This thesis work addresses the effect of trees on the urban energy balance and examines two methods for accounting for their impact in micro-climate modeling. The primary objective was to explore the ways that trees influence the exchange of radiation within the urban environment, namely the obstruction of the sky to nocturnal longwave radiative cooling and the interception of direct short-wave solar radiation. In pursuit of this goal, this study employed computational methods involving high resolution digital models of the city of Vienna, Austria. They were used to calculate continuous sky view factor (SVF) maps and estimate global solar radiation in urban street canyons. Both methods were compared to on-site measurements for validation. Additionally, the strength of the relationship between SVF and urban heat island intensity (UHII) was also explored. In order to achieve this, 5 urban study areas and one rural reference area were selected. Each study area was centered around a longterm weather station. In each area it was necessary to distill a single area mean value of SVF from the continuous SVF maps to compare against the UHII value. Several methods for area mean sampling were compared in view of the strength of their relationship to UHII.