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Arizona Climate
Temperature
Two characteristics define Arizona in terms of its temperature regimes: a wide variation
in temperature across the state due to its great elevational differences and extremely high
summertime temperatures in its deserts.
Whereas in many other states daytime high temperatures may vary by 10 F or 20 F
across the state, Arizona commonly records differences of 40 F to 45 F. In late June it
is common to have temperatures of 115F in Bullhead City on the same day that Greer or
Alpine record a temperature of only 75 F.
This large difference in temperature across the state is due largely to differences in
elevation. As elevation increases atmospheric temperature decreases due to a decrease in
atmospheric pressure. Lower atmospheric pressure means that there are fewer air
molecules in a given volume of air. This in turn means that there are fewer collisions of
molecules that we register as temperature. On average atmospheric temperature decreases
3.6 F for every 1000’ of elevation gain – this is known as the average environmental
lapse rate. However, this rate does fluctuate day to day and with the seasons. In summer
the environmental lapse rate is usually higher than average and in winter it is usually a
little lower than average. This means that in summer the mountain peaks of southeastern
Arizona may be 25 F to 35 F cooler than the surrounding desert valleys while in winter
they may be only 10 F to 15 F cooler. Compare the maps below of average January
and July temperatures.
Arizona temperatures are controlled not only by elevation, but also to a lesser degree by
latitude. The latitude of a location marks its angular distance from the earth’s equator.
Places farther from the equator receive incoming solar radiation at a more oblique angle
and therefore receive less energy from the sun per unit area. The latitudinal control on
atmospheric temperature is stronger in winter than in summer. In winter places on the
Colorado Plateau in northeastern Arizona are almost 10 F colder than places at the same
elevation but farther south in the mountains of the Basin and Range.
The extremely high temperatures recorded in Arizona’s desert regions in summer are
due not only to low elevation, but also to the dry clear skies characteristic of the area.
Dallas, Texas and Atlanta, Georgia lie at roughly the same elevation and the same
latitude as Phoenix. Why then does Phoenix have summer temperatures averaging 10 F
to 15 F hotter than these other cities? Why do some places in Arizona reach
temperatures over 120 F? It is due to the fact that the humid cloudy skies of these other
cities, while making it feel stickier, actually reflect incoming solar radiation and keep the
temperature cooler. The southwest U.S. in general has some of the most cloudless skies
on earth. Why? It seems that our planet has five cells of what are called sub-tropical high
pressure (STH). These STHs are all centered at roughly 30 latitude, and are located just
off the west coasts of continents. High-pressure air is sinking air and as air sinks it
compresses and heats up – just like compressing air into a car tire. Heating the air
increases its water vapor capacity and therefore dries it out.
Winter temperatures
Average temperature values in January range from 20 to 25 F in the high regions of
the San Francisco Peaks, White Mountains, and Kaibab Plateau (all of which are on the
Colorado Plateau), while average temperatures in January approach 55 F in the region of
the lower Gila and Colorado Rivers (which are found in the Basin and Range).
Winters are coldest on the high elevation plateaus of the Colorado Plateau province,
particularly
above
7000'.
Temperatures below freezing are
common in this region from
November
through
April.
Temperatures below -30 F have
been recorded at three places on the
Plateau all fairly distant from one
another: -32 F at Chinle in the far
northeastern corner of the state; 33 F at Fort Valley near Flagstaff;
and -40 F at Hawley Lake in the
White Mountains. Temperatures as
low as -50 F must certainly have
occurred near the summits of the
San Francisco Peaks and in the
highest portions of the White
Mountains.
Winter temperatures are far
warmer in the low-lying basins of
the Basin and Range
province, especially in southwestern
Arizona. The warmest winter
temperatures are found in the low-lying valleys of the lower Colorado River, the
lower Gila River, and along the Santa Cruz River where average winter temperatures
reach 50 F.
Average minimum winter temperatures over practically the entire desert floor of
southwestern Arizona do not go below freezing. Some places are warm enough at night
to have average winter minima of up to 45 F. It is these places which support Arizona's
citrus industry. Surprisingly, the warmest spots at night in winter are not at the absolute
lowest elevations, as you would think. On winter nights, heavy cold air from surrounding
mountain ranges sinks/slides downhill to occupy the lowest layers of the valleys in
between the mountains. These cold air layers are relatively thin (500' to 1000').
