From a January 2004 study of Urban Heat Island (UHI) in the Midwest
(based on 56 years of data at climate stations), average UHI per year
was .002 C per year or 0.0037 F per year.
From a July 2006 study of UHI at Minneapolis MN (based on 115 years of
July average minimum and maximum temperatures at Minneapolis and at
Farmington MN), UHI had a large effect on daily minimum temperatures
but little or no effect on daily maximum temperatures. Data plots are
at 1. UHI Urb: