In the midst of an already record-breaking heat wave, Phoenix, Arizona, set a particularly eye-popping record: the temperature only dropped to 97 degrees Fahrenheit overnight between Tuesday and Wednesday, setting an all-time record high for a nighttime low. When temperatures stay high overnight, they place a particularly heavy burden on the body, raising the risk of heat illness and death.
The U.S. —and the world—has seen a spate of extreme heat so far this year, including the planet's hottest-ever June and hottest week on record during the first week of July. Rising global temperatures from burning fossil fuels are the main driver of more frequent and more intense heat waves. And an El Niño event is also boosting global temperatures this year.
A heat dome has been in place for weeks over the U.S. Southwest and Texas, and it has fueled many heat records. Phoenix has now seen 20 days in a row with a daytime high of 110 degrees F or higher, a record that is likely to continue for several more days. A heat dome is an area of high pressure that parks over a region. High-pressure ridges, as they are also called, feature sinking air, which compresses and heats up. These ridges' typical clear skies also allow the sun's rays to beat down on the ground, further raising temperatures.
Prolonged heat extremes pose a major public health threat because heat is the number-one weather-related killer in the U.S.; it causes more human deaths than hurricanes, tornadoes and floods combined. Heat can cause dehydration, which leads the blood to thicken and makes the heart pump harder. That organ and others can be damaged by too much exposure to heat.
The soaring, triple-digit high daily temperatures grab the headlines, and they definitely are a concern—but when temperatures only drop into the 80s and 90s at night, the body doesn't get a chance to cool down. This is particularly a concern for those who lack air-conditioning, including unhoused populations. And heat is especially a health risk for the very young, the elderly and those with preexisting health conditions such as asthma and heart disease.
