A heatwave that is pushing temperatures toward 120 degrees Fahrenheit (49°C) is hitting Arizona residents and travelers where it hurts: It’s just too hot to fly.

Phoenix, Arizona in the southwestern United States is used to hot temperatures. In June, the average high temperature is 104° Fahrenheit (40° Celsius), and almost every summer day tops 99°F (37°C). Summer nighttime temperatures don’t fall below 70°F (21°C). People do their errands in the early morning or in the evening. If they drive in the heat of the day, they have one eye on the engine temperature gauge. At home, they keep their windows closed and their air conditioning on. “You get used to it,” residents say, taking a certain amount of pride in their ability to deal with heat.

Sky Harbor International Airport in Phoenix announced the cancellation Tuesday, 19 June, of 20 American Eagle flights, in response to a crippling heatwave caused by an unusually strong high-pressure system, according to the National Oceanic and Atmospheric Administration. The temperature Tuesday was recorded at 118°F (48°C) at the time of cancellation. By June 20th, more than 40 flights had been canceled. By 21 June, cancelled flights totalled more than 50. The flights were scheduled to take off between 3 and 6 p.m., the hottest time of day. The excessive heat warning for south and south-central Arizona is expected to last until Sunday, 25 June.

Heat of this intensity has an impact on the ability of an airplane to take off. Flight depends on air pressure being sufficiently high below the wing and low above the wing, in effect pulling the plane into the air. The shape of the wing enables swiftly-moving air to create the difference in pressure that causes an airplane to leave the ground. However, hot air is less dense than cooler air, and in order to achieve the pressure required to lift an airplane into the air, greater speed is required – speeds that may not be achievable in the length of runway available.

According to Bianca Hernandez, a meteorologist with the National Oceanic and Atmospheric Administration, “We are used to 100 to 110 degrees, but once it gets this much above average, it poses a threat,” Hernandez says.

American Airlines released a statement explaining that the Bombardier CRJ aircraft commonly used on the airline’s shorter routes specify a maximum operating temperature threshold of 118°F (48°C), though large jets can operate at temperatures up to approximately 127°F (52°C).

The unusual heat in the southwest United States normally occurs less than once a year on average, and is considered dangerous. However, as global climate changes, heat waves like this will become more common and intense, according to the journal Nature Climate Change. The study predicts that, globally, the number of people affected by 20 days or more of intense heat (dangerous heatwaves combining temperature and humidity) will increase from the current estimate of 1 in 4 people to 3 out of 4 by 2100.

A National Academy of Science report from 2016 asserted that modern, extreme heatwaves are certainly linked to climate change. In addition to the health and environmental threats posed by climate change, disruptions to daily life, such as transportation safety, are also at risk if global warming continues unabated.

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