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Posted at 10:30 AM ET, 08/31/2009

Making Hotter Augusts Not Quite as Hot in D.C.

By Ann Posegate

Wx and the City

* Cool Down Is Here: Full Forecast | Hurricane Tracking Center *

The abundance of paved surfaces in urban areas often leads to temperatures several degrees warmer than in nearby rural locations. The so-called Urban Heat Island effect could exacerbate the impacts of climate change. Courtesy UCAR/EPA.

Last week, Andrew Freedman's article brought up a critical issue for residents of the D.C. metro area: the potential for increasingly hotter Augusts. In the comments section, he posed the question: "Does anyone have any ideas for practical ways to minimize the impacts of extreme heat in the future, short of following Congress' lead and getting out of town?"

Bombojea47 and meteoscott responded with two suggestions: more green space and painting our rooftops white. They were right. Besides escaping to the countryside, most solutions boil down to one simple principle: Less concrete = less heat.

Adapting to hotter summers in the future involves reducing the existing Urban Heat Island (UHI) effect by increasing our vegetation and use of lighter-colored pavement.

Keep reading for more on the Urban Heat Island effect and what can be done to minimize its impact...

The UHI is just like it sounds: Cities act like islands that produce and trap their own heat, leading to ambient air temperatures that are often warmer than in nearby rural areas. We can feel the effect of UHI on our feet and faces when we walk across a paved parking lot or street in the evening after it has been baking in the summer sun all afternoon and is emanating heat.

Cities are built with an abundance of dark, paved surfaces like buildings, streets and roofs that have a low albedo, or ability to reflect the sun's radiation. Thus, paved surfaces absorb and retain more heat than grassy fields or forested parks, and likewise emit more heat into the air immediately above them, a process that continues long after the sun goes down. This surplus heat can leave temperatures in cities several degrees warmer than surrounding areas, especially at night.

Typical urban development also includes loss of tree cover (meaning less shade and less evaporation of water into the atmosphere by plants -- both natural cooling processes), tall buildings that trap heat and reduce air flow, and a lot of vehicles that release hot gases. All of these activities contribute to warmer temperatures which, in turn, affect our health, energy use, weather, wildlife habitat, and quality of air and water.

Most paved surfaces and buildings serve essential roles in our communities. The good news is that the impact of UHI can be reduced by increasing natural sources of cooling and using lighter-colored building materials. Here's a breakdown of some "cool" solutions that are being implemented in D.C., Maryland, Virginia and other urban areas around the country:

Urban Tree Canopy and Vegetation

Ten years ago, the Washington Post published two NASA Landsat satellite images from an American Forests report that showed the Washington metro area's recent loss of tree canopy. Between 1973 and 1997, the metro area lost over 70,000 acres of heavy tree cover (the District alone experienced a 64 percent loss of its area of heavy tree cover) as a result of more impervious surfaces (concrete, asphalt, buildings and other non-porous surfaces). Today, 35 percent of the District's land area is covered by tree canopy; over 40 percent of its land area is covered in impervious surface (source and maps).

Shade trees planted along streets and on the east and west sides of houses are especially effective at cooling. According to the U.S. Department of Agriculture, "the net cooling effect of a young, healthy tree is equivalent to ten room-size air conditioners operating 20 hours a day" (source). Casey Trees, a nonprofit in the District, operates an interactive tree calculator to help you calculate the ecological and economic benefits of trees on your property. If trees are not feasible, other types of vegetation can be planted - they do not provide as much cooling as trees, but are certainly an improvement over plain pavement.

Green Roofs

Temperatures on a dark-colored asphalt rooftop in the city can reach 150 to 175 degrees Fahrenheit on a hot summer day. After installing a green roof, these temperatures can drop by as much as 40 to 50 degrees. Green roofs consist of layers of waterproof lining, insulation, gravel, soil and sun-loving plants (take a virtual tour). The District's "20-20-20" vision is to install 20 million square feet of green roofs, or about 20% of the roof area of all city buildings over 10,000 square feet, by 2025. District government is offering a pilot green roof subsidy program for residents. For residential properties, green roofs remain a more expensive cooling option. But, they are extremely effective in cooling the air, reducing home energy use and reducing stormwater runoff.

