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The presence or scarcity of vegetation is an essential factor in determining how much urban areas heat up, according to a NASA study Using data from multiple satellites, the researchers found that areas covered in part by impervious surfaces such as asphalt, concrete, and steel had an average summer temperature 3.4 degrees F higher than nearby rural areas. The highest U.S. urban temperatures compared to surrounding areas were along the Interstate-95 corridor from Boston to Washington and around Atlanta and the I-85 corridor in the Southeast. In desert cities such as Phoenix, the urban area was actually cooler because irrigated lawns and trees provide cooling that dry, rocky areas do not, the researchers explain. The urban heat island effect occurs primarily during the day, when impervious surfaces in cities absorb more sunlight than surrounding vegetated areas.