Flagstaff’s climate type on the Köppen climate classification system is variously reported as a warm dry-summer Mediterranean climate (Csb), a Hemiboreal climate (Dsb and Dfb), and a cold semi-arid climate (BSk). It is consistently described as “semi-arid”. Flagstaff’s Köppen type is recorded as Dsb in the city center, with areas of BSk, Csb, Csa (hot-summer Mediterranean), Cwa (humid subtropical), and Dsa (humid hot summer) on the outskirts and bordering the city; it is mostly Dsb, BSk, and Csb.
Northern Arizona experiences a summer monsoon season from July to September, with Flagstaff’s wettest months being July and August, and its driest being June, all in the summer; Mediterranean climates have wet season only in the winter. Mediterranean climate categorization does not consider snowfall. Semi-arid climates will receive 10–20 inches (250–510 mm) of annual rainfall, while Flagstaff experiences more.
Flagstaff’s hardiness zone is mostly 6a, with some areas 5b, meaning plants withstand temperatures down to −15 °F (−26 °C). It is in the Transition life zone; the concept of life zones was first observed in the Flagstaff area. Wind in Flagstaff typically blows southwesterly throughout the year, based on topographical features. The city’s climate data is observed from Flagstaff Pulliam Airport.
The city receives precipitation every year, with two distinct wet periods in the summer and winter; the summer monsoon season accounts for 34% of annual rainfall, with the winter producing 28%. The summer monsoon season, originating from the Mexican monsoon period, is also wetter than winter, with an average 7 inches (180 mm) compared to the winter’s 6 inches (150 mm). Before the summer monsoon each year there is a dry period in May and June. Long-term average precipitation is 21.6 inches (550 mm) annually, with much heavier rainfall attributed to El Niño events. Comparatively, La Niña events have caused below-average rainfall.
Since 1996 the city has been experiencing its driest period, known as the Early-21st Century Drought. Richard Hereford of USGS speculates that the effects of the drought, predicted to last until the late 2020s, may be severe because of Flagstaff’s growing population and global warming. The summer wet season is more reliable and consistent than other times of year, but due to the high temperatures of the area this rainfall is quickly lost to evaporation. During the Early-21st Century Drought, rainfall has been consistently below average in all but the summer season, with temperature averages for all but the summer (which has remained consistent) also increasing.
Equally, snowfall has been lower during the extended dry period, though the city set a new record for its highest daily snowfall on February 21, 2019, with 35.9 inches (910 mm) and was still ranked as the United States’ third-snowiest city in 2020 (based on 2018–19 National Oceanic and Atmospheric Administration data). Flagstaff has consistently been among the snowiest cities in the United States, and snow and winter culture is argued by Michael Weeks to be a large part of Flagstaff’s identity. The Arizona Snowbowl is a major attraction, though has had to make artificial snow during warmer seasons, and the city tried to launch a bid to be the host city of the 1960 Winter Olympics. The maximum daily snow cover was 83 inches (210 cm) on December 20, 1967, although the mean maximum for a full winter is only 20 inches (510 mm). However, due to the infrequent and scattered nature of the snowstorms, persistent snowpack into spring is rare. One notable exception occurred during the severe winter of 1915–16, when successive Pacific storms buried the city under over 70 inches (178 cm) of snow, and some residents were snowbound in their homes for several days.
Though one of the least-sunny cities in Arizona, Flagstaff still ranks among the United States’ sunniest cities, having sunshine for an average 78% of the year. The city receives much more sunshine than other snowy cities, which are primarily in the north of the country.
There are four seasons in Flagstaff, with cool winter temperatures averaging 45 °F (7 °C) and warm summer temperatures averaging 80 °F (27 °C), pleasant compared to the rest of Arizona; the average annual snowfall is 97 inches (250 cm). Spring begins in April with pleasant weather. Sometimes, snow reappears in May, but the spring period is typically mild and dry, lasting until early June. The summer is measured between days when freezing temperatures occur, beginning in June – the last freezing temperature generally in early June – and ending in September. Flagstaff’s summer will receive a few days of daytime temperatures above 90 °F (32 °C), with cooler nighttime temperatures typically in the region of 40 °F (4 °C). Flagstaff’s summers are also notable for the monsoon season in July and August, when thunderstorms occur almost daily. Thunderstorm activity happens mostly during the daytime.
Freezing temperatures (below 32 °F (0 °C)) return towards the end of September, with the mild fall season having daytime temperatures around 60 °F (16 °C). With its many trees, leaves do change color in Flagstaff’s fall, with the change starting at the end of September and happening throughout October. Fall lasts only until the snow comes in November, with winter marked between periods of snowfall, typically from November until mid-April at the latest. Temperatures in winter are usually below freezing, going no higher than around 40 °F (4 °C) in the day, even in sunshine. A combination of snow cover and high pressure occurring during winter months will cause the temperature to drop further, once reaching a record low of −30 °F (−34 °C). Flagstaff’s winter wet season is caused by Pacific storms and lasts from November through April.