Michigan as two other reporting stations selected for comparison â Waukegan, Illinois and Milwaukee, Wisconsin. Waukegan is located in a more rural area ...
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URBAN INFLUENCES ON VISIBILITY Michael R. Witiw*, Kenneth W. Fischer*, Jeffrey A. Baars** *Terabeam Corporation, Redmond, WA, USA; **University of Washington, Seattle, WA, USA Abstract An important part of the urban environment is visibility. It is related to both health and transportation, and more recently, to the effectiveness of free space laser communications. During the 1940s and 1950s, very low visibility episodes were common in the industrialized world. Well known are the Donora, Pennsylvania (USA) fog of 1948, and the Great Smog of 1952 in London. More recently, an examination of several locations in the United States indicates that visibilities in the urban core actually tend to be higher than those in the surrounding area. In New York City, low visibilities occurred in midtown Manhattan less than 40% as often as at La Guardia airport. At Washington Reagan Airport, one of a few urban airports in the USA, we saw that low visibilities occurred less than one third as often as at airports farther from the urban core like Philadelphia. In Chicago, observations in the city core revealed low visibilities occurred less than 15% of the time that it occurred in the more rural locations. Since 1985, the United States Environmental Protection Agency has reported steady decreases in urban particulate matter. We believe that the decrease in the frequency of occurrence of dense fog in urban cores may result from a combination of the decrease in urban fine particulate matter coupled with the effects of the Urban Heat Island (UHI). Key words: visibility, fog, urban environment, urban heat island 1. INTRODUCTION 1.1 Historical Background There are many factors that influence visibility in the urban environment. In many locations in the industrialized world, low visibility episodes appear to have peaked in the 1940s and 1950s. Two famous episodes include the Donora, Pennsylvania smog of 1948 (EPA, 2003), and the Great Smog of 1952 in London. In Donora, although there were no formal reports of visibility, anecdotally it was reported to be extremely low. In London, a smokeenhanced fog persisted from 5 through 9 December, 1952. Chimney smoke from residences was identified as a prime cause for the severity of this event. Visibility in London remained below 50 meters for 48 hours, and below 10 meters at Heathrow for nearly 48 hours (Met Office, 2003). Visibilities this low are almost unheard of today. These two events were the catalysts for clean air legislation in both the United Kingdom and the United States. Clean air acts were passed in the United Kingdom in 1956 and 1968. In the United States, a federal clean air act was passed in 1963, but states were not required to put pollution standards in place until further legislation was passed in 1970. These acts appear to have significantly affected the fog and visibility climatology of many urban areas. Today, there is little difference in the amount of sunshine received in London compared to the nearby countryside – a difference from 40 years ago, when the city received 30% less sun than its surroundings. 1.2 Urban Environment Influences on visibility in the urban environment are complicated. Combustion processes add moisture and particulate matter into the air, while the Urban Heat Island (UHI) warms the air, distorts the wind pattern and lowers the relative humidity in the urban core. In a study completed in California, Trijonis (1982) found a high inverse correlation of visibility to relative humidity and aerosol concentrations. In fog, visibility is inversely proportional to the total liquid water content and the number of water droplets. Since the scattering coefficient varies as the third power of the droplet concentration, an increase of water droplets by a factor of 10 doubles the scattering coefficient, reducing visibility by half (Hudson, 1980). Although visibilities in cities are frequently lower than the surrounding countryside, occurrences of very low visibilities often are less frequent (Oke, 1978). TRENDS IN URBAN VISIBILITY Since the disastrous low visibility events of the mid-twentieth century, in much of the world there has been an overall slow but steady improvement in urban visibility. Trijonis (1982) noted a decrease in mean visibility in both the San Francisco Bay area and the Los Angeles basin from 1949 to 1966. From 1966 to 1976, he noted an improvement in both areas. Witiw, Baars and Ramaprasad (2001) note a further significant improvement in Los Angeles from 1975 to 1990. Although this improvement was attributed to a shift in phase of the Pacific Decadal Oscillation, it is likely a decrease in particulate matter was at least partly responsible. A further study of visibility trends in 12 U.S. cities, found improvement in seven cities, no significant change in three cities, and a decrease in visibility in two cities (Witiw and Baars, 2003). In Sao Paulo, Brazil, Araujo, Freitas, and Goncalves (2002) reported a decadal decrease in total fog occurrence from the 1930s through the 1990s. Urban development and
sea surface temperature trends were seen as the most important factors, with the influence of El Nino and La Nina being important. In Buenos Aires, Hoffman and Nunez (2001) reported a decrease in fog days in metropolitan Buenos Aires, but no change in the surrounding rural areas. In Canada, over the past 30 years, a 20% to 30% reduction in fog days has been observed (Muraca et al., 2001). The trends in parts of the world, however, are toward lower visibility. This is true in some rapidly developing countries where it is extremely difficult to cope with increased particulates. In Beijing, for example, a decrease has been recently seen in urban visibility (Wang et al., 2001). In the case of Yerevan, Armenia, an energy crisis during the 1990s resulted in residents using particulate producing heating stoves. Much more frequent dense fog resulted (Hovsepaya, 2001). 3. CASE STUDIES OF URBAN VISIBILITY In the United States, visibilities at airports are reported in statute miles. (One statue mile is slightly more than 1600 meters.) However, since the mid and late 1990s, with the widespread deployment of the Automated Surface Observation Sytem (ASOS), visibilities below 400 meters have been only irregularly reported. Also comparing pre-ASOS data to previously manually collected data was made more difficult by the rounding algorithm used by ASOS. For example, prior to ASOS, a visibility reading of 400 meters could be interpreted as a visibility between 400 and 799 meters. It implied the observer was able to visually recognize the 400 meter visibility marker, but not the 800 meter marker. ASOS, however, rounds the reported visibility to the nearest 400 meters. As a result, visibilities between 600 and 799 meters, which would have previously been reported as 400 meters, are reported as 800 meters by ASOS. In this report, all tabular data are for the period September 2001 through August 2002. Except as noted, the National Climatic Data Center in Asheville, North Carolina, USA was the source for data. 3.1. Chicago The Chicago area has a fog climatology complicated by the frequent occurrence of marine fog originating on Lake Michigan that reaches its maximum intensity in late winter and early spring. The intensity and frequency of this marine fog decreases with increasing distance from Lake Michigan. For a year long period from September 2001 through August 2002, Terabeam collected standard meteorological data including measurements of visibility, temperature and dew point in the Chicago Loop using a Vaisala PWD11 Present Weather Detector. The instrumentation was sited on a building top roughly 95 m above lake level and at a similar distance from Lake Michigan as two other reporting stations selected for comparison – Waukegan, Illinois and Milwaukee, Wisconsin. Waukegan is located in a more rural area north of Chicago. The Milwaukee reporting station is located well to the south of that city's urban core. Data collection was interrupted from 14 November through 5 December, and data for those dates are missing. Visibility less than 600 meters at the Chicago downtown location occurred at less than 15% of the frequency it occurred at the more rural Milwaukee and Waukegan locations. A look at temperatures reveals the downtown location to be notably warmer than surrounding reporting stations. In January, downtown averaged 2.4C warmer during the day and 2.8C warmer at night. In July, the city averaged .6C warmer during the day and 3.3C warmer at night. The largest difference was seen in the first 13 days of November where the downtown site reported average minimum temperatures 5.4C warmer than the surrounding reporting stations. Average daytime high temperatures were 1.6C higher at the downtown site. Tables 1, 2 and 3 show the seasonal variation in high temperatures, low temperatures and visibilities
