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Weather Almanac for July 2001
HOT TOWN -- SUMMER IN THE CITY
"Hot Town, Summer In The City," the opening line of a 1966 hit song by the rock group The Lovin' Spoonful, describes a familiar condition to urban residents. To meteorologists, this condition is known as the urban heat island and distinguishes the city's climate from the surrounding rural areas through all seasons. Most of us are aware of its presence in the colder months when the local weather forecast announces: "Low tonight, 4o, cooler in the suburbs," or "risk of frost in the outlying areas."
The urban heat island is an area centered on the city core of warmer air temperatures than those in the surrounding countryside. On an infrared satellite picture, cities show up as islands of heat (red colour) amidst a sea of cooler temperatures (black areas).
The warmest temperatures are usually located at the city centre with temperatures dropping off toward the city edge. A further drop is usually apparent in the surrounding suburbs, with minimum temperatures found in rural surroundings. The diagram below shows an idealized schematic of the urban heat island structure, but in reality the picture is not always as smooth. As infrared satellite images show, pockets of warm air and cool air (usually parks) make up the thermal topography of the heat island.
The urban heat island has significant impacts on the climate of a city when compared to the encircling countryside. It reduces heating needs, snowfall amounts and snow cover duration during the winter in many cities. It alters the wind speed and direction (in conjunction with other urban elements), cloud cover, precipitation and air quality, year round. But it is during the summer that we most feel impact of the urban heat island as it makes "walking on the sidewalks, hotter than a match head," and for that reason, I limit this discussion to the summer impacts of the urban heat island.
That the environment of cities differs from the surrounding countryside has been recognized for millennia. But at first, it was poor air quality -- mostly smoke from heating fires or industrial processes -- that characterized the city environment. In 1818, when a familiar name in the history of meteorology Luke Howard published the first scientific study of urban climate entitled The Climate of London, more details of the character of urban climate emerged.
Howard, who is most known as "The Man Who Named The Clouds," was the first to propose that the urban centre was warmer at night than the surrounding countryside and have the data to back up the finding. In The Climate of London beneath a table of a nine-year comparison between temperature readings in London and the surrounding country, he commented: "Night is 3.70 [F]o warmer and day 0.34 [F]o cooler in the city than in the country." He attributed this difference to the extensive use of fuel in the city, with the daytime cooling a result of the urban smoke haze reducing the penetration of sunlight.
Since Howard's ground-breaking report on London, that city and many others around the world have been studied extensively to determine the full urban impacts on local and regional climate. While individual situations vary in detail, the general picture that emerges showed cities are noticeably and significantly warmer than the surrounding countryside, particularly at night. Furthermore, the urban heat island effect increases as cities grow in size and population.
[Ironically, many who argue that human influences have no impact on current global warming use the fact that the growth of cities and their heat island has "contaminated" the temperature record over the past century or so, thus masking any greenhouse gas-induced global warming trend.]
Storms of "Hot-Lanta"
Besides subtly altering climate normals such as mean temperature and precipitation, many cities are capable of altering their hazardous weather as well. Once a subject of hot debate, the evidence today is more convincing. One strong proof arose out of an international gathering of athletes. The 1996 Summer Olympics held in Atlanta, Georgia provided more than just fun and athletic competition; they also provided an interesting confirmation of what some had believed prior to the Games: that Atlanta was experiencing more thunderstorms than it had a few decades earlier.
In support of the Games, a dense weather observation network across Atlanta was established with a team of meteorologists to provide Olympic officials and athletes with up-to-date weather conditions at the many venues and quick advisories of any weather hazards that might arise. Following the Games, a team of meteorologists from California's San Jose State University studied the weather records collected from the Atlanta network and came up with a startling conclusion: Atlanta generated its own thunderstorms!
To see the full picture emerge, we must go back to the early 1970s when metropolitan Atlanta began a rapid expansion into one of America's major cities. From 1970 to 1980, the population grew by 27% and then exceeded that figure the following decade with a further 33% increase in population. To accommodate the growth, its suburbs doubled in size, and nearly 142,000 hectares (350,000 acres) of forest were cleared over the 13 metropolitan counties. As dark roofs and pavement replaced green vegetation, the Atlanta heat island grew rapidly, at times recording an urban-rural temperature contrast as high as 5.6 Co (10 Fo).
