Around three quarter of the European population already lives in urban areas and by 2050 this proportion isexpected to rise over 80%. Since climate change is expected to bring rising temperatures and more frequent heatwaves, mitigating the impact of extreme heat events is one of the most important issues in urban planning. Nature-Based Solutions (NBS) is a recently introduced concept in environmental research and management that promotesnature as a means to address the challenges brought about climate change. Researches of human-biometeorologydemonstrated that radiation heat load, quantified as mean radiant temperature (Tmrt), is the main source ofdaytime heat stress in summer. In European cities—especially in those with dense historic urban cores—carefullyplanned and properly maintained shade trees constitute the most effective NBS for mitigating extreme thermalconditions, while also offering several co-benefits.This study was conducted with the following two aims: (a) to assess the impact of woody vegetation anddifferent façade orientation on the radiation heat load in a complex urban setting; and (b) to evaluate the performanceof SOLWEIG, a radiation model, in reproducing the measured short- and long-wave radiation flux densities.The observation was conducted over a 26-hour long period at the rectangular Bartók Square in Szeged (Hungary)on a clear and warm late-summer day. The investigation utilized two tailor-made human-biometeorologicalstations equipped with rotatable net radiometers. The model–measurement comparison is based on data collectedfrom five locations within the square: from the center and from four sites next to the bordering façades.The measurements confirmed that on clear summer days Tmrt can reach extreme level at exposed locations(65–75C). However, shade trees are able to reduce daytime Tmrt to 30–35C. Shading SE-, S- and SW-facingfaçades and adjacent sidewalks is extremely important for the reduction of pedestrian heat stress and thus, therisk of heat stroke. When a measurement point adjacent to a facade became exposed to direct solar radiation,the radiation load increased significantly due to the additional lateral components—the reflected short-wave andthe emitted long-wave fluxes—emanating from the wall. In this respect, SOLWEIG was unable to reproducethe prolonged heat emitting effect of walls following solar exposure. Additionally, the measurement–modelcomparison revealed minor inconsistencies that originated from the model’s treatment of tree crowns: representingthem as perfectly shaped and homogeneous bodies. Due to this simplification, the observed brief penetrations ofdirect sunbeams through the canopy at most locations were not reproduced.