To study spatio-temporal characteristics of urban heat island effect of Xi′an City

the study inverted brightness temperatures of three time periods based on three Landsat TM/ETM+images of 1990

2002 and 2007

which were then normalized and scaled into six grades to calculate urban-heat-island ratio index (URI).The impacts of vegetation on urban heat island were further discussed through the relationship between NDVI values and urban heat islands.The results show that URI increased and the effects of urban heat island were strengthened in Xi′an City from 1990to 2007.During this period

the urban heat islands evolved from scattered spots to flakes

and then to connected flakes

and the distribution of urban heat island approached from the districts of Lianhu

Xincheng

Yanta

Beilin to the districts of Weiyang and Baqiao. The NDVI values were negatively correlated with urban heat island.Appropriately increasing urban green areas might be able to reduce significantly the urban heat island effect