Arizona-Sonora Desert Museum Digital Library

This northeastern subdivision is the highest and coldest part of the Sonoran Desert. Located in south-central Arizona and northern Sonora, the terrain contains numerous mountain ranges, and valleys narrower than those of the Lower Colorado River Valley subdivision. Trees are common on rocky slopes as well as drainageways, and saguaros grow on slopes above the cold valley floors. This community is also called the "saguaro-palo verde forest." It is the only subdivision that experiences frequent hard winter frosts, so many species of the lower elevation and more southerly subdivisions cannot survive here. Nevertheless it is a rich area. The small range that is the Desert Museum's home, the Tucson Mountains, has about 630 taxonomically distinct kinds of plants. This richness is partly explained by the two equal rainy seasons which total twelve inches (305 mm) per year on average. The hilly terrain provides a multitude of microhabitats on north and south slopes and deep, shaded canyons. The proximity to chaparral, woodland, and grassland communities contributes still more species to the flora.

Biologists are increasingly concluding that the Arizona Upland's climate, vegetation density, and biodiversity resemble thornscrub more than desert. Don't be surprised if this subdivision is reclassified in the near future.

Tucson is the only major city located in Arizona Upland, although much of metro Phoenix's parks and land above 2000 feet (600 m) in elevation share its characteristics. Residents who have moved to this area from temperate climates often complain about the lack of seasons. Actually Arizona Upland has five seasons which, though more subtle than the traditional temperate four, are distinct if one learns what to look for.

The following description is for Tucson, but is fairly applicable to the rest of Arizona Upland and to the eastern one-half of the Lower Colorado River Valley subdivision as well. The seasons are a little later at higher latitudes and elevations, earlier at lower ones. The monsoon is later and more sporadic farther west; in some years it fails to reach the Colorado River.

Summer monsoon or summer rainy season (early July to mid-September)

In local native tradition, the year begins with the most dramatic weather event of the region — the often abrupt arrival of the summer rains. A tropical air mass brings humidity and moderates the temperatures from June's extremes; frequent thunderstorms occur; this is the main growing season for many of the larger shrubs and trees. (Monsoon is derived from an Arabic word for "season," and was applied to a wind that changes directions seasonally. Be aware that it does not refer to rain or storms per se, but rather to the shift of wind direction which brings moist air that can generate storms — in our case, a southerly wind in July. The word is often misused, even by some weather reporters.)

A sixth season, late summer, lasting from mid August through September, is sometimes added; this is a hot and dry period after the monsoon ends — nonexistent in some years.

Autumn (October & November)

Warm temperatures; low humidity; little rain; few species in flower, but the growing season for winter annuals begins if there is enough rain. Late summer and autumn occasionally receive heavy rains from the remains of Pacific hurricanes (tropical storms).

Winter (December & January; sometimes February)

Mostly sunny, mild days, with intermittent storms that bring wind, rain, and cool-to-cold temperatures; February often warm and dry, more spring-like.

Spring (early to late February through April)

Mild temperatures; little rain; often windy; one of two flowering seasons; winter annuals may start blooming in February in warm, wet years.

Foresummer (May & June)

High temperatures; very low humidity; no rain in most years; May is very warm and often windy; June is hot and usually calm. There is little biological activity except for the flowering and fruiting of saguaro and desert ironwood. Most plants and many animals are dormant until the rains arrive.