Climate Information

(US Weather Service Spotter Station AZ-709)

Weather and Climate have a critical effect on growth of cacti and succulent plants. I am fortunate to live in a succulent rich ecosystem with a climate favorable to the growth of many types of cacti & succulents from around the world. This is the Arizona Upland subdivision of the Sonoran Desert, which is characterized by 5 seasons. Although the climate is generally warm to hot, dry, and sunny, it presents its own set of complications for growing cacti and succulents. In particular, the combination of heat, aridity, and sun intensity are either fatal or disfiguring to unadapted (or unadaptable) plants exposed to the elements. Because of the extremely high UV content of sunlight, most growers in this region keep plants under at least 25--50% shade cloth all year round.

According to contemporary interpretation, the Arizona Upland subdivision of the Sonoran Desert has 5 seasons. The 5th season is a splitting of summer into a hot dry portion, known as the "arid fore-summer" and a relatively short, hot humid portion known as the Arizona Summer Monsoon. The Monsoon involves a seasonal shift of the prevailing winds from westerly to generally easterly or southeasterly, thereby importing moisture from the Sierra Madre of Mexico. It is during the ~2 month period of July and August that we receive about 1/2 our annual precipitation in the form of short but intense thunderstorms. These thunderstorms are convective and occupy small cells as compared with frontal thunderstorms found in most areas of the world. There is a secondary peak of rainfall during the December to February period and the remaining months are generally dry. In fact, August usually brings a number of stretches of 2 to 5 days during which the prevailing wind switches to westerly and the dewpoint drops back into the 20's or 30's Fahrenheit. Although the dewpoint reaches 54˚F and above during July and August, the remainder of the year is characterized by very dry--often bone dry--air. The average daily high at the coldest time of year is in the low 60's and the average daily high in summer is in the low to mid 100's Fahrenheit. A typical June or July daytime high (in the shade) is about 105--108F (40--43˚C). The annual temperature extremes are between ~25˚F (-4˚C) on at least 1 night per winter and sometimes up to 120˚F (~50˚C) in late June. These temperatures are measured by the standard method using a thermometer in the shade and shielded from direct solar radiation. In the sun, at ground level, it can reach ~180F (~80˚C) on some days. Somewhere I once read Arizona summer described as "oven-like heat". This is true.

Precipitation

2007:

Days with measurable precipitation 0.01" or more:  2.

Total precipitation, 2007 calendar year:  0.18".

2006:

Days with measurable precipitation 0.01" or more:  22.

Total precipitation, 2006 calendar year:  7.35".

2005:

Days with measurable precipitation 0.01" or more:  21.

Total precipitation, 2005 calendar year:  10.52".

2004:

Days with measurable precipitation 0.01" (0.25mm) or more:  27.

Total precipitation, 2004 calendar year:  9.11".

2002:

Days with measurable precipitation 0.01" or more:  24.

Total precipitation, 2002 calendar year:  6.24"

Notice the peak rainfall during the Arizona Summer Monsoon. During July and August, there were 15 days with precipitation during the ~60 day period of the "rainy season".

This graph compares the monthly average precipitation at recording stations near me and at the Weather Service Office at Tucson Airport. These are 50 year averages. The name "Anvil" is a ranch 5 miles southwest of my location and "Tucson" refers to the Weather Service Office at the Tucson Airport, 25 miles east.

 Temperature Profile

This graph shows the temperature profile on a typical June day. Time is measured in Mountain Standard Time (GMT - 7h).

The two curves compare the temperature at my location (blue, 3201 feet elevation) and the Weather Service Office (red, ~2500 feet elevation at the airport). Notice the difference between my location in the open desert and the airport which is 10 miles from the city center but still within the temperature bubble known as the "urban head island". Because of the bone dry air and lack of thermal inertia, my location heats up faster, beginning earlier in the morning but also cools faster, beginning earlier in the afternoon. Although the daily high is usually similar at the 2 sites, the nighttime low at my location is typically 8˚F (5˚C) cooler and is reached earlier in the evening.

Arrival of the Monsoon Airmass

An important feature of the climate is the "5th season" known as the Arizona Summer Monsoon, or just the "Monsoon". This weather pattern is described above. The graph below shows the abrupt arrival of moist air just before 8AM local time on 8 July 2002. For keeping records, the monsoon season technically lasts from June 15 through September 30, even though the average date of onset is 3 July and typically the pattern dissipates by around 5 September. During the monsoon, there are also usually 3 or 4 breaks of 2 to 3 days in which the humidity drops to or near to regular levels. Some years the monsoon has trouble both keeping the humidity up and offering more than a few thunderstorms during that 2 month period.

The National Weather Service in Tucson makes observations at the Tucson International Airport, about 25 miles to the east.