Arizona's climate varies widely by altitude. Southern Arizona, including Phoenix and Tucson, has very hot summers and mild winters, with cooler summertime temperatures in the Chiricahua Mountains in the southeast corner of the state. Best times to visit are fall, late winter and early spring or higher elevations in summer. Northern Arizona, including Flagstaff, Sedona and the Grand Canyon, has four seasons but weather is mild enough to visit any time.
Arizona covers 113,909 square miles, with about 350 square miles of water surface. The state has three main topographical areas: (1) a high plateau averaging between 5,000 and 7,000 feet in elevation in the northeast; (2) a mountainous region oriented southeast to northwest with maximum elevations between 9,000 and 12,000 feet; and (3) low mountain ranges and desert valleys in the southwestern portion of the state.
From the White Mountain area across the Mogollon Rim to the San Francisco Peaks lies an unbroken stand of Ponderosa Pine. The Kaibab Plateau north of the Grand Canyon continues this timbered strip into southern Utah . The highest point in the state is Humphreys Peak , located northwest of Flagstaff , with an elevation of 12,611 feet. Baldy Peak , in the White Mountains of eastern Arizona , is the second highest in the state with an elevation of 11,490 feet. The desert valleys of southwestern Arizona are an extension of the Sonora Desert of Mexico, with elevations as low as about 100 feet above sea level in the Lower Colorado River Valley .
Cold air masses from Canada sometimes penetrate into the state, bringing temperatures well below zero in the high plateau and mountainous regions of central and northern Arizona . The lowest readings can dip to 35 degrees F below zero in the Desert. High temperatures near or above 100 degrees F are common throughout the summer months at the lower elevations. Great extremes occur between day and night temperatures throughout Arizona . The daily range between maximum and minimum temperatures sometimes runs as much as 40 to 50 degrees F during the drier portions of the year. During winter months, daytime temperatures may average 70 degrees F, with night temperatures often falling to freezing or slightly below in the lower desert valleys. In the summer the pine-clad forests in the central part of the state may have afternoon temperatures of 80 degrees F, while night temperatures drop to 35 or 40 degrees F.
Precipitation throughout Arizona is governed to a great extent by elevation and the season of the year. The higher elevations of the state, running diagonally from the southeast to the northwest, average between 25 and 30 inches of precipitation (rain plus melted snow) annually, while the desert southwest averages as low as three or four inches per year. The plateau country in the northeastern corner of the state receives approximately 10 inches of precipitation annually.
From November through March, winter storms occur frequently in the higher mountains of the central and northern parts of the state and sometimes bring heavy snows. Summer rainfall begins early in July and usually lasts until mid- September. Usually occurring in the form of thunderstorms, summer rains are often accompanied by strong winds and brief periods of blowing dust prior to the onset.
The average number of days with measurable precipitation per year varies from near 70 in the Flagstaff area to 15 at Yuma . The air is generally dry and clear, with low relative humidity and a high percentage of sunshine. April, May and June are the months with the greatest number of clear days, while July and August, as well as December, January and February have the cloudiest weather and lowest percent of possible sunshine (but with more than 300 days a year of sunshine, they are few and far between). Humidity, while low when compared to most other states, is higher throughout much of Arizona during July and August, which is the thunderstorm season.
During any season in Arizona , some part of the state enjoys near-perfect weather.
Generally, the “peak,” or busiest, season in the desert areas (southern half of the state) lasts from January through March; the next most popular “shoulder” season is from April to May and September through December; and the season when visitors can find the greatest values is June through August. Cooler temperatures may be enjoyed at higher elevations in southeast Arizona.
Peak and value seasons in the mountainous regions (in the northern half of the state) are the opposite of the desert areas. Generally, peak season is from June through August, shoulder season is April to May and September through December, and value season is January through March.
Peak season in some mid-climate areas of the state, such as Sedona, is from March to May and September through October, with shoulder season from January to February.
Depending on the time of year and where one plans to travel in Arizona, anything from a swimsuit to a winter jacket may be appropriate. Overall though, dress is fairly casual in Arizona, with an emphasis on comfort. Few restaurants require jackets and ties for dining, so throughout the year just about anything goes. Jeans and western wear are always in style. If you plan on attending a cultural event or dining at a fine restaurant, it is advisable to bring along more formal outfits.
During the summer throughout the state, shorts and sandals are appropriate during the day and a sweater or light jacket is perfect in the higher elevations for the evenings. In the cooler months, a sweater or light jacket is perfect in the desert areas for evenings and a winter jacket is recommended in the ligher elevations.
Also, wide-brim hats, sunglasses and sunscreen are recommended year-round.