The combination of green roofs and green walls can lower ambient temperatures in typical "urban canyon"-like streets, achieving up to a 12.8˚C (23˚F) ambient temperature difference in an arid climate, and 8.4˚C (15.1˚F) in a humid climate. This combination had the greatest effect compared to no vegetation, green roofs only, and green walls (walls covered in vegetation, such as climbing vines) only. Green roofs alone had the second greatest ambient temperature effect, likely due to roofs' higher exposure to sunlight.
Alexandri, Eleftheria, and Phil Jones. (2008) Temperature decreases in an urban canyon due to green walls and green roofs in diverse climates. Building and Environment 43(4), 480-493.