As the hot, hazy days of July melt into August, you’ll hear about a major contributor: The Bermuda High. This is a semi-permanent area of high pressure in the North Atlantic. It shifts east or west depending on the season and the central pressure. As this high pressure system shifts west during the late summer, it funnels hot humid tropical air north over the United States. The further west the high pressure shifts, the further west the flow of the tropical air. This, combined with the shift of the jet stream to northern Canada, leads a stagnant weather pattern where one hot humid day follows another.
Mary Knapp, Weather Data Library