As streets usually cover more than a quarter of the urban area, canyon street morphology plays an important role in creating the urban climate. It directly influences the air temperature, moisture and wind flow within the streets as well as the urban surrounding area and has been the topic in several urban climatology studies. Recently, studies based on the street cluster thermal time constant (CTTC) model have been carried out by the authors with a view to assessing the thermal effects of alternative architectural designs of the flanking buildings and inner courtyards. The effect of green spaces, especially that of shade trees which plays a significant role in solar radiation penetration, has not yet been considered. In the CTTC model, passive cooling of the street by solar heating attenuation is governed mainly by the street orientation and its geometry as measured by the aspect ratio of flanking buildings height to street width. The tree shading coverage largely offsets the contribution of these two factors. Moreover, significant thermal effects are provided by the tree canopy, in addition to the direct solar radiation. Accordingly, adjustments are called for in the currently used canyon street models. The present paper discusses the geometry and orientation aspects of the canyon street climate and how these aspects are affected and can be reconciled in the presence of shade trees. Some consequences of environmental design of urban spaces and their effects on outdoor thermal comfort are also considered. (C) 2003 Elsevier Science B.V. All rights reserved.