Over the past thirty years there has been increasing interest in what have been called urban heat island ( UHI ) .
It is only since 2007 that thought has been given to the rising columns of warm air , or ' thermal plumes ' that they produce .
Over the past thirty years there has been increasing interest in what have been called urban heat island ( UHI ) , but it is only since 2007 that thought has been given to the rising columns of warm air , or ' thermal plumes ' that they produce .