Precipitation Modification By Major Urban Areas

Abstract

Historical weather records at eight American urban areas of varying size, type, and climate were studied for indications of inadvertent precipitation modification. The six largest cities all had experienced warm seasonal rainfall increases of 9 to 17% during the 1955-70 period. The increases in the Midwest cities occurred largely with cold frontal systems, but in the coastal cities they were largely during air mass (non-frontal) conditions. The Midwest increases also were found to occur as enhancement, not initiation, of moderate to heavy rain days. Significant increases in summer thunder-day frequencies (13 to 41%) and hail-day frequencies (90 to 450%) were found at the six largest cities, and the increases occurred largely in the morning hours. The typical locations of maxima in the Midwest cities were thunder over and near the city, and rain and hail 25 to 55 km downwind. The maxima of all events in coastal cities were in or near the city. Overall, the results suggest that urban precipitation enhancement is related to city size, industrial nuclei generation, and urban thermal effects. The alterations have considerable relevance to urban design, local area forecasting, local water supplies, agricultural production, hydrologic design, and to planned weather modification.

Urban effect on the characteristics of cloud-to-groundlightning over Belo Horizonte-Brazil

“…In general, they have attributed the effect to the urban heat island circulation, even though a possible role of air pollution has been suggested. Several authors have found evidence of urban effects on the local weather phenomena (Myrup, 1969;Braham, 1974;Huff and Changnon, 1973;Semonin and Changnon, 1974;Changnon et al, 1981;Landsberg, 1981;Karl et. al., 1988, Balling andIdso, 1989).…”

Some variations of the rainfall in Mexico City from 1954 to 1988 and their statistical significance

“…Those changes cause the phenomenon known as "heat island" and associated effects (Jáuregui, 1997(Jáuregui, , 2000(Jáuregui, , 2004. According to Huff and Changnon (1973), the heat island effect has increased convective precipitation. In large urban areas, as in the Federal District, changes in the albedo of the surface may result in a warmer surface, thus a more unstable atmosphere and adequate conditions for intense storms to occur.…”

Enhancement and Initiation of a Cumulus by a Heat Island

“…Many climatological studies have shown increases in precipitation and aggravation of convective storm activity (Huff and Changnon, 1973;Schickedanz, 1974;Changnon, 1978 and others), though there have been a few investigations that questioned the nature and the magnitude of urban effects on precipitation (e.g., Pittock, 1977). Another urban anomaly which is most noticeable is that a city has a higher temperature than its suburbs; this phenomenon is called a heat island.…”