Canopy-layer urban heat island (UHI) magnitude, or "intensity," is traditionally defined as a simultaneous screen-level air temperature difference between “urban” and “rural” measurement sites. This definition is seemingly intuitive and clear, yet its operational use in empirical UHI literature is both confusing and inconsistent. The methodological rigor of this literature is therefore of critical concern. These problems are historically rooted in urban climatology's simplistic standard for site classification, the urban-rural dichotomy. This paper reports on progress being made toward a new and more refined approach to UHI definition and site classification. The solution, it is argued, lies in a newly developed series of standardized, local-scale “thermal climate zones.” The zones are locally homogenous in surface thermal, moisture, radiative, and aerodynamic properties, and are classified by their thermal “responsiveness” to diurnal heating and cooling cycles. Born conceptually of Oke's (2004) Urban Climate Zone (UCZ) scheme, the new assemblage offers an expanded and detailed portrayal of “city,” “agricultural,” “natural,” and “mixed-use” landscapes. The proposed scheme is universal in scope and is designed to capture the continuum of natural and built surface-types characterizing UHI magnitude in cities and regions worldwide. UHI magnitude is ultimately defined, measured, and reported more objectively through “inter-zone” temperature differences than through arbitrary “urban-rural” differences.