As the city expanded, local weather observers began to notice an increase in the frequency of non-frontal, or air-mass, thunderstorms and their attendant precipitation. Most striking, however, was a shift in the time of occurrence of these thunderstorm events. Non-frontal thunderstorms usually occur from mid to late afternoon, following the daily peak in solar heating and maximum air temperature needed for their formation. Atlanta weather observers, however, were frequently recording early morning thunder, a time when such storms should have been infrequent.
Using the data from the Olympic network, the San Jose State research team discovered convincing evidence that Greater Atlanta urban heat generation was indeed triggering thunderstorms south of the city. The city heat was replacing solar heat as the initiation mechanism for cumulus growth and storm development, and its greatest contrast with surrounding regional conditions occurred in the early morning hours.
It is most unlikely that Atlanta is a special case, and other urban areas in the US and around the globe should also trigger urban thunderstorms. Researchers and public safety specialists have thus become increasingly concerned that urban-generated storms could trigger flash flooding and increase lightning and wind damage.
Urban Heat Hazards
Summer in the city can be especially discomforting. In fact it can become extremely deadly, for according to US National Weather Service statistics, US heat-related deaths from 1989 to 1998 exceeded fatalities from any other severe weather category. Averaged over the ten-year period, hurricanes killed 14 people annually, tornadoes claimed 57 lives, and lightning strikes and floods killed 58 and 99 people, respectively. Excessive heat, however, killed an average of 193 during the same time period, about three times the number of deaths due to the much-feared tornadoes and hurricanes combined. Most of these deaths occurred in the cities.
In the past fifty years, heat waves in the US have been blamed for over 400 deaths on four occasions. During a prolonged 1963 heat wave, more than 4,600 deaths occurred in the eastern United States. A stifling, lingering heat wave killed 1,700 people in the East and Midwest US in 1980. In 1988, another East/Midwest heat wave killed 454. In 1995, a heat wave claimed a total of 716 lives in Chicago, Philadelphia, Milwaukee and St. Louis, 600 of those in the Windy City. In the heat waves of 1936, more than 4,768 Americans died from heat and high humidity.
Most fatalities occurred during these heat waves in urban areas where trapped heat brought unrelenting dangers to many, particularly the elderly and the poor who could not afford air conditioning in their homes. Such figures worry bioclimatologists and public health officials who fear continued climate warming could increase heat-related fatalities, particularly in northern US and Canadian cities where episodes of extreme heat are infrequent. Similar concerns have been raise in large cities outside North America such as Rome, Italy.
A number of cities including Toronto, Washington DC, Philadelphia and Rome now plan to initiate "Hot Weather Health Watch, Alert and Warning" programs when weather forecasts indicate potential prolonged hot and humid conditions. The warning system is based on a Weather Stress Index (WSI) developed by Dr Laurence Kalkstein and colleagues at the Center for Climatic Research at the University of Delaware. The WSI includes not only the absolute level of heat danger but also incorporates an adjustment for the relative frequency of such conditions in the region. Kalkstein believes such warning systems could save as many as 300 lives a year in the US by issuing timely alerts.
Cooling The Cities?
Even without the spectre of death associated with excessive urban heat, urban heat islands cost a city's residents money. The need for additional air conditioning can cost the typical household $100 per year or more, depending on location and local energy rates. In addition, power companies must maintain extra generating capacity that it may only use a few days each year, and this cost is always passed on to the consumer.
For example, if hot days are too common in California's urban areas this summer, the number of rolling, and perhaps full, blackouts in the state may increase. Power problems might also strike other parts of the US if a prolonged heat wave were to settle over the eastern half of the nation, because approximately 1/6 of the United States electricity consumption is used to cool buildings.
When the economic and health consequences of the summer urban heat island are all tallied, the total is great enough to concern many urban governments and citizen groups. Several cities such as Toronto, Ontario and Chicago, Illinois have begun to look into ways of reducing the urban heat island's summer impact. One of the most promising initiatives is through the greening of the city; that is, by increasing the vegetation cover within the urban limits.
Adding greenery over an area as big as a city park or as small as an individual tree, will help reduce the area covered by dark, heat-trapping surfaces, and thus favorably move the heat balance toward cooler conditions. In addition to added vegetation, replacing dark, absorbing rooftops and pavement with lighter, more reflective materials can increase the city's albedo (solar reflectance) and thus reduce the amount of sunlight absorbed by buildings and pavement. Rooftop gardens have also been proposed as a positive influence for heat island reduction as well as for air pollution reduction.
As cities heat under the summer sun, it is no wonder that urban residents flock to lake and sea shores or into the country each summer. But, while such journeys are good for both mental and physical heath, the heat generated by traffic and road systems further adds to the discomfort of those left behind.
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