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Houston
Pilot Cities
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definition
Houston
is situated on the Southeast Texas Coast Plain and is
located 50 miles north of the Gulf of Mexico. Houston
has a population of nearly 2 million residents and is
the fourth largest city in the United States. The
Houston
Metropolitan
Statistical Area
covers almost 9,000 square miles and has about 4.7
million inhabitants.
Houston's Heat Island
In August 2000, the National Aeronautics and Space Administration (NASA) took aerial photos of Houston using the Advanced Thermal and Land Applications Sensor aircraft data. These flyover photos represent a typical view of the city (left) and a thermal readout of metropolitan hot spots (right).
In the thermal image, red and white areas indicate "hot spots" and generally correspond with roads and roofs. Blue, green, and purple areas are cool and indicate water and vegetation. The temperature ranges are from approximately 149°F (65°C) for the hot spots to 77°F (35°C) for the cooler areas. (The images have not been calibrated. Absolute temperatures will change after calibration, but relative temperature differences between surface types will not.)
Aerial view of Houston (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) is modeling
Houston's near surface heat island, which represents
near ground air temperatures as opposed to surface
temperatures measured by thermal images. LBNL is
conducting this modeling analysis over a large area,
several times larger than the city center. LBNL staff
have determined that Houston's urban heat island often
is around 3°F (1.7°C) but can peak at up to
6°F (3.3°C).
Houston's Climate
Houston's climate is subtropical and humid. The sun shines for most of the year, and Houston has an annual growing season of nearly 300 days. Houston receives an annual average rainfall of about 50 inches.
The average low temperature in Houston is 72°F
(22°C) in the summer and 40°F (4°C) in the
winter. The average high temperature is 93°F
(34°C) in the summer and 61°F (16°C) in the
winter. Humidity in June is typically around 93% at 6
a.m. and 63% at 6 p.m. Based on 1961-1990 National
Climatic Data Center data, Houston has, on average,
2,700
cooling degree
days
and 1,599
heating degree
days
based on a reference temperature of 65°F
(18°C).
Local climate data, such as cooling and heating degree days, can help researchers estimate the potential energy savings and air quality impacts of 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.