Cabeza Prieta Natural History Association
Climate

The climate of the Sonoran Desert in the area of Ajo falls into the lower Colorado River Valley subdivision to the west and the Arizona Upland subdivision to the east. The line of demarcation lies near the eastern border of the Cabeza Prieta National Wildlife Refuge.

The Lower Colorado River Valley subdivision stretches into southern California, and surrounds the northern end of the Gulf of California. It is among the driest and hottest places in N. America rivaled only by the Mohave Desert's Death Valley. The summer highs can reach 120° F (49° C) with surface temperatures nearing 180° F (82° C). Winter high temperatures often reach the low to mid 70's° F (18-22° C) and nights can produce frost. The region is generally dryer in the western regions where only two or three inches (5-8 cm) of rain falls in an average year. The eastern edge may have over twice as much rain. The rain here is divided between the late summer monsoon and several winter storms. The summer rains tend to be violent downpours of up to an inch (2.5 cm) or more per hour but the winter rains may occur in gentle storms of an inch (2.5 cm) or less over a day or more. The land receives almost uninterrupted daylight sun which is unrelenting in the summer heat and offers a pleasant warmth in the winter. Sunscreen, a hat and long sleeves are a necessity year round.

The Arizona Upland climatic subdivision reaches east beyond Tucson and far down into Mexico ending north of Hermosillo. Summer temperatures can reach 100° F (39°C) and frost and even snow can form in winter. The low winter temperatures fall into the teens. This is the highest and coolest part of the Sonoran Desert covering numerous mountain ranges. It is characterized by five seasons. Like the Lower Colorado River Valley subdivision there is a summer monsoon, the local natives' traditional beginning (July to mid September), when an abrupt rainy season breaks the dry heat. The rains often come as thunderstorms and produce the major growing season for trees and larger shrubs. The name monsoon does not refer to any one storm but to the onset of the southern wind which can bring those rains. This is followed by a dry but warm autumn during October and November, but in some years the pacific hurricanes may produce heavy rains. Winter is December and January, mild and sunny with intermittent storms, occasional frosts and cool temperatures rarely reaching 70° F (21° C). Mid-February brings spring with early blooming in mild wet years and spectacular blooming in April. The seasonal year ends with the foresummer of May and June which is dry and hot in most years.

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