Climate of Phoenix Summary

ABSTRACT

The purpose of this publication is to provide comprehensive data on the climate of Phoenix. It is hoped that these data will help residents, visitors, prospective residents, agriculturalists, engineers, community planners, Chambers of Commerce, the movie industry, etc., make more skillful decisions affecting their lives, their plans for the future, and hence the whole economy of the area. Data in this revision are for the period January 1, 1896 through December 31, 1995. This marks 100 years of continuous weather records for Phoenix.

The assistance given by Mr. Robert S. Ingram, former Meteorologist in Charge, National Weather Service Office, Phoenix, Arizona, Mr. Paul C. Kangieser, former NOAA Climatologist for Arizona, and other staff members is gratefully acknowledged. The writer is gratefully indebted to Mr. Harold C. Bulk, former Assistant State Climatologist, Office of Climatology, Arizona State University, for his article, “An Overview of Phoenix Climate”. Ms. Brazel and Mr. Balling’s research paper, “The Myth of Increasing Moisture Levels in Phoenix”, is also included in this Technical Memorandum.

I. GENERAL GEOGRAPHICAL AND CLIMATOLOGICAL SUMMARY

Phoenix is located in about the center of the Salt River Valley, a broad, oval-shaped, nearly flat plain. The Salt River runs from east to west through the valley, but, owing to impounding dams upstream, it is usually dry. The climate is of a desert type with low annual rainfall and low relative humidity. Daytime temperatures are high throughout the summer months. The winters are mild. Nighttime temperatures frequently drop below freezing during the three coldest months, but the afternoons are usually sunny and warm.

At an elevation of about 1100 feet, the station is in a level or gently sloping valley running east and west. The Salt River Mountains, or South Mountains as they are commonly called, are located 6 miles to the south and rise to 2600 feet MSL. The Phoenix Mountains lie 8 miles to the north with SquawPeak rising to 2600 feet MSL. The famous landmark of Camelback Mountain lies 6 miles to the north-northeast and rises to 2700 feet MSL. Eighteen miles to the southwest lie the Sierra Estrella Mountains with a maximum elevation of 4500 feet MSL, and 30 miles to the west-northwest are found theWhite Tank Mountains with a maximum elevation of 4100 feet MSL. The Superstition Mountains are approximately 35 miles to the east and rise to 5000 feet MSL.

The central floor of the Salt River Valley is irrigated by water from dams built on the Salt River system. To the north and west of the gravity flow irrigated district, there is considerable agricultural land irrigated by pump water.

There are two separate rainfall seasons. The first occurs during the winter months from November through March when the area is subjected to occasional storms from the Pacific Ocean. While this is classified as a rainfall season, there can be periods of a month or more in this or any other season when practically no precipitation occurs. Snowfall occurs very rarely in the Salt River Valley, while light snows occasionally fall in the higher mountains surrounding the valley. The second rainfall period occurs during July and August when Arizona is subjected to widespread thunderstorm activity whose moisture supply originates in the Gulf of Mexico, in the Pacific Ocean off the west coast of Mexico and in the Gulf of California.

The spring and fall months are generally dry, although precipitation in substantial amounts has fallen occasionally during every month of the year.

During the winter months, the temperature is marginal for some types of crops. Areas with milder temperatures around the edges of the valley are utilized by these crops. However, the valley is subjected to occasional killing and hard freezes in which no area escapes damage.

The valley floor, in general, is rather free of strong wind. During the spring months southwest and west winds predominate and are associated with the passage of low-pressure troughs. During the thunderstorm season in July and August, there are often local, strong, gusty winds with con­siderable blowing dust. These winds generally come from a northeasterly to southeasterly direction. Throughout the year there are periods, often several days in length, in which winds remain under 10 miles per hour.

Sunshine in Phoenix area averages 86 percent of possible, ranging from a minimum monthly average of around 78 percent in January and December to a maximum of 94 percent in June. During the winter, skies are sometimes cloudy, but sunny skies predominate and the temperatures are mild. During the spring, skies are also predominately sunny with warm temperatures during the day and mild pleasant evenings. Beginning with June, daytime weather is hot. During July and August, there is an increase in humidity, and there is often considerable afternoon and evening cloudiness associated with cumulus clouds building up over the nearby mountains. Summer thunder­showers seldom occur in the valley before evening.

