Hot 'Lanta (NASA, school kids team to study how trees help cities keep their cool)
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how trees help cities keep their cool
how trees help cities keep their cool
original posting: May 8, 1997
Everyone talks about the weather, but it turns out we have been doing something about it for decades. The change - for the worse - was so gradual that we didn't notice it. And fixing problems that we have caused may start with planting a few trees downtown and using the same observation techniques developed for studying other planets in our solar system.
This month, dozens of school kids in Atlanta will team with NASA's Global Hydrology and Climate Center to determine the extent of the changes. This Urban Heat Island Experiment is part of a larger study of past and future land use impacts on climate and air quality of Atlanta.
In turn, it may help cities reshape themselves for comfort - and lower heating bills - and help developing nations avoid the same mistakes.
|ATLAS imager aboard NASA's Lear 23 jet will take images of Atlanta like the ones here of a shopping mall in Huntsville, Ala. The thumbnail at left combines day (left) and night (right) images. Click here to get the full picture.|
For the convenience of newspaper and magazine editors, we have posted Photoshop TIFF copies of these images (in PC and MAC versions) for downloading and use free of charge.
"We are looking at the impact of Atlanta's growth on its air quality and temperatures over the 1973-97 period," said Dr. Dale Quattrochi, the experiment's principal investigator at the Global Hydrology and Climate Center in Huntsville, Ala. "Plus, Atlanta is looking at where to grow over the next 20 years."
While 5th and 6th grade students take thermometers into parking lots and schoolyards to measure temperatures at noon, NASA will send its Lear 23 jet aircraft overhead to scan the area and map heat patterns in Atlanta and surrounding urban areas. Combining the two sets of data will help paint a picture of how the city - and trees - affect local weather and air conditioning bills.
"Urban forests are important to keeping cities cool," said co-investigator Dr. Jeff Luvall. "What's important are both the extent and arrangement of these forests."
Urban sprawl Atlanta has been one of the fastest growing cities in America for the last two decades, so much so that the city is considered as having four central business districts rather than one. While the population of Atlanta proper dropped from 495,039 in 1970 to 394,017 in 1990, the suburban population in 20 counties around the city soared from 1.8 million to 3 million.
Atlanta can also be taken as a model for what is happening in the rest of the world.
"Developing countries are expanding so rapidly that you have to ask how they affect local and regional weather and, possibly even global, climate," Quattrochi said.
That means more houses, buildings, roads, and parking lots. Satellite images of Atlanta readily show how urban sprawl has pushed into what had been farmland and woodlands, largely along interstate and other major highways.
This growth of cities has produced "urban heat islands", or hot spots within the otherwise cool countryside. Heat islands are caused by heat buildup during the day in buildings, pavement, and other urban surfaces. This heat is released after sunset and forms a dome of higher temperatures over the city.
But Quattrochi and co-investigator Jeffrey Luvall, also at GHCC, have found some hope in the midst of the sweltering cities. Trees can help reduce temperatures. That's part of what they want to define and study in Atlanta.
A hot place to shop
For example, pictures of a shopping mall in Huntsville show the mall and other buildings, parking lots, and roads are very warm during the day, and the parking lot is still "glowing" at night. By contrast, wooded areas, even small trees islands in the parking lot, show as cool spots.
The difference has to do with how materials absorb and release heat. Asphalt absorbs heat from the sun and quickly releases it as heat radiation. Temperatures in the parking lot, in the summer of 1994, were as high as 120 degrees F during the day, while tree islands in the lot were at 89 degrees F - a difference of 31 degrees! Nearby wooded areas were only 85 degrees F.
At night, the parking lot stayed warm, about 75 degrees F. The tree islands were 63 degrees F, and the woods were only 63 degrees F. Even grassy areas near the woods appear hotter than the woods because a meadow has less vegetation and shade.
It's important to understand that just as much sunlight falls on cities as on woodlands of the same size. The difference is in how urban materials react to the absorption of solar radiation. Asphalt in parking lots and on rooftops, in particular, soaks up everything and reradiates it as thermal infrared radiation.
On the other hand, water absorbs a large amount of heat before its temperature rises 1 degree, and takes a long time to release it. That means that trees, which have a large water content and release water into the atmosphere to keep themselves cool, also absorb a lot of the incoming heat and release it over a longer period of time.
|Inframetrics thermal infrared camera (145K image); shows how trees make a difference in the middle of a parking lot. The image at left was taken during the day, while at night (right half) the shadow of the tree is still visible. Click to get larger views.|
Luvall offers another striking view of the difference that trees make. He took an Inframetrics camera (which is not part of the Atlanta study) to Athens, Georgia, last year and videotaped heat patterns during the day and evening. Not only was a tree in a mall parking lot cooler, the asphalt in the tree's shade was cooler even after sunset.
Luvall showed how the difference is readily "visible," to the right camera. The Huntsville pictures were taken with ATLAS, the Airborne Thermal/Visible Land Application Sensor aboard a Lear 23 jet operating out of NASA's Stennis Space Center in Bay St. Louis, Miss. ATLAS, which will also be used in the Atlanta experiment, sees in 15 colors.
"This is the same basic instrument as Galileo has," Luvall said, referring to the unmanned NASA spacecraft now orbiting Jupiter. Many of the computer tools used for analyzing the images also are derived from methods used to study the planets.
False colors for true images
The red, green, and blue we know in human vision are just broad channels within the 400 to 700 nanometer (nm) wavelength band that covers visible light. ATLAS sees in six visible light channels, two near-infrared (just beyond 700 nm where human vision stops), and six in the thermal infrared band, 8,000 to 14,000 nm (or, 8-4 microns) which represents radiated heat energy.
ATLAS has a resolution of 10 meters (33 feet), meaning that objects about the size of a small house or building will show up as only one or two dots in the images. That's detailed enough to show the heat patterns in the city.
|To provide a better understanding of data collected by imaging sensors, data may be overlaid on topographic maps to provide 3D images, such as this view (85K image) across central Huntsville, Ala., looking into Jones Valley and on to Huntsville and Green Mountains. The image is a frame from a fly-around video compiled from aircraft and map data.|
ATLAS lets scientists see how different objects reflect or absorb and emit heat. It's related to the greenhouse effect that made life possible on Earth: incoming sunlight passes through the atmosphere and is absorbed at the ground. The now-warm objects reradiate this energy at longer wavelengths - thermal infrared radiation - that the atmosphere absorbs rather than passing back into space.
(The current climate debate is not over whether a greenhouse effect is taking place, but whether it has become a "runaway greenhouse," one that will heat beyond comfort.)
What scientists see, through ATLAS and other instruments, is that cities follow this rule to the extreme. Asphalt parking lots and roofs soak up virtually all of the radiation that falls on them and reradiate it as heat.
The NASA Lear 23 will carry ATLAS plus a standard 9x9-inch aerial mapping camera taking pictures on false color infrared film (blue is filtered out; green shows a blue; red as green; and near infrared as red) to help in matching the Atlas images with what is on the ground.
ATLAS will be flown about solar noon (1 p.m. EDT) and again about midnight to get true "before and after" pictures. The aircraft will fly north and south to cover a 30-mile (48 km) square centered on Atlanta.
In addition to kids at 10 schools - the heat island experiment will involve water vapor data taken by instruments carried aloft by balloons, and sunlight measurements from shadow band radiometers placed at several locations around the study area.
The Atlanta experiment was scheduled to start no earlier than May 5, depending on weather conditions and aircraft availability. Results will become available later in the fall. Even the kids will participate - they'll get CD-ROMs with copies of the images taken over their schools.
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