shown that a rise of 3°F increases air-conditioning use and costs as much as 20% compared to nearby shaded or rural areas.

In the light (or heat) of the above, consider energy costs in many cities where there is nothing but buildings and pavements, where impervious coverage is near 100%. Some with no shade trees either.


In 1994, NASA did a study at a mall in Alabama where they read daytime temperatures in the middle of the parking lot at 120°F, and then a spot nearby under a couple of shade trees at 89°F. Those few shade trees made a difference of 31°F!

Recall that unshaded asphalt lot earlier with temperature of 160°F? The same paved surface with shade trees could have temperatures kept below 100°F. Such a difference is particularly important to all ‘Downtown’ areas -aka Central Business Districts, and efforts to make them enticing, welcoming places for customers and visitors.

The really good news then is that tree canopy cover does reduce urban heat islands and reduces on-site heat build-up substantially (Akbari, et al. 1992, Asaeda, et al, 1996).

Trees are the least expensive means of reducing heat island effects as their canopies cool paving by direct shading of the ground surface, as in the image right.

They also cool streets and parking areas indirectly through transpiration of water through their leaves, much like perspiring does for our bodies.

Helpful to remember:

We get a 1°F drop in temperature, with each additional 10% amount of tree canopy cover (Simpson, et al, 1994).


The challenges of establishing and maintaining street trees in dense cities are often extreme. But hey, here’s more good news! By knowing a few basic care measures anyone can be an effective tree steward on city streets for neighborhood and work places. Below is a list of some tree challenges and common sense ways to overcome them, and, beat the heat.

Watering With all the heat and paving in cities, newly planted trees require more water for establishment than in less dense places. Remove weeds from tree beds. Water weekly for 2 years to get trees off to a vigorous, established start. Coordinate watering with neighbors to insure new trees are not overwatered. And remember to gives trees a deep soaking during long hot dry spells. A few inches of mulch will retain the moisture that trees need consistently.

Pruning Trees need pruning quickly after 2-year establishment period to grow vigorously, be structurally sound and look their best. Consider becoming a Citizen Pruner with, or, hire an arborist. Avoid dense thickets or tangled main branches where they start at base of crown. Look here for an example of before and after pruning.

Protection|Preservation Be vigilant on all construction activity, especially scaffolding over city sidewalks , which often kills or disfigures tree canopies, stripping away what little shade some streets have. This a big problem on arterial streets/avenues (more on them in next post).


Even more so than city parks (which now have private conservancies taking care of them), urban street forests are seriously underfunded, from their establishment to their maintenance. Corporations and philanthropists should consider large donations for city street shade trees, because along with mitigating urban heat, robust trees shading streets, plazas, and parking lots extend all the benefits of trees throughout a community for generations.

PLUS Please curb your dog, stay on pavements, and don’t chain bikes to trees, etc., etc.

This is MVR, NY Street|High Line Arborist and TD, EcoDoc...signing off from Green In!

Image Sources

HI Profile

Solar reflectance cool pavements dark pavements

Heat batteries

Air Quality Web image compilation

HI Energy Costs

Tree Canopy Outrigger-napili-shores trip advisor

Tree transpiration

Cash $$$

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