The radiant environment in open spaces is very sensitive to the surrounding built form, which determines their openness to the sky and exposure to the sun. This paper presents the analysis of 132 urban forms in London and Paris, two cities at similar geographical latitude, but of different urban geometry, focusing on the relationship between urban geometry and insolation of open spaces at neighbourhood scale. The method consists of three stages: (1) the geometric analysis of the urban forms, (2) their solar access analysis and (3) the statistical exploration of the results. Special emphasis is on the sky view factor (SVF), which is employed as an integrated geometry variable and environmental performance indicator. The comparative analysis of the two cities underlines the significance of urban layout for modifying the outdoor radiant environment, and reveals temporal characteristics of the relation between urban geometry and insolation of urban forms, induced by the varying solar geometry. Indicatively, the average mean ground SVF (mSVF) was found to be primarily affected by the quantitative characteristics of the open space, and able to predict average daytime insolation on March 21 and June 21 (R²> 0.8), in both cities.