Nine of the 10 warmest years on record for the 48 contiguous states have occurred since 1998, resulting in more intense heat waves each year.[1] In the last decade, the number of extreme heat days rose, with rates in some states five times higher than 40 years ago. Due to what’s known as the urban heat island effect, this increase is magnified in urban areas, where over 80% of Americans live.
What is extreme heat?
Extreme heat is generally defined as temperatures that are much hotter than the historical record in the region. Technical definitions vary at the county, state, and national levels.
