Introduction

In Chapter 4, we examined the possible effects of particulate and gaseous emissions on precipitation and weather on the regional scale in a general sense rather than specific urban-induced changes. In this chapter, we examine the evidence suggesting that pollutants as well as other urban effects are causing changes in the weather and climate in and immediately surrounding urban areas.

There is considerable evidence which suggests that major urban areas are causing changes in surface rainfall, increased occurrences of severe weather, especially hailfalls, and alterations to surface temperatures (Ashworth, 1929; Kratzer, 1956; Landsberg, 1956, 1970; Changnon, 1968, 1981a; Changnon and Huff, 1977, 1986; Hjelmfelt, 1980; Oke, 1987). Some of the hypothesized causes of those changes include:

• urban increases in CCN concentrations and spectra, and IN concentrations;

• changes in surface roughness and low-level convergence;

• changes in the atmospheric boundary layer and low-level convergence caused by urban heating and land-use changes; and

• addition of moisture from industrial sources.

A major cooperative experiment was carried out in the St. Louis, Missouri area in the 1970s to identify urban-induced changes in weather and climate and to identify the primary causes of those changes. A comprehensive review and summary of the experiment and its results are described in the monograph METROMEX: A Review and Summary (Changnon, 1981b). In this section we draw heavily on those findings to discuss the potential mechanisms causing urban-induced changes in weather and climate.