The autumn season, beginning during the latter part of September, is characterized by sudden changes in temperature. The change from the heat of summer to the mild winter temperatures usually occurs during October. The normal temperature change from the beginning to the end of this month is the greatest of any of the twelve months in central Arizona. By November, the mild winter season is definitely established in the Salt River Valley region.

An Overview of Phoenix Climate

By Harold Bulk, Office of Climatology, Arizona State University

The climate of a location is the synthesis of several elements. The temporal variations of several of these elements is shown in the graph on the following page.

The temperature of the air is probably the element that most people are aware of. Yet air temperature is the result of many other climatic elements. The most important is the receipt of solar energy, for solar energy is the force that drives most of the other climatic elements. The daily amounts of solar energy that are received at the top of the atmosphere (the extra-terrestrial radiation, or ETR) is shown in curve A. The amounts vary from nearly a thousand Langleys (1 Langley = 1 calorie per square centimeter) on the day of the Summer Solstice to about 400 Langleys on the day of the Winter Solstice. Clouds reflect a substantial portion of the solar energy. More is absorbed by water vapor in the air, and even the atmosphere itself will scatter a portion of the solar energy back to space as well as absorb a portion.

Curve C represents the amount of energy that can reach Phoenix on a clear, dry day. (Rosendahl, 1976). It is apparent that only about 70% of the ETR reaches the surface under these conditions. The ten-year average daily receipt of solar energy at Phoenix is shown in curve D.

Some of the energy reaching the earth’s surface is reflected back toward space by the earth itself, some is used to evaporate water, and the remainder warms the air. The large drop in energy receipt during July is directly traceable to the increase in cloudiness (curve E) during this period. (The depletion of solar energy due to clouds is also apparent during the winter months, although less spectacularly so). The continued depression of the averaged receipts of solar energy into August is due to the increased water vapor in the atmosphere (curve F, from Reitan, 1960). The increased water vapor in the atmosphere is due to a shift in the winds from a predom­inantly westerly direction to a southerly direction, the so-called “Arizona Monsoon”. Although the dry bulb temperatures may be depressed during this period, the “sensible temperatures” seem higher due to the increased humidity of the air.

Also shown is the ten-year average daily precipitation at Phoenix (curve G). It is seen that the largest average daily receipts occur in July and August. Rainfall plays a significant role in that a portion of the solar energy reaching the ground is used to evaporate moisture.

Curve B is the average daily temperature at Phoenix. This curve lags the curves for ETR (A), that of clear-day receipts (C), and that for averaged receipts (D). This is due primarily to the thermal lag of the earth. The flattening of the temperature curve during August is due to the energy absorbed by the enhanced rainfall during that time.

Clearly, the daily average temperature at Phoenix is the result of primarily the solar energy reaching the earth’s surface and the precipitation regime.

References

Local Climatological Data, Monthly for Phoenix, Arizona. NOAA, EDIS, Asheville, North Carolina, 1971-1980.

National Weather Service Forecast Office, Phoenix, Arizona. Daily total Horizontal Solar Energy Receipts, 1971-1980.

Reitan, C.H., 1960: “Distribution of Precipitable Water Vapor over the Continental United States”. Bulletin American Meteorological Society, 41, 79-87.

Rosendahl, H. 1976: “Table of daily Values of Maximum Possible Solar Energy, in Solar Radiation and Sunshine Data for the Southwestern U.S.”. R. Durrenberger, Editor, Tempe, Arizona, Laboratory of Climatology.

II. CLIMATOLOGICAL SUMMARY BY MONTHS

 

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III. HISTORY OF WEATHER OBSERVATIONS

In the 1800s when communications in the United States were improved by the development of the railroads and telegraph, the practice of predicting weather from purely local signs and the haphazard measuring of meteorological phenomena began to decline. Scientists had noted correlations between the weather in one section of the country on a particular day and that in another section on the succeeding day. It was soon realized that a simul­taneous knowledge of weather conditions all over the country could conceivably enable man to predict storms of major consequences, and that warnings from such predictions could save countless lives and protect property investments. But it was not until the late 1860s that mounting public interest in a national weather service culminated in the signing into law by President Grant on February 9, 1870, of a resolution providing for meteorological observations at all military stations within the United States.