Therefore, on winter nights the warmest spots in the desert are located on the bajadas and
pediments of desert mountain ranges around the edges of valleys and up off the desert
basin floor. For example, winter nighttime temperatures are warmer in White Tank
Regional Park 15 miles west on Olive than here in Glendale due to Glendale's lower
elevation.
Summer temperatures
Average temperatures increase from 30 F to 40 F from January to July.
The high sections of the Colorado Plateau now enjoy average temperatures of 70 F,
with higher sections such as the SF Peaks, the Kaibab Plateau, and the White Mountains
having average temperatures of only 60 F. These very high areas may still have winter
minima below freezing, but most of the Colorado Plateau has minima above 40 F.
The rest of the state is quite warm in the summer except for the isolated high peaks of
the southeastern and northwestern Basin and Range region. Practically all areas below
5000' have summer maxima in excess
of 90 F. In fact, practically all of the
southwest section of the state report
daily maxima in summer of over 100
F. Temperatures over 120 F have
been recorded at all towns on the
Colorado River south of Lake Mead,
and all towns on the Gila and Salt
Rivers from Phoenix downstream.
These river valleys have the lowest
elevations and therefore the hottest
temperatures of all. The highest
temperature on record for Arizona is
128 F registered at Lake Havasu
City in June of 1990.
The southeastern portion of the state
contains higher elevation basins
within the Basin
and Range province. Therefore the
summer temperatures in these desert
basins are cooler than in southwest Arizona.
The warmest summer temperatures in Arizona usually occurs in late June and early
July. This period directly precedes the summer monsoon season which brings with it
cloud cover and rain. Once the monsoon season begins, increasing cloud cover reflects
solar radiation and temperatures begin to decrease slightly. The intense solar radiation of
late June heats the desert surface to such a degree that hot rising air creates a thermal
low-pressure system. It is this thermal low-pressure system that brings on the moistureladen winds of the monsoon season.
Precipitation
Precipitation amounts
Arizona tends to be very dry in general due to its west coast location and 30 North
latitude – which put it under the influence of sub-tropical high pressure.
As we discussed, these locations on all continents tend to be fairly dry due to
subsiding(sinking) air from the upper atmosphere. Subsiding air is compressing, heating,
and drying out and resulting in intense
solar radiation.
Roughly half of Arizona receives
less than 10 inches of rain a year (see
map at left). This includes most of
southwestern Arizona and most of the
northeast as well. The most arid
region, receiving less than 5 inches of
rain a year, are the low elevation areas
in the west and southwest portions of
the state along the valleys of the Gila
and Colorado rivers. Yuma receives
less than 3 inches of rain per year.
Certain regions in the central portion
of the state, however, receive
significant amounts of precipitation.
The Kaibab Plateau, the San
Francisco Peaks, the Mogollon Rim
and the White Mountains, all of
which lie on or near the southern/western edges of the Colorado Plateau physiographic
province, receive more than 20 inches of precipitation per year with some very local
areas receiving up to 30 inches per year. Other rainier regions of the state include the
higher mountain ranges of the Transition Zone province (Mazatzals; Bradshaws; Sierra
Anchas) and of the Basin and Range (Catalinas; Rincons; Santa Ritas; Huachucas;
Chiricahuas, Pinalenos). (see map) All these regions lie at high elevations. High
elevations – being cooler – have an easier time reaching saturation. Just as atmospheric
temperature in Arizona is linked to elevation, elevation exerts a strong influence on
precipitation as well.
Two ingredients are necessary to create clouds and precipitation – a moisture source to
put humidity into the air and a lifting mechanism to cool the air toward saturation.
For much of the year Arizona is under the influence of sub-tropical high-pressure which
keeps Arizona dry. However, twice a year low pressure (rising air) makes its way into the
state and brings Pacific moisture with it. These two rainier times of year will be discussed
below.
Precipitation seasons
The graph (below) and table (below) show the seasonality of Arizona's precipitation.