Cool Roofs and Cool Pavement

Dark-colored asphalt rooftops both absorb and retain a lot of heat. Cool roofs that make use of new technologies in roof design and lighter-colored paint reflect rather than absorb the sun's rays, leading to temperatures 50 to 60 degrees Farenheit cooler than those above dark-colored roofs. Secretary of Energy Steven Chu recently suggested cool roofs as a means to reduce global temperatures.

According to the Environmental Protection Agency, reflective "cool" roofing materials are often comparable in cost, or even less expensive, than traditional roofing materials, plus they reduce energy use on hot summer days (you can calculate energy savings you would receive if you replaced your current roof with a cool one). Like cool roofs, cool pavements are made of materials that better reflect sunlight.

No doubt, if projections for warmer summers were to come to fruition, the impacts would be exacerbated by UHI and changes in the urban landscape. Increasing our green space, avoiding paving when it is not needed, driving less and cooling our rooftops are practical solutions to minimize impacts of future warming. Plus, they'll make us and our environment healthier in the process.

To have a significant impact, these initiatives need to happen on a city-wide (and nationwide) scale. The District's plan to minimize UHI impact is to increase tree canopy. Northern Virginia's focuses on planting trees and other vegetation, as well as increasing the area's number of cool roofs and green roofs. Maryland's plan includes all of these solutions and more.

Where and when do you notice the Urban Heat Island effect? Do you plan to implement any of the above solutions on your own property or in your neighborhood? Let us know with a comment below.

Learn more:
Cooling Summertime Temperatures -- strategies to reduce UHI
Weather and the Built Environment -- a free online course
Benefits of a Green Roof
Tree Benefits (more)
D.C. Green Infrastructure Maps

By Ann Posegate  | August 31, 2009; 10:30 AM ET
Categories:  Climate Change, Extreme Heat, Local Climate, Posegate, Wx and the City  
Save & Share:  Send E-mail   Facebook   Twitter   Digg   Yahoo Buzz   StumbleUpon   Technorati   Google Buzz   Previous: Forecast: A Wonderful Week Ahead of Weather
Next: PM Update: Hot August Ends With Cool Day


Your entire column is built upon the premise/assumption that there will be "hotter summers in the future". What if your assumption is wrong? Have you considered that possibility?

Mr. Q

Posted by: Mr_Q | August 31, 2009 10:46 AM | Report abuse

Hi Mr. Q,

Unfortunately, Urban Heat Island has been and will continue to be an issue regardless of exactly how hot future August temperatures turn out to be. These suggested solutions reduce air temperatures in the immediate area, restore air and water quality, and improve our health and quality of life. So, even today, they are helpful measures to take.

Posted by: Ann-CapitalWeatherGang | August 31, 2009 11:22 AM | Report abuse

Then why not write your column that way??? Why start out by forecasting future temperatures? Why did you feel the need to write, "Adapting to hotter summers in the future involves reducing the existing Urban Heat Island (UHI) effect by ..."?

If reducing UHI makes sense right now, regardless of future August temperatures, then maybe you should have written your column to reflect that. As your column is written, you propose to reduce UHI to fight future temperatures, not current ones.

Mr. Q

Posted by: Mr_Q | August 31, 2009 11:39 AM | Report abuse

The Urban Heat Island can have additional nasty effects. When conditions promote instability, the UHI may actually lead to the formation of additional "popcorn" air-mass thunderstorms, and it can increase the severity of pre-existing thunderstorms when they move in here.

On the other hand,temperature differentials due to the UHI may sometimes induce mesoscale eddies in the general circulation, which could alter local precipitation distribution. Lately I've been wondering if some of our "Washington Split" precipitation anomalies are due to heat-island induced changes in the general circulation.

In addition the type of precipitation we get in winter when the rain/snow "freeze line" wavers around our area might just be altered by the UHI. I'm beginning to suspect that we are losing up to five snow events per winter because the Urban Heat Island is changing several events that might have been all-snow in, say 1809/1810, into sleet events or even plain old all-rain events.

