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Recent rapid growth and expansion of urbanized areas in Phoenix, Arizona has brought an increase in the effects of the urban heat island (UHI) and lowered comfort levels in outdoor spaces within the city, especially at night. This study investigates outdoor human comfort, in relation to the UHI, utilizing output from the microclimate model ENVI-met. Outdoor comfort levels are evaluated during different seasons of the year utilizing ENVI-met at three unique landscapes within central Phoenix. Analysis of modeling output focused on mean radiant temperature and predicted mean vote outcomes at key points within each of the three areas. Results indicate that areas in downtown during the summer are more comfortable during the early part of the day (cool island) and less comfortable than surrounding areas from mid-afternoon into the evening hours (heat island). In contrast, areas outside of downtown, in open agricultural fields, have higher discomfort during the daytime and higher levels of comfort in the evening hours. During cooler times of the year, heat retention within downtown raises comfort levels during the late day and early evening, as opposed to heat release in open and exposed areas away from downtown. In Phoenix, model results confirm that increasing the amount of shade trees (and shade structures), as well as increasing the amount of vegetation and permeable surfaces can dramatically decrease heat and increase outdoor human comfort. These results confirm findings from similar studies in cities located in hot and arid environments. (C) 2013 Elsevier Ltd. All rights reserved.