Namely, extreme heat and stubborn pockets of high pressure and hot air colloquially called heat domes. During a heat dome, hot air is trapped over an area of land for an extended period of time, similar to how a lid on a boiling pot of water keeps the heat inside of the vessel. Having such high temperatures over one area for a long strength of time causes dangerous and extreme heat. The old moniker “it’s not the heat, it’s the humidity” holds true with heat domes as well. “Usually after two or three days of extended extreme heat, heat illnesses really start to set in, because the body hasn’t had a chance in 72 hours to cool off,” says Kelsey.