Jul 21, 1998

Salt Lake City shows hot and cold spots

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Mountains to east provide cool refuge

July 21, 1998: Salt Lake City, the capital of Utah, has rooftops and other structures that reach a blistering 71 deg. C (160 deg.F) in the summer heat, according to data from the last of the overflights planned in the Urban Heat Island Pilot Project (UHIPP).

Sacramento and Baton Rouge were observed in earlier overflights. Several other U.S. cities are participating through ground-based and satellite observations. Houston, Chicago, Los Angeles, Nashville, Phoenix, and Tucson will participate with satellite data only.

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In this "quick look" image, below, - which has not been calibrated or corrected - white and red are hot and generally correspond with roads and buildings. Blue and green are cool and generally correspond with water and vegetation. North is up. From surface temperature estimates, the white areas are about 71 deg. C (160 deg.F), said Dr. Jeff Luvall, a NASA scientist, and the principal investigator of UHIPP at the Global Hydrology and Climate Center in Huntsville, Ala. Dark areas (vegetation) are approximately 29 to 36 deg. C (85-96 deg. F). Absolute temperatures will change after calibration, but the relative temperature differences between surface types will stay the same.

The image spans Salt Lake City from about 300 West Street on the western (left) edge of the image to the foothills of the Wasatch Mountains on the eastern (right) edge. Two rivers snake in from the Wasatch to the northeast, Parley's Fork of the Red Butte Creek (from the right side of the top edge) and the Lithograph Fork (from the top of the right edge of the image). State road 186 runs across west to east across the image, then takes a leisurely turn south; E 800 South Street runs the full width of the image. (Links to ; credit: NASA/Marshall Space Flight Center.)

The image was taken Monday, July 13, 1998, at 12:34 p.m. local time by the Airborne Terrestrial and Land Applications Sensor (ATLAS) imager aboard a NASA Lear 23 jet equipped with various sensors and cameras for UHIPP.

Analysis of the data collected in UHIPP is under way, but at least one preliminary result shines through.

"One thing's for sure," said Luvall, "every city I have looked at is hot! - and can use a lot of trees and reflective rooftops."

Atmospheric profiles of temperature, relative humidity, and pressure were measured with a balloon-borne instrument package called a radiosonde to calibrate the ATLAS measured surface temperatures. Additional roof surface temperatures were taken with a handheld "heat spy," an infrared thermometer to help calibrate the ATLAS thermal measurements. Scattered around the city on three rooftops were instruments which measured the visibility or transmissivity of the atmosphere to aid in the calibration of the visible data.

The image clearly demonstrates the principle behind UHIPP, that the differences in cooling and heating between the natural and manmade surfaces can affect city temperatures.

"Urban forests are important to keeping cities cool," Luvall said in an earlier interview "What's important are both the extent and arrangement of these forests."

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Luvall also noted the interesting cooling effect that the streamside vegetation has as streams flow from the mountains. You can see the streams as cool blue "fingers" from the eastern mountains in the image above. Luvall said there is the tendency for engineers to ditch and concrete these areas in the city, which effectively removes the cooling streamside vegetation.

Luvall said that it is important to note that this is a quick-look image that has not been corrected for atmospheric interference or fully calibrated with ground sensor data.

In addition to ATLAS and the "heat spy" instruments on the ground, the study uses images from a 23x23 cm (9x9 in) film camera aboard the Lear 23 and sensors aboard weather satellites.

The Global Hydrology and Climate Center (GHCC) in Huntsville, Ala., working with the U.S. Environmental Protection Agency and several local governments, is conducting UHIPP. The GHCC is a joint venture by NASA/Marshall, the Universities Space Research Association, and the Space Science and Technology Alliance of the State of Alabama. UHIPP follows the successful Urban Heat Island Experiment in Atlanta in May 1998.