In the midst of an already record-breaking heat wave, Phoenix, Ariz., set a particularly eye-popping record: the temperature only dropped to 97 degrees Fahrenheit overnight between Tuesday and Wednesday, an all-time high for an overnight low.
When temperatures remain high at night, they place a particularly heavy burden on the body, increasing the risk of heat illness and death.
The US – and the world – has seen a wave of extreme heat so far this year, including the hottest June on Earth and the warmest week on record during the first week of July. Rising global temperatures from burning fossil fuels are the main cause of more frequent and intense heat waves. And an El Niño event is also raising global temperatures this year.
A heat dome has been in place over the southwestern US and Texas for weeks, fueling many heat records. Phoenix has now seen 20 days in a row with daytime temperatures of 110 degrees F or higher, a record streak that will likely continue for several more days. A heat dome is a high-pressure area that sometimes parks over a region. High-pressure ridges, as they are also called, are provided with sinking air, which compresses and heats up. The distinctively clear air of these ridges also causes the sun’s rays to strike the ground, further raising temperatures.
Long-term heat extremes are a major threat to public health, as heat is the number one weather-related cause of death in the US; it causes more human deaths than hurricanes, tornadoes, and floods combined. Heat can cause dehydration, which thickens the blood and makes the heart pump harder. That organ and other organs can be damaged by too much heat exposure.
The skyrocketing, triple-digit high daily temperatures make headlines, and they’re certainly a concern, but when temperatures drop into the 80s and 90s at night, the body doesn’t get a chance to cool down. This is a particular concern for those who do not have air conditioning, including unhoused populations. And heat is especially a health concern for the very young, the elderly, and those with pre-existing health conditions such as asthma and heart disease.
According to Climate Central, a nonprofit research and news organization, nighttime lows in the US are rising twice as fast as daytime highs. a 2022 Climate Central Analysis found that nighttime lows in the US have risen an average of 2.5 degrees F since 1970. In Phoenix, they are up 5.7 degrees F over the same period.
Phoenix’s all-time low record of 97 degrees F supplanted the previous record of 96 degrees F, which was set in 2003. It may not be another 20 years before the new record is surpassed, as climate change continues to make summers hotter.
A sign of how remarkable the recent record was came from the response the National Weather Service’s office in Flagstaff, Ariz., sent on Twitter to the bureau’s Phoenix office. “So you’re saying your all-time record warm low temperature is our all-time record high temperature?” the read tweets. “That’s just nasty.”