Posted by: Bombo47jea | August 31, 2009 11:44 AM | Report abuse

I heard about some areas reverting paved roads to gravel to save money, but I hadn't previously thought about gravel roads as a mechanism to reduce the UHI effect. I suppose light colored gravel, a permeable surface, would be considered a type of cool "pavement."

Posted by: spgass1 | August 31, 2009 12:23 PM | Report abuse

If, by adopting various techniques, the August temperature of D.C. were reduced by an average of 7 degrees Fahrenheit, what would that mean for the average temperature of D.C. during the winter months? Would it also decrease?

Which would result in overall annual higher energy consumption for the D.C. area -
1. current D.C. annual temperatures
2. significantly reduced D.C. annual temperatures

Do more people die from heat related causes or cold related causes?

It is called the law of unintended consequences. Just something to consider.

Mr. Q.

Posted by: Mr_Q | August 31, 2009 1:02 PM | Report abuse

I'm so glad to see this "cool roofs" thing starting to gain steam (so to speak). I've been trying to talk to people about this for years and they just kind of shrug it off as something that would only help around the margins (or they don't get why I bring it up at all). I think the effects could be very significant.

I bought a house in the Maryland suburbs eight years ago, and one end of it has a (nearly) flat roof that is coated with EPDM (a kind of rubber). The thing is black and a huge heat sink in the summer months. I looked and looked for a reflective coating that was designed to adhere longterm to this material, and couldn't find one. I ended up putting bright 4x8 polystyrene panels on the roof, weighted down by (lots!) of white bricks. It definitely cooled off that end of the house, but it wasn't an ideal longterm solution, as they have finally degraded into dust after 5 years. If anyone knows of something (new?) that is made for EPDM, I'd love to hear about it.

MrQ - cooling buildings costs far more in energy than heating them, per degree-day. So the tradeoff of any permanent whitening of a roof is in the positive direction, unless maybe you're in the extreme northern or southern lattitudes. Hudson Bay or Antarctica, say. And with current predictions of longterm temperature changes, DC is not going to be hurting in the winter anyway.

Posted by: B2O2 | August 31, 2009 2:15 PM | Report abuse

B202 wrote, "cooling buildings costs far more in energy than heating them, per degree-day".

Where did you get that information?

If you look at annual energy consumption, you will find that cold states use far more energy (per household) than hot states. Even Texas!

Go here and scroll about half way down the page. Go to the "Energy Consumption and Climate Section". Check out the graphic for yourself. He lists his DOE references if you wish to check his data.

--begin quote from the link--
Clearly there is a distinct regional pattern to home energy use. The states with the coldest winters (northern states in the continental interior) have the highest overall totals, while southern and coastal states have the lowest. This is not surprising, in view of the aforementioned fact that space heating is the biggest energy user in a typical home.
--end quote from the link--

And, to make matters much worse, if you scroll up, he has chart detailing how energy is consumed in the average residence *AND* how that energy is produced. You will note -
1. far more energy is used in heating than cooling
2. the energy used to heat is predominantly derived from petroleum and natural gas.

It is a double whammy.

Mr. Q.

Posted by: Mr_Q | August 31, 2009 2:55 PM | Report abuse

Mr. Q - Comparing total energy use per household in cold states to that in warm states is a very poor way of checking the basic assertion I made - which I had thought was pretty common knowledge, since using electric A/C's to cool a house is less efficient than simply burning natural gas (the most common heating source).

Without checking through his interpretation of the EIA's data, you could have all kinds of other confounds that skew a gross comparison like that. For simple starters, houses in the north could be bigger on average, making them consume more energy in both the summer and winter than in the south. Throw in differences in lifestyle, bigger appliances, whatever. It's simply not an apples to apples way of getting that answer.

The line of demarcation where the tradeoff occurs - making cooling the predominant energy user - may well be more equatorial than I implied earlier. If so, my bad. But the cool roof phenomenon certainly makes sense in southern regions, where you are barely heating at all in the winter and grinding away at the A/C for many months.

