Urban heat islands — microclimates within a highly developed city — can be up to 20 degrees warmer than surrounding areas that are more rural, posing a serious threat to the environment, public health and quality of life. Heat islands develop when green spaces are replaced with roads, parking lots and large buildings that retain more heat during the day than natural landscapes. One of the best defenses against the effects of urban heat islands are trees, which can cool the environment, filter the air and improve air circulation.