The selection of the U.S. Army Signal Service to take such observations was dictated by the availability of communications facilities which the Signal Service had developed during the Civil War and were continuing to develop for protection against the Indians after the war. The original weather services provided by the military organi­zation covered only the Gulf andAtlantic Coasts and the Great Lakes. Another Act of Congress, on June 10, 1872, extended these services throughout the entire United States.

Weather observations had been taken at many Army posts in Arizona prior to these formalities by Army Post Surgeons. Observations are available from some of these locations today:

Station County Data Began
Fort Defiance Apache December 1, 1851
Camp Crittenden Santa Cruz December 1856
Fort Mohave Mohave June 1859
Fort Grant Graham December 1, 1860
Camp Goodwin Graham August 1864
Fort Whipple (Prescott) Yavapai January 1865
Fort McDowell Maricopa September 1, 1866
Camp Wallen Cochise November 1866
Camp Date Creek Yavapai January 1867
Fort Bowie Cochise August 1, 1867
Camp Willow Grove Mohave November 1867
Camp Reno Gila February 1, 1868
Fort Verde (Camp Verde) Yavapai February 1, 1868
Camp Hualapai Yavapai December 1869
Fort Yuma Yuma January 1, 1870

Observations from these stations were primarily temperature and rainfall. It wasn’t until 1891, when the U.S. Weather Bureau was established, that development of reporting stations proceeded with cautious economy.

The Bureau directed its attention mainly toward establishing a network of field stations. Faced with the growth of public interest, civic pride and the need to provide the best coverage for its forecasting and warning services with limited funds, the Weather Bureau could only slowly grant requests to establish weather stations in a rapidly expanding Nation.

The first Weather Bureau Office to open in Arizona was in Yuma where the duties were transferred from the Army at Fort Yuma in July 1891. Tucson followed in September of that year, and it was not until four years later that the small community of Phoenix rated a full station. Records had been kept in Phoenix by the Signal Service beginning on January 28, 1876, and Signal Service personnel continued to take observations until they transferred the station on the corner of Center and Washington Streets to the Weather Bureau on August 6, 1895.

In 1901 the office was moved to the southwest corner of 1st Avenue and Adams where it remained until it moved into the Federal Building on the southwest corner of 1st Avenue and Van Buren in March 1913. Three years later in June 1916, the office moved to the Water User’s Building on the southeast corner of 2nd Avenue and Van Buren. It remained there until September 1924 when it moved to the Ellis Building at 2nd Avenue and Monroe. On October 21, 1936, it moved to the Federal Building at Central and Fillmore where it stayed until it was closed on october 22, 1953.

Meanwhile, the development of air transportation and teletype communications in the 1920s and 1930s altered and redirected somewhat the purpose of the Weather Bureau as first defined by the law in 1890. This law provided for “the distribution of meteorological information in the interest of agriculture and commerce…” as one of the Weather Bureau’s major functions. “Commerce” now included the mushrooming aviation industry — and in 1940, to meet this partial change in emphasis, the Weather Bureau was transferred from the Department of Agriculture to the Department of Commerce where it remains today. In support of this new means of transportation, another Weather Bureau office was established atSky Harbor Airport on May 2, 1933, and observations were taken there also until July 1935 when Department of Commerce radio operators took over the program. The Weather Bureau returned again to this station in January 1939 and has managed the station ever since that time.

In July 1965 the Weather Bureau was incorporated as an integral part of the Environmental Science Services Administration (ESSA). In October 1970, the name was changed to the National Weather Service, and it became an integral part of the National Oceanic and Atmospheric Administration (NOAA).

