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BUTTerwICK & dANIeL: FLOrA OF SOUTHerN BLACK MTS., ArIzONA 273
is surrounded by a variety of Tertiary volcanic rocks (gray et al. 1990a; Nations and Stump 1996).
The topography is more complex on the west side of Black Mesa where it consists of lower-elevation
buttes, ridges, and rocky hills in the vicinity of McHeffy Butte, wrigley Peak, and Boundary
Cone, a prominent landmark in the area (Fig. 2).
Perennial springs occur occasionally throughout the flora area on slopes (e.g., Shaffer, Fig, and
Cave springs), in narrow, boulder-strewn canyons (e.g., Cottonwood, grapevine Springs, and Twin
Springs canyons), and along a few of the major tributaries including Meadow Creek, warm Springs
Creek, and Silver Creek wash.
CLIMATe
Climatic patterns in Arizona are affected by elevational gradients and by large-scale meteorological
systems that affect weather in western North America. As a result of the latter, a biseasonal
pattern characterizes precipitation in Arizona (Sellers and Hill 1974). winter rains generally
occur from October to April when the westerlies move moist Pacific air masses eastward. These
rains take the form of widespread gentle showers that may last for several days. Summer monsoon
rains, concentrated from July to September, are created when moist air masses from the gulf of
California and the gulf of Mexico move northward into the state. rapid cooling and condensation
occur as moist superheated air rises over mountainous terrain. Summer rains tend to form as localized
heavy thunderstorms of short duration.
The character of the biseasonal rainfall pattern varies across the state. In general, as one moves
westward, a decreasing proportion of the annual precipitation occurs during the summer. In southeastern
Arizona, dependable summer rains contribute up to 70% of the annual total (Hastings and
Turner 1965). However, descriptions of mapped ecological sites indicate that summer rains in the
flora area account for only 35% of the annual precipitation (U.S. department of Agriculture 2016).
Thus, most of the annual precipitation (ca. 65%) is in the form of winter rains.
There are no site-specific climatic data for the Black Mountains. The nearest climate stations
are in Bullhead City to the west, which has hotter and drier conditions, and in Kingman to the east,
which has cooler and wetter conditions. However, the average annual precipitation in the portion
of the desert region of the Basin and range Province, which includes our flora area, ranges from
12.7 cm (5 inches) to 25.2 cm (10 inches) (Sellers and Hill 1974). Increases in elevation generally
correlate with greater average amounts of precipitation. This general pattern is reflected in the ecological
site descriptions for the flora area with the lowest elevation sites being in the 7.5–15 cm (3–
6 inch) precipitation zone, the mid-elevation sites being in the 15–23 cm (6–9 inch) precipitation
zone, and the highest-elevation sites being in the 25–33 cm (10–13 inch) precipitation zone (U.S.
department of Agriculture 2016).
According to Sellers and Hill (1974), average daily temperatures for the region that includes
our flora area range from 7° to 10° C (45° to 50° F) during January and from 29° to 32° C (85° to
90° F) during July. Like precipitation, temperatures also follow elevational gradients. Thus, higher
areas tend to be cooler and more mesic. Average daily temperatures for January and July are
10.5° and 35° C (51° and 95° F) in the 7.5–15 cm (3–6 inch) precipitation zone, 7° and 30.5° C
(45° and 87° F) in the 15–23 cm (6–9 inch) precipitation zone and 6° and 28° C (43° and 82° F) in
the 25–33 cm (10–13 inch) precipitation zone (U.S. department of Agriculture 2016).