Many inadvertent weather changes are intermittent and depend on the type of natural weather conditions existing. For example, in one summer 7 added storm days occurred over the city, whereas 16 added storm days were created in the following summer, revealing the large year-to-year variability that exists in most inadvertent weather changes. Studies of individual summer days revealed those days when the weather conditions were changed sufficiently by St.
Louis, MO, revealing a localized increase from about 16 storm days per summer in the region’s rural areas to a peak of 28 days in and east of the city. Figure 1 shows the average summer pattern of thunderstorms around St. The weather changes accidentally wrought by human activities translate into climate changes when the weather change becomes frequent and significantly different from the climate values expected in a natural, unaffected environment. Interest in and concern over inadvertent weather changes grew as the recognition of the widespread and significant magnitude of these changes developed among the citizenry and public officials. Major advances in meteorological knowledge now allow use of numerical modeling to estimate urban heat islands and other weather anomalies. Progress in detecting and understanding inadvertent weather modification came rapidly in the latter half of the twentieth century as new remote sensors, including satellites and radars, made it much easier to detect and measure conditions. The growth of meteorological knowledge about the atmosphere’s behavior, which expanded rapidly during the twentieth century, brought forward investigations of how other land use changes by humans and the emissions from factories and vehicles altered the weather. For example, by 1880 scientists in England discovered that more rainfall occurred on work days, Monday through Friday, than on nonwork days (Saturday and Sunday), a result correctly assessed as an effect of industrial pollutants. The industrial revolution began in the nineteenth century and led to many more weather changes.
Serious studies of urban effects on weather developed in Europe in the nineteenth century and intensified as networks of surface weather stations yielded data that allowed scientists to define the patterns of temperature, humidity, and precipitation in and around cities. Knowledge that humans were altering the weather became commonplace about 200 years ago as residents of many of Europe’s larger cities recognized that urban areas had poorer visibility due to the increase in haze and smoke and higher temperatures than did their surrounding rural areas. However, many such changes in the atmosphere from human changes in the biosphere have been relatively localized, and their effects typically create weather changes occurring on scales ranging from 1 km 2 up to 10 000 km 2.
The list of accidental weather changes is almost endless. For example, major cities change every facet of their weather, including the number of thunderstorms large irrigated areas create clouds and rainfall when conditions are right and jet aircraft often create cirrus clouds. Human activities have accidentally been able to change all facets of the weather at and near the Earth’s surface. Inadvertent weather modification refers to any changes in the weather resulting from human actions done for purposes other than changing the weather. Changnon, in Encyclopedia of Atmospheric Sciences (Second Edition), 2015 Introduction