Temperature records from Phoenix Sky Harbor International Airport indicate that Arizona's summers are becoming hotter and longer. Data shows that overnight temperatures are rising faster than daytime highs, with the number of days exceeding 100 degrees increasing from an average of 94 days per year to 111 days over the past 30 years. In 2020, Phoenix recorded a record 145 days at or above 100 degrees. The average number of days reaching 110 degrees has also increased from 13 to 21 days annually, with 2024 experiencing 70 such days.
Scientists attribute these changes to global climate change and the urban heat island effect, where urban development retains heat. The average summer overnight low temperature has risen from approximately 71.5 degrees between 1896 and 1960 to about 78 degrees today, with nights above 85 degrees becoming more common. In July 2023, Phoenix set a record for the warmest overnight low at 97 degrees.
The warming trend is not limited to summer; winter temperatures have also increased, making hard freezes less common. Health experts warn that elevated overnight temperatures hinder the body's ability to cool down, raising the risk of heat-related illnesses, particularly for vulnerable populations. The combination of climate change and urban growth is altering Arizona's temperature patterns, leading to more extreme heat and fewer opportunities for recovery during heat waves.