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Sacramento
Pilot Cities
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definition
Sacramento is located in California's central valley
against the Sierra foothills. The city has a population
of approximately 400,000 people and covers over 96
square miles. The Sacramento
Metropolitan
Statistical Area
includes about 1.8 million people in a six-county area
and covers approximately 984 square miles.
Sacramento's Heat Island
In July 1998, the National Aeronautics and Space Administration (NASA) took flyover photographs of Sacramento using Advanced Thermal and Land Applications Sensor aircraft data. These flyover shots represent a typical view of the city (left) and a thermal readout of metropolitan hot spots (right).
Aerial view of Sacramento (courtesy of NASA-Marshall Space Flight Center- Global Hydrology and Climate Center).
- select image for a larger view
The images are visible (left) and thermal infrared (right). The white areas, mostly rooftops, are about 140°F (60°C) and the dark areas, primarily vegetative areas or water, are approximately 85-96°F (29-36°C). The hottest spots are buildings, seen as white rectangles of various sizes.
Sacramento's rail yard is the orange area east of
the Sacramento River, which flows from top to bottom.
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.)
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 Sacramento'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 Sacramento's heat
island with the eastern area being consistently cooler
due to the higher elevations of the Sierra foothills.
LBNL simulations indicate that after 12:00 p.m., the
temperature distribution in the Sacramento area becomes
a strong function of elevation. From noon until 5:00
p.m., Sacramento's simulated heat island, which
represents air temperatures near the ground, is
approximately 0.9-1.8°F (0.5-1°C).
Sacramento's Climate
Sacramento's climate is characterized by mild year-round temperatures. Summers are dry with little humidity and an abundance of sunshine. Winter months can be cool and often rainy. The highest temperatures occur throughout July and August and can average around 88°F (31°C). The coldest days are usually in January when the temperature can drop to an average low of 38°F (3°C). Average monthly temperatures for Sacramento range from 53.5°F (12°C) in January to 88°F (31°C) in July.
Sacramento receives an average annual rainfall of
about 18 inches per year and has an average relative
humidity in June of 78% in the morning and 32% in the
afternoon. Based on 1961-1990 National Climatic Data
Center data, Sacramento has, on average, 1,237
cooling degree
days
and 1,869
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 of implementing heat island reduction 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 also can 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.