METROMEX, a field project designed and now in progress at St. Louis, involves 4 research groups planning and working cooperatively to study inadvertent weather modification by urban-industrial effects, and, in particular , man-made changes of precipitation. Urban areas affect most forms of weather and some, such as winds, temperature, and visibility, are obvious and their changes are easily measured. Inadvertent precipitation changes are harder to measure, and except for the well-documented La Porte anomaly, urban-related rain changes have had only limited study. Examination of historical data at St. Louis has revealed summer increases in the immediate downwind area of: 1) rainfall (10-17%); 2) moderate rain days (11-23%); 3) heavy rainstorms (80%); 4) thunderstorms (21%); and 5) hailstorms (30%). METROMEX field measurements in the summer of 1971 involved 220 raingages and hailpads, 3 radar sets, 70 rainwater collectors, 14 pibal stations, 4 meteorological aircraft, unique atmospheric tracers, and a wide variety of standard and unusual meteorological equipment. These measurement tools were used to provide information on 1) the processes of cloud and precipitation formation, 2) the chemistry of aerosols and rain-water, 3) the urban heat budget, 4) the 3-D patterns of precipitation elements, and 5) the airflow and cloud development for numerical models.

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Concern about the apparent increase in the acidity of rainfall from the 1950s to the 1970s prompted reexamination of data from the intermittent, short-term sampling networks that are the basis of the trend estimates. A reassessment of precipitation chemistry data for the mid-1950s reveals excessively high values of calcium and magnesium in comparison with current measurements. The most likely explanation is the severe drought and duststorms that much of the United States experienced in the 1950s. When these excess soil loadings are adjusted within reason to nondrought conditions, newly calculated pH values for this period are not much different from those in recent years. These results suggest that the downward pH trend due to the increase in acid-forming emissions since the mid-1950s is much smaller than previously estimated. must be compared with data obtained at another time. Obviously, the individual measurements must be comparable in quality for a valid comparison. Preferably, identical instrumentation for sample collection and identical chemical analytical methods should be employed throughout the period over which the trend is to be calculated. A number of advances have been made in precipitation sampling, handling, and analysis methods since the 1953-56 data were acquired. A factor of major importance, because of the relatively short existence of sampling networks and stations, is the climatic representativeness of the periods of sampling. All of these variables will be considered to present an interpretation of changing precipitation quality in the eastern United States over the past 25 years.