Thermal diversity in open spaces is deemed highly desirable as it can enhance humans’ thermal experience and thus, the use of the urban space. This study explores the occurrence of spatial thermal diversity in open spaces in London and Paris, focusing on two parameters affecting the thermal environment. The method employed is based on the mapping of combined availability of sun and wind using DEMs and image processing techniques. The aim is to contribute to establishing a methodology for the assessment of thermal diversity, and to examine the relationship between this and the urban density and coverage variables. A mathematical formula for quantifying thermal diversity is proposed, and used for computing average and instantaneous values for 132 urban forms. The relationship of thermal diversity with the two variables is found to be statistically significant (R2>0.5), and best fit is achieved by polynomial curves. Solving the equations, the density and coverage values that maximize thermal diversity are identified for the two cities. To interpret the findings, the effect of built obstruction on the occurrence of the four microclimatic combinations considered (i.e. sunny-windy, sunny-lee, shaded-lee, shaded-windy) is examined. Lastly, examples of urban forms are discussed in relation to their geometry and performance.