Doubt exists among atmospheric scientists about current estimates of local and regional effects of urbanization on climate, but not generally about the existence of these urban effects. This paper presents a framework for discussion of various estimators, uses the framework to make the case for a particular estimator, and then uses the framework to examine possible shortcomings of other estimators which appear in the literature. The measure recommended consists of differences between observations, from urban and pre-urban periods, first stratified by synoptic weather type. The measures whose shortcomings are examined are 1) contemporaneous urban-rural differences, 2) contemporaneous upwind-downwind differences, 3) contemporaneous urban-regional ratios, 4) time trends of differences and ratios and 5) contemporaneous weekday-weekend differences. The paper is designated as a “problem analysis” because its goal is general facilitation of discussion about the problem of empirical estimation of urban effects on climate.

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