ELEVATIONS AT
NATIONAL WEATHER SERVICE FORECAST OFFICE
PHOENIX, ARIZONA

LATITUDE 33 deg 26′ North
LONGITUDE 112 deg 01′ West

ELEVATION OF AIRPORT 1128 Feet
ELEVATION OF IVORY TIP OF BAROMETER 1109.31
ELEVATION OF STATION PRESSURE 1107
ELEVATION OF GROUND AT HYGROTHERMOMETER 1110
ELEVATION OF GROUND AT WIND VANE AND ANEMOMETER 1110
ELEVATION OF CLIMATOLOGICAL STATION 1107
ELEVATION OF GROUND AT OFFICE 1106
ELEVATION OF HYGROTHERMOMETER Above Ground, 5
ELEVATION OF WIND VANE AND ANEMOMETER Above Ground, 33
ELEVATION OF SUNSHINE SWITCH Above Ground, 7
ELEVATION OF PYRANOMETER Above Ground, 6
ELEVATION OF RAIN GAGE Above Ground, 5

Acceleration of Gravity at Phoenix: 979.428 cm/sec2

Boiling Point of Water at Phoenix: 210oF

Temperature Conversion F to C (C = (F-32)*5/9)

Precipitation Conversion I to M (M = I*2.54)

Pressure conversion I to M (insert here)

IV. TEMPERATURE

NORMAL MAXIMUM, MINIMUM, AND MEAN BY MONTHS 1961-1990

Month MAXIMUM MINIMUM MEAN
January 65.9 41.2 53.6
February 70.7 44.7 57.7
March 75.5 48.8 62.2
April 84.5 55.3 69.9
May 93.6 63.9 78.8
June 103.5 72.9 88.2
July 105.9 81.0 93.5
August 103.7 79.2 91.5
September 98.3 72.8 85.6
October 88.1 60.8 74.5
November 74.9 48.9 61.9
December 66.2 41.8 54.1
Annual 85.9 59.3 72.6

NORMAL MAXIMUM, MINIMUM, AND MEAN BY MONTHS 1971-2000

Month MAXIMUM MINIMUM MEAN
January 67.3 44.8 56.1
February 71.4 48.4 59.9
March 76.1 53.0 64.5
April 84.7 57.6 71.1
May 93.9 67.4 80.6
June 103.9 75.6 89.8
July 106.6 82.9 94.7
August 104.5 81.6 93.0
September 99.0 75.6 87.3
October 87.7 62.1 74.9
November 75.0 50.4 62.7
December 67.1 43.9 55.5
Annual 86.4 61.9 74.2

HIGHEST MEAN AND LOWEST MEAN BY MONTHS AND YEAR OF OCCURRENCE 1896-2007

Month HIGHEST YEAR LOWEST YEAR
January 62.0 2003 43.2 1937
February 66.0 1991 48.6 1939
March 72.3 2004 54.3 1897
April 80.1 1989 62.4 1967
May 87.0 1984 69.0 1917
June 94.6 2006 79.0 1965
July 97.7 2003 85.4 1912
August 96.2 2007 84.6 1918
September 92.2 2001 78.9 1900, 1912
October 82.7 2003 66.2 1916
November 70.0 2007 55.0 1922
December 61.3 1980 46.6 1911
Annual 77.0 1989 67.8 1964

HIGHEST AND LOWEST MEAN MAXIMUM AND HIGHEST AND LOWEST MEAN MINIMUM
BY MONTHS AND YEAR OF OCCURRENCE – 1896-2008

MEAN MAXIMUM    MEAN MINIMUM HIGHESTYEARLOWESTYEAR HIGHESTYEARLOWESTYEAR

January 74.6 2003 53.5 1949 49.8 1993 31.9 1937
February 78.7 1991 60.5 1939 53.2 1991 33.4 1964
March 86.7 1972 67.3 1897
1958
59.5 2004 40.3 1917
April 94.2 1989 75.1 1900 65.9 1989 47.5 1967
May 101.2 1984 83.7 1917 73.7 1997 54.2 1917
June 108.6 1974 96.0 1967 81.8 2006 60.6 1965
July 109.8 1989 97.7 1912 86.6 2003 73.1 1912
1913
August 107.3 1962 97.0 1955 86.1 2007 71.3 1900
September 104.6 2001 93.6 1900 80.6 1983 63.6 1912
1965
October 95.8 1952 80.7 1919 70.8 2003 50.8 1923
November 82.3 1949 67.4 2000 58.3 2007 37.3 1916
December 74.9 1980 59.2 1909 48.2 1977
1991
32.6 1916
Annual 89.8 1989 81.4 1913 64.9 2003 53.1 1917

Greatest number of consecutive months with average temperature below normal:

13 Months from May 1916 through May 1917

Greatest number of consecutive months with average temperature above normal:

