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Active Management Area Climate

Climate in the AMA Planning Area varies widely due to its large geographic extent, with significant temperature and rainfall differences between some AMAs.  Average annual temperatures range from 72.9°F in the Phoenix AMA to 53.3°F in the Prescott AMA compared to the statewide average of 59.5°F.  Phoenix and Tucson climate stations report the warmest temperatures with the exception of the summer monsoon season when Tucson receives a significant amount of its annual rainfall and associated cooler temperatures (Figure 8.0-6).

Figure 8.0-6 Average monthly temperature from 1952-2007 in the AMA Planning Area

Figure 8.0-6

Average annual precipitation (1971-2000) ranges from 8.3 inches at Phoenix Sky Harbor Airport to 18.7 inches at Nogales and Prescott. The AMA Planning Area exhibits a bi-modal precipitation seasonality that is characteristic of Arizona (Figure 8.0-7).  During the winter and spring, frontal storm systems move west-to-east, guided by the jet stream. Summer monsoon thunderstorms also deliver significant amounts of precipitation, particularly in the Prescott and Santa Cruz AMAs.  While precipitation amounts vary widely across the planning area, there are also strong year-to-year variations, due primarily to the influence of the El Nino-Southern Oscillation (ENSO), as well as long-term wet and dry periods that are linked to multi-decadal ocean variations.

                 Figure 8.0-7 Average monthly precipitation from 1948-1952 to 2006-2007

Figure 8.0-7

As shown in Figure 8.0-8, many of the wettest and driest periods since 1960 were synchronous throughout the AMAs with notable wet periods in the late 1970s, early 1980s and early 1990s.  Notable dry periods were the early 1960s, the early 1970s and the period from 1996 through 2006. The greatest year-to-year precipitation variations during this period occurred in the Phoenix AMA and the least variation in the Prescott AMA, with the exception of 1965 when Prescott received almost double its annual rainfall.

The planning area encompasses parts of five of Arizona’s seven climate divisions. A climate division is a region within a state that is generally climatically homogenous. Long-term climate data for Arizona’s climate divisions have been reconstructed from tree ring and instrumental data. These data show that since 1000 A.D., Climate Division 7 experienced more years (compared to the other planning area climate divisions) in which precipitation was less than that measured in 2002, one of the driest years in the instrumental record (CLIMAS, 2008).  Climate Division 7 encompasses most of the Tucson AMA and all of the Santa Cruz AMA.

Figure 8.0-8 Annual percent of average precipitation measured between 1960-2007

Figure 8.0-8

Average annual temperatures in the AMA Planning Area have been increasing since 1960, a phenomenon observed throughout the state. Figure 8.0-9 shows that all of the major urban locations in the AMAs have seen temperature increases, reflecting both a regional temperature trend and the influence of urban expansion and development. The effect of urban areas on temperature, precipitation and other climate phenomena is an important consideration in the planning area.  Phoenix, for example, has experienced the greatest increase in temperatures during the time period shown. Figure 8.0-10 illustrates an increase in daily minimum temperatures during the summer months in Phoenix and Tucson, and is contrasted with modest increases measured at Casa Grande National Monument, a relatively non-urbanized area between the two cities.

Research on urbanization and warming in the Phoenix metropolitan area shows that, from 1948-2000, urbanization has increased the nighttime minimum temperature in central Phoenix (Sky Harbor Airport) by approximately 9° F and the average daily temperature by approximately 5.5° F (Baker and others, 2002).  The number of days with temperatures between 59-100°F at Sky Harbor Airport has increased by about 30 days since 1948, most notably during the spring and fall. During the period 1990-2004, the Phoenix urban heat island expanded substantially, commensurate with increasing population and urban development. Recent research shows that temperatures in areas characterized by urban infill development, and areas in the core of the city were approximately 2° F and approximately 4° F warmer, respectively, than temperatures outside of urban areas (Brazel and others, 2007). Similarly, in central Phoenix the hours per day that exceed 100° F during the months of May through September have doubled since 1948 (Baker and others, 2002). 

Tucson’s urban heat island effect increased by approximately 5.5° F during the 20th century, with most of the warming since the late 1960s (Comrie, 2000).  In the Tucson area, urban temperatures increased at almost 3 times the rate of rural temperatures. Temperature changes are not, however, uniform. Within the urban zone, variations in temperatures are caused by differences in housing density, the amount of green space, topography, and localized cold air flows downslope from mountains.

Figure 8.0-9 Average annual temperature measured between 1960 and 2007

Figure 8.0-9

The impacts of urban warming are varied and include increases in energy consumption, predominantly from longer usage of air conditioning, and stress to animals and humans.  Since 1948, the total number of cooling degree days (CDD) in Phoenix has increased by 569 while the heating degree days (HDD) has declined by 331 (Baker and others, 2002). The CDD and HDD are indices that reflect the demand for energy needed to cool or heat a structure, respectively. Research conducted in 2003 in Phoenix found that distinct neighborhoods experience up to 7° F difference in temperature.

Two studies suggest that urbanization and large irrigated areas in the Phoenix metro area increase precipitation to the northeast of the city (Diem and Brown, 2003; Shepherd, 2006). Average precipitation in the northeastern suburbs and exurbs of metropolitan Phoenix has increased by 12-14%, from the first half of the 20th century (Shepherd, 2006).  The study suggests that urban heating, from built surfaces and buildings, affects upward motion in the atmosphere and can increase storminess beyond the urban area. Irrigation increases local water vapor in the atmosphere, and probably contributes to the increased precipitation (Diem and Brown, 2003).

Figure 8.0-10 Average Daily Minimum June, July and August temperature measured between 1960 and 2007

Figure 8.0-10


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