Phoenix has a hot desert climate (Köppen: BWh), which is common in the Sonoran Desert. It is the largest city in the Americas with this type of climate. Phoenix experiences very hot summers and short, mild winters. The city is located in one of the sunniest regions on Earth, with sunshine levels similar to the Sahara Desert. Each year, Phoenix receives about 3,872 hours of bright sunshine, more than any other major city. Summer high temperatures in Phoenix are the highest of any major city in the United States. On average, there are 111 days each year when the temperature reaches at least 100 °F (38 °C), mostly from late May through late September. High temperatures exceed 110 °F (43 °C) on about 21 days annually. The highest recorded temperature in Phoenix was 122 °F (50 °C) on June 26, 1990.
In 2024, Phoenix-Mesa, AZ, was ranked fifth in the United States for the most ozone pollution, according to the American Lung Association. Vehicle emissions contribute to the formation of ozone. Phoenix also has high levels of particulate pollution, though cities in California have higher levels of this type of pollution. PM2.5 and PM10 particles, which come from sources like diesel exhaust and dust, can reach concerning levels in Phoenix. Exposure to high amounts of PM10 dust, often from dust storms or construction sites, can increase the risk of Valley Fever, a fungal lung infection.
Unlike many desert areas, where temperatures change greatly between day and night, Phoenix’s urban heat island effect reduces daily temperature differences. As the city has grown, summer nighttime temperatures have risen. Pavement, buildings, and sidewalks absorb heat during the day and release it at night. On average, summer nighttime temperatures stay at or above 80 °F (27 °C) for about 74 days each year. On July 19, 2023, Phoenix recorded its warmest daily low temperature of 97 °F (36 °C).
Phoenix receives about 300 days of sunshine each year, or over 85% of daylight hours. Rainfall is very low, with an average annual total of 7.22 inches (183 mm) at Phoenix Sky Harbor International Airport. The dry, sunny weather is occasionally interrupted by winter storms from the Pacific Ocean and summer monsoons. Historically, the monsoon season began when the average dew point reached 55 °F (13 °C) for three days in a row, usually in early July. However, the National Weather Service changed the official start date to June 15 in 2008, and the season ends on September 30. During the monsoon, humidity increases, and heavy rain, flash floods, hail, strong winds, and dust storms—sometimes called haboobs—can occur.
August is the wettest month, with an average of 0.93 inches (24 mm) of rain, while June is the driest, with only 0.02 inches (0.51 mm) of rain. On September 8, 2014, Phoenix recorded its highest rainfall of 3.30 inches (84 mm), breaking a 75-year-old record. This storm was caused by the remnants of Hurricane Norbert, which flooded major roads and stranded many drivers. Dew points in Phoenix range from 31 °F (−1 °C) in April to 58 °F (14 °C) in August. Occasionally, dew points can drop as low as 0 °F (−18 °C) or rise above 70 °F (21 °C) during monsoon seasons, creating humid conditions.
Desert areas around Phoenix are increasingly at risk for wildfires. Fire risk is highest during dry spring and summer months after wet winters, when invasive plants like buffelgrass grow densely. Difficult terrain often complicates firefighting efforts. Many desert plants are not adapted to fire, making wildfires a serious threat to the local ecosystem.
The lowest annual temperature in Phoenix is usually in the mid-to-low 30s. It rarely drops below 32 °F (0 °C), which happened only 17 times between 1991 and 2020. Some parts of the Phoenix area may experience frost in winter. The earliest recorded freeze was on November 4, 1956, and the latest was on March 31, 1987. The lowest temperature ever recorded in Phoenix was 16 °F (−9 °C) on January 7, 1913, and the coldest daily high was 36 °F (2 °C) on December 10, 1898. The longest period without frost in Phoenix lasted nearly eight years, from December 27, 1990, to December 23, 1998.
Snow is rare in Phoenix. The first recorded snowfall was in 1898, and snow accumulations of 0.1 inches (0.25 cm) or more have occurred only eight times within the city. The heaviest snowstorm on record was in January 1937, when 1 to 4 inches (2.5 to 10.2 cm) of snow fell in parts of the city and took three days to melt. On December 6, 1998, snow fell in the northwest part of Phoenix, and Sky Harbor Airport reported a light dusting. On February 21–22, 2019, northern and northeastern areas of the metro region received several inches of snow, while Sky Harbor reported record rainfall. A similar storm occurred on March 1, 2023. Between 2010 and 2021, Phoenix experienced three major winter storms with snow-like graupel.