What is a heat dome? Behind the hot and humid conditions

Dangerously hot and humid conditions are choking nearly the entire eastern half of the United States this week, caused by a “heat dome” that's parked over parts of the country

Dangerously hot and humid conditions are choking nearly the entire eastern half of the United States this week, caused by a “heat dome” that's parked over parts of the country.

Heat alerts were in effect Wednesday for around 150 million people. Temperatures over 95 degrees Fahrenheit were widespread across the Ohio Valley and through the East Coast. “Extremely dangerous heat” is expected to persist through Thursday, according to the National Weather Service.

But what is a heat dome, and why does the phenomenon bring oppressive heat with it?

A heat dome occurs when a strong and persistent region of high pressure sits over land and traps hot air, like a lid on a pot.

Heat domes are usually affected by the behavior of the jet stream, a ribbon of fast-moving air high in the atmosphere that flows from west to east in the Northern Hemisphere and affects weather systems.