38 Months from January 1988 through February 1991

HIGHEST MAXIMUM AND LOWEST MINIMUM
BY MONTHS AND DAY AND YEAR OF OCCURRENCE
1896-2008

HIGHEST LOWEST
MAXIMUM DAY YEAR MINIMUM DAY YEAR
January 88 19 1971 16 7 1913
February 92 25
27
1921
1986
24 7
8
1899
1933
March 100 26 1988 25 4 1966
April 105 20
29
1989
1992
35 10 1922
May 114 30 1910 39 3 1899
June 122 26 1990 49 4 1908
July 121 28 1995 63 4
5
1912
1912
August 116 1
4
1972
1975
58 20 1917
September 116 1 1950 47 22
20
21
1895
1965
1965
October 107 1
2
1980
1980
34 30 1971
November 96 1
2
1924
1924
27 23 1931
December 87 10 1950 22 31
26
1900
1911
Annual 122 JUN 26 1990 16 JAN 7 1913

 

Temperature

LOWEST MAXIMUM AND HIGHEST MINIMUM
BY MONTHS AND DAY AND YEAR OF OCCURRENCE
1896-2008

LOWEST HIGHEST
MAXIMUM DAY YEAR MINIMUM DAY YEAR
January 39 6
21
1913
1937
61 27 1988
February 46 6
8
1899
1903
65 21 1996
March 49 2 1915 74 28 1986
April 52 1 1949 77 27 1987
May 54 1 1915 86 29 1983
June 68 2 1899 93 27 1990
July 79 1 1911 96 15 2003
August 73 27 1951 95 12 2003
September 66 23 1895 90 3
31
1982
19832007
October 56 30 1959 83 2 1997
November 40 28 1919 73 4 2001
December 36 10 1898 65 3 1986
Annual 36 DEC 10 1898 93 JUN 27
JUL 20
1990
1989

GREATEST AND LEAST MONTHLY TEMPERATURE RANGE
BY MONTHS AND YEAR OF OCCURRENCE
1896-1995

GREATEST LEAST
RANGE YEAR RANGE YEAR
January 69 1971 33 1995
February 61 1921 31 1993
March 67 1966 35 1980
April 62 1949 44 1931
May 66 1910 40 1981
June 63 1929 38 1986
July 50 1905 33 1959
1981
August 51 1918
1962
31 1955
September 58 1945 35 1981
October 66 1917 33 1983
November 67 1931 38 1986
December 58 1911 35 1992
Annual 96 1990 77 1984
1991
1992

HOTTEST AND COOLEST SUMMERS 1896-1995
(June, July, August, and September Combination)

HOTTEST Year COOLEST Year
93.4 1981 84.1 1912
93.3 1989 84.2 1913
92.7 1988 84.4 1923
92.1 1994 84.6 1965
92.0 1977 85.1 1964

HOTTEST THREE
SUCCESSIVE MONTHS
HOTTEST TWO
SUCCESSIVE MONTHS
Temp (Combination) Temp (Combination)
94.8 June, July, August 1981 95.6 July, August 1989
94.4 June, July, August 1988 95.5 July, August 1981
94.4 June, July, August 1989 95.1 July, August 1988
93.9 June, July, August 1985 94.8 July, August 1991
93.8 June, July, August 1994 94.7 July, August 1985
93.5 June, July, August 1977 94.6 July, August 1977
93.2 June, July, August 1986 94.6
94.6
94.6
July, August 1994
July August 1995
July, August 1995

HOTTEST MONTH

Temperature Month Year
97.4 July 1989
96.2 July 1988
95.8 August 1981
95.6 July 1980
95.5 July 1983
95.3 August 1994

WARMEST AND COLDEST WINTERS 1896-FEBRUARY 1995
(December, January, February Combination)

WARMEST Year COLDEST Year
60.6 1980-1981 49.3 1963-1964
59.4 1985-1986 49.8 1948-1949
58.6 1991-1992 50.0 1916-1917
58.5 1990-1991 52.2 1945-1946

COLDEST TWO
SUCCESSIVE MONTHS
COLDEST MONTH
48.0 January, February 1964 43.2 January 1937
48.2 December, January 1936-1937 44.6 January 1949
48.3 December, January 1948-1949 46.6 December 1911
48.4 December, January 1931-1932 47.0 January 1932
48.7 January, February 1949 47.1 December 1916

