The variability of urban canyon energy budgets is investigated using a numerical simulation of urban canyon energy exchange and climate. The simulation scheme consists of three coupled model subcomponents: (i) a surface energy budget model, (ii) a windfield model based on the "k-epsilon" turbulence model, and (iii) a scalar dispersion model for heat. The model is two-dimensional, for dry surfaces and is most appropriate for conditions under which forced convection dominates over buoyancy effects. Energy budgets are evaluated for a cross-canyon "slice" at the midpoint of its length, and energy budget characteristics are illustrated and discussed both for the individual canyon facets and for the total canyon system, expressed relative to the equivalent flux densities through the canyon top. Net radiation, sensible heat flux and the conductive heat exchange with the substrate are compared with different ambient weather conditions and with different canyon aspect ratios, materials and orientation.