From these it
should
be
apparent
that
Arizona's
precipitation
tends to arrive in
two
different
seasons. There is
a summer rainy
season (July and
August are the
rainiest months
for all portions
of the state) and
a winter rainy
season (lasting
from December
through
the
middle of March).
The table demonstrates that eastern Arizona benefits most from the summer rains, while
the western portion of the state gets more total rain in the winter.
The arid low-lying deserts of the southwestern portion of the state are not the driest
portion of the state in all months however. During the winter, the cold high plateaus in
the northeastern portion of the state are even drier due to surrounding mountain ranges
blocking arriving moisture. Winter up here in this region is very cold and very dry.
Arizona's dry seasons are the spring and the fall. Drought conditions are greatest in May
and June. The rainy parts of the state may go weeks without rain while the low-lying
southwest deserts receive rain during May and June only once every three years on
average. The second dry season is the fall. This period of drought is not as severe as the
May-June drought. The fall drought seems more pronounced in October in southern
Arizona and more pronounced in November in northern Arizona.
Causes of precipitation
Four major factors favor precipitation: moisture/humidity in the air; mountainous
terrain; convergence of air flow from the south; and intense thermal heating.
Moisture/humidity in the air - this factor seems fairly obvious. We can not get rain
unless there is water in the air to form rain. Because Arizona is dominated by high
pressure, by subsiding air from the upper atmosphere, our air is usually very dry. Special
conditions must arise to bring moisture to our state. These conditions will be discussed in
the next section.
Mountainous terrain forces air to rise and cool. Because cool air can not hold as much
moisture in the form of invisible gaseous water vapor as warm air can, cool air becomes
saturated by water vapor more easily. Saturated conditions are what leads to clouds
(water in the visible liquid state).
Convergence of air from the south crowds the air and forces it to rise and cool.
Intense thermal heating of the air by hot desert ground surfaces forces air to rise and
cool (see above)
Arizona's two rainy seasons
Keeping the above causes of precipitation in mind it is now possible to explain Arizona's
two seasons of increased precipitation.
July and August (monsoon season)
In July and August the ground surface of the desert becomes so hot that it superheats the
air above and causes it to rise. This creates what is called a thermal low-pressure
system centered in the Yuma area. A low-pressure cell is an area of rising air. This lowpressure cell pulls in air from the surrounding region as it rises. One region providing air
to this rising low-pressure cell is the tropical Pacific Ocean west of Mexico. This oceanic
air is very humid and provides a good moisture source for Arizona. When this humid air
arrives in Arizona it is forced to rise due to intense heating and due to large mountain
ranges in the southeast corner of our state and in central Arizona. This rising air cools, the
invisible gaseous water vapor condenses to clouds (liquid water) and it rains. The
portions of the state receiving the most rain are the mountains of the southeast corner
(they get the first crack at the moist air) and then the mountains to the north in the
transition zone and on the southern edge of the Colorado Plateau. Because this moisture
arrives from the south, the southern slopes of many mountain ranges often receive the
highest amounts of rain. Phoenix, because of its low elevation and lack of nearby large
mountains, receives very few of these summer rainstorms. Summer rainstorms occur
most often in the late afternoon, as this is the
time of the most intense heating.
December through March (winter rains)
In winter our moisture source is the North Pacific Ocean. At this time of year we can not
rely on intense heat for air to rise. However, all year long warm moist air from the
subtropical latitudes collides with cold dry air from the polar latitudes. These collisions
result in intense large storms known as mid-latitude cyclones. The warm air rises over the
top of the cold air forming centers of low pressure. These low-pressure cells (mid-latitude
cyclones) are born over the North Pacific Ocean and move from west to east across the
United States most of the year. However, they usually stay far to the north of Arizona.
However, occasionally these storms will veer southward if atmospheric conditions
change. These storms usually are moving fairly quickly and linger no longer than two or
three days. However, these storms do last longer than the short thunderstorms associated
with our summer rains. Several storms days or weeks apart may track over Arizona
before the normal atmospheric conditions return and the subtropical cyclones once again
pass to the north. It is these winter storms that provide the higher elevations with snow.