Sorry I don't have better data to show you, maybe someone else can chime in to clarify this.

Posted by: B2O2 | August 31, 2009 3:49 PM | Report abuse

An additional point, Even living in FL, we only would run the AC from late May to Oct. On average. We'd rarely ever turn on the heat. I think the longest period that we had the heat on constantly was maybe 5-7 days. While at my home in NY I know my parents use the heat from Oct through early May, and the AC June-Sept. So you're drawing heating/cooling energy nearly year-round as opposed to a much longer season of comfortable temperatures in the more southern states.

Posted by: Brian-CapitalWeatherGang | August 31, 2009 3:57 PM | Report abuse

--begin quote--
The answer, experts say, is that both heating and cooling your home take large amounts of energy, more than we use for any other appliance. But according to researchers at National Geographic's The Green Guide, you will probably consume more energy heating your home than cooling it.
--end quote--

source of the above quote

Mr. Q.

Posted by: Mr_Q | August 31, 2009 4:07 PM | Report abuse

I would think that in many colder climate areas, roofs would be white (covered with snow) for much of the winter anyway. Therefore, lighter colored roofs might not hurt during the winter as much as otherwise expected.

That said, our local winters don't typically provide long-lasting snowcover so that doesn't really apply to us.

Speaking of snow, only a couple more weeks until the HPC Snow models go live again.

Posted by: spgass1 | August 31, 2009 4:41 PM | Report abuse

@spgass, We're already begining to see significant snows again on the North slope of Alaska and NW Canada. The mts. in Northern Labrador are snow-capped again also, as well as a large portion of the higher terrain in NE Russia.

Posted by: Brian-CapitalWeatherGang | August 31, 2009 5:13 PM | Report abuse

as an architect who has designed heating/cooling systems, i can say that in this area, heating dominates. systems are designed for about 4500 heating degree days, and about 1300 cooling degree days.

**these figures assume a 65 degree base temperature - which i hope is unrealistic. i hope most people are not setting the thermostat at 65 degrees in the summer. if the thermostat is set at 78 degrees in the summer, it would lower the cdds probably to under 1000 - making heating even more dominant.**

just think about it: your heater is on for more of the year than your air conditioner. and in the dead of winter, you're trying to warm the air probably about 45 degrees, whereas in the heat of summer you're only cooling it about 25 degrees.

i think the "magic line" (where heating/cooling demand are equal) on the east coast is probably around southern south carolina. (?)

Posted by: walter-in-fallschurch | August 31, 2009 6:30 PM | Report abuse

walter-in-fallschurch, the figures you cite (degree-days) are part of the equation, but the other part is then "how much energy does it take to accomplish this heating/cooling degree?".

I don't have figures on hand, but it has long been my understanding that air conditioning is a much less energy-efficient process than heating (as long as you're heating by natural gas). The full explantion for that is hazy to me, but I believe in part it is all the "middlemen" involved in A/C, which is the power station having to turn coal into electricity, transmission losses, and then the efficiency of the condensor/circulator units themselves. On top of that I believe there is a sizable energy cost to removing the moisture from the air before you even lower its temperature. Contrast that with the heating end of things where you simply combust natural gas that's pumped to your house, and circulate that heat with a fan.

I just don't know what the bottom line of that is, but I know that A/C is costlier than heating, per degree-day. So that has to be figured in.

Posted by: B2O2 | August 31, 2009 8:48 PM | Report abuse

Solve all problems = Nuclear Energy (for 20-30 years) until Fusion comes online...



Posted by: meteoscott | August 31, 2009 10:48 PM | Report abuse

I agree meteoscott. Nuclear power has no toxic emissions and it is reliable (unlike wind and solar). But apparently it makes too much sense. There wasn't a penny of stimulus money for nuclear power.

Mr. Q.

