How anomalous is this week's extreme heat?

More than 150 million Americans have been under heat advisories and warnings this week — about 45% of the country’s population.

Extreme heat is typically defined by two or more days of anomalously high temperatures for a given area. For example, 100°F may not be extreme for the people and infrastructure of southwest Texas, but those living in Maine, a state far less equipped to handle the heat, 100°F could lead to energy system failures and mass casualties.

My slice of Maryland — just north of Washington DC — has only seen temperatures this high in June once in the last 75 years.