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Salt Lake City
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- Activities
- Air Quality
- Energy Savings
- Urban Fabric
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definition
Salt
Lake City is located in the high-desert terrain of
northwestern Utah. The city has a population of
approximately 182,000 people and covers over 109 square
miles. The Salt Lake City
Metropolitan
Statistical Area
includes over 1.3 million people and covers
approximately 1,617 square miles.
Salt Lake City's Heat Island
In September 1998, the National Aeronautics and Space Administration took (NASA) aerial photos of Salt Lake City using Advanced Thermal and Land Applications Sensor aircraft data. These flyover photos represent a typical, visible view of the city and a thermal infrared readout of metropolitan hot spots.
The white areas in the visible photo, mostly rooftops, are about 160°F (71°C). The dark areas, primarily vegetative areas or water, are approximately 85-96°F (29-36°C).
In the thermal image, red and yellow areas indicate "hot spots" and generally correspond with urban development, while blue and green areas are cool and generally correspond to the natural environment. (These images have not been calibrated. Absolute temperatures will change after calibration, but the relative temperature differences between surface types will not.)
Aerial view of Salt Lake City (courtesy of NASA-Marshall Space Flight Center- Global Hydrology and Climate Center).
-select image for a larger view
Before determining how heat island reduction strategies impact an area, researchers need to evaluate existing surface characteristics. Aerial photos are useful for estimating the proportions of vegetative, roofed, and paved surface cover relative to the total urban surface in a city. Having this urban fabric information can help researchers simulate the meteorological and air quality impacts of heat island reduction strategies.
Surface cover data also help scientists determine an area's heat island. The Department of Energy's Lawrence Berkeley National Laboratory (LBNL) modeled Salt Lake City's near surface heat island, which represents near ground air temperatures as opposed to surface temperatures measured by thermal images.
LBNL conducted this modeling over a large area,
several times larger than the city center. They found
that topography greatly influences Salt Lake City's
heat island with higher elevations having consistently
lower temperatures than the lower elevations. LBNL
simulations indicate that Salt Lake City's heat island
can reach 7.2°F (4°C) at night and about
3.6°F (2°C) during the afternoon. The aerial
extent of Salt Lake City's heat island is greater in
the afternoon compared to earlier in the day.
Salt Lake City's Climate
Salt Lake City's climate is characterized by hot, dry summers, and cold, mild winters. The Great Salt Lake, which lies to the northwest of the city, greatly influences the climate. The average summer temperature in Salt Lake City is 92.2°F (33.4°C), and the average winter temperature is 36°F (2.2°C). Salt Lake City receives most of its rain in the spring and heavy snow in the winter. Average annual rainfall for the City is about 15.7 inches per year.
Salt Lake City has an average relative humidity of
68% in the morning and 43% in the afternoon. The area's
summer daytime relative humidity averages less than
30%. Salt Lake City receives 67% annual sunshine. Based
on 1961-1990 National Climatic Data Center data, Salt
Lake City has, on average, 1,047
cooling degree
days
and 5,765
heating degree
days
.
Local climate data, such as cooling and heating degree days, can help researchers estimate the potential energy savings and air quality impacts from implementing heat island mitigation strategies. For example, areas with long, sunny, hot summers and high cooling degree day values generally can achieve substantial energy savings.
Information on an area's local climate can also help communities focus on heat island reduction activities that best suit their region. For example, cities with predominantly dry climates may achieve greater benefits from increasing vegetation than would cities in humid climates. Dry-climate cities more effectively capture the cooling benefits of evapotranspiration – or evaporation of water from leaves. However, dry-climate cities also need to consider the availability and cost of water to maintain vegetation.