Posted by: Mr_Q | August 31, 2009 11:28 PM | Report abuse

Meteoscott and Mr. Q: The idea of widespread deployment of nuclear power simply is not tenable in terms of economics. There is a reason why no new nuclear plant has been built in this country since the 70s, and it's not just the "not in my backyard" phenomenon or the waste issue, but rather has to do with the unbelievably high expenses associated with plant construction and operation. Huge subsidies are required, along with lengthy delays due to liability concerns and more. It's just not a viable option at this time, despite the fact that it has vast advantages over fossil fuels in terms of carbon dioxide emissions.

It would be greatly appreciated if you two could collaborate and solve the vexing fusion problem for us all. So please get to work on that :)

Posted by: Andrew-CapitalWeatherGang | September 1, 2009 12:46 AM | Report abuse


I agree with you if we are talking about crappy 30 year old American reactors, which have many problems with expenses, efficiency and reliability. Your argument might be valid decades ago, but with current technology and knowledge of 4th generation nuclear technology, prices have dramatically dropped (of course subsidies are required, but subsidies are going to be major part of any future "green" technological revolution). If you look at the european reactors (3rd and 4th generation) with upwards of 95%+ reprocessing, efficient energy production and much more reasonable costs, it really does come down to the american public's will.

The cold hard fact is that nuclear technology is the only current energy technology that can meet our future energy and carbon reduction goals. The quicker we can all realize this, the faster we can learn to use this technology responsibly and efficiently.

Just some thoughts…



PS - GOOGLE: American Nuclear Engineering University Departments, and see how many come up. Americans have given up on this technology long ago, and I feel that we need to reenergize this area of science in America in order for us to make sure we can develop our own plentiful, clean and cheap energy… until Fusion Energy comes online in 30+ years…

Posted by: meteoscott | September 1, 2009 9:27 AM | Report abuse

I just laugh when I hear all the excuses for not increasing nuclear energy in this country. Funny how the French have managed to do just that, now getting at least 70% of their electricity from nuclear plants safely and efficiently (they reprocess their spent fuel). French-produced nucelar energy is now displacing fossil-fueled energy in Germany, that's how cheap it is.

We can throw away millions of dollars of subsidies at windmills -- and blissfully ignore the cost of transmitting wind energy from source to load, which will require hundreds of millions of high-voltage transmission wires and resulting losses -- but we can't subsidize nuclear? What a bunch of liberal BS. Face it, your greenies just don't like nuclear energy, despite all the evidence that it is cheap, clean, and efficient, and you invent excuse after excuse to avoid facing the facts. I've worked in the energy field for over 20 years now, and I know BS about energy when I hear it.

Posted by: WashingtonDame | September 1, 2009 11:03 AM | Report abuse

Hi WashingtonDame,

Although I tend to agree with much of what you said, claiming that this is liberal BS is just wrong. These restraints regarding US Nuclear ambition spread across a wide range of political and scientific ideologies. And some of the concerns and restraints are legitimate arguments for not implementing a widespread deployment of nuclear technology.

Excuses are always used in any debate when differing options are present. The point is to bring moderate voices to the table in order to discuss all of our options using the most accurate and up-to-date knowledge, information and technology available in order to make decisions that have the highest probability for success. If we can do this, I think that we will come to the conclusion that nuclear power must be strongly considered over most other technologies, with that said, there is no silver bullet and a wide range of technologies should be considered.



Posted by: meteoscott | September 1, 2009 11:39 AM | Report abuse

Mr. Freedman wrote, "There is a reason why no new nuclear plant has been built in this country since the 70s, and it's not ..."

You are incorrect. Did you know that we currently have 104 licensed and operable commercial nuclear power reactors at 65 plant sites. And those 65 plant sites are spread across 31 states?

The 80's saw a HUGE increase in nuclear power plants.

I take it you have abandoned your stance about not discussing policy here? That would be awesome! I eagerly welcome that development.

Mr. Q.

Posted by: Mr_Q | September 1, 2009 1:26 PM | Report abuse

walter ifc,
The average degree days for DC are:
4055 (heat)
1531 (cool)
This season has been slightly below average.

Posted by: CapitalClimate | September 1, 2009 9:15 PM | Report abuse

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