RECORD HIGH DEW POINTS IN DEGREES AND DATES OF OCCURRENCE
1896-1995

HIGHEST HOURLY DEW POINTS

Dew Point (F) Month Day Year
79 July 19 1957
78 August 9 1977
78 August 20 1978
77 August 1 1951
76 July 10 1899
76 August 10 1913
76 August 4 1943
76 July 31 1945
76 July 17 1953
76 July 22 1966
76 August 19 1966

HIGHEST DAILY AVERAGE DEW POINTS

Dew Point (F) Month Day Year
74 August 4 1943
73 August 19 1966

HIGHEST MONTHLY AVERAGE DEW POINTS

Dew Point (F) Month Year
68 August 1955
67 August 1943

RECORD LOW DEW POINTS IN DEGREES AND DATES OF OCCURRENCE

1896-June 1995

LOWEST HOURLY DEW POINTS

Dew Point (F) Month Day Year
-23 November 28 1976
-22 December 21 1977
-15 January 29 1970
-14 February 2 1972
-13 Februrary 3 1972
-13 April 13 1974
-12 March 10 1977
-11 February 4 1972

LOWEST DAILY AVERAGE DEW POINTS

Dew Point (F) Month Day Year
-10 December 21 1977
-5 February 3 1972

LOWEST MONTHLY AVERAGE DEW POINTS

Dew Point (F) Month Year
20 February 1972
21 March 1977

RECORD LOW HOURLY HUMIDITIES IN PERCENT AND DATES OF OCCURRENCE

1896-June 1995

Relative Humidity (%) Month Day Year
2 May 8 1904
2 May 16 1907
2 May 13 1976
2 December 21 1977
2 April 21 1979
2 June 4 1982

HEAT INDEX
(Apparent Temperature)

Most people are familiar with the term “wind-chill factor” which gives the combined effects of wind and temperature as an equivalent calm air temperature. For example, if the temperature is 0_F and the wind is 5 mph, the wind-chill factor is -5_F; at 10 mph, it is -22_F; and at 20 mph it is -39_F. Just as an increase in wind makes the cold air more unbearable, so does an increase in the moisture content of the air make the high summer temperatures more uncomfortable.

In most sections of the country, people look forward to summer. In the desert southwest, however, summer is the most undesirable time of the year. The term “Heat Index” is an apparent temperature based on the actual temperature and the amount of moisture in the air. The Heat Index Graph, devised by the National Weather Service, uses temperature and humidity values to determine the heat index. The areas of the graph are labeled: very warm, hot, very hot, and extremely hot. Most of the typical sunny summer days in the high country of Arizona fall into the very warm category. At the 5000-foot level, they fall into the hot, and in the lower deserts, they are in the very hot area of the graph. The chart also gives the heat syndrome for each classification.

The dew point, or the temperature to which the air must be cooled before condensation can take place, gives a true value of now much moisture is actually in the air. By knowing the temperature and dew point, the humidity can be determined. Using the temperature and humidity, the heat index can be arrived at by using the graph.

The prolonged summer head with maximum temperatures generally between 105 and 110 degrees in the Phoenix area causes some degree of fatigue in most people. Exhaustion and even heatstroke and sunstroke are possible with prolonged outdoor activity. This is especially true during much of July and August when the atmosphere becomes laden with tropical moisture.

Phoenix records were checked back to 1896 to find the highest humidity ever for each temperature from 100 through 118 degrees.

 TEMPERATURE DEW POINT HUMIDITY HEAT INDEX
100 75 45 112
101 74 43 112
102 69 35 110
103 69 34 110
104 68 32 111
105 69 32 113
106 67 29 112
107 66 27 112
108 68 28 116
109 66 26 116
110 62 22 113
111 63 22 116
112 59 18 113
113 59 18 115
114 57 16 114
115 56 15 115
116 56 15 117
117 56 14 117
118 56 14 118

It is interesting to note that with high moisture content, with humidities in the 30% and 40% range, the temperature never reached over 105 degrees. It is only with very dry air that temperatures climbed over 112 degrees. This is nature’s way of not allowing conditions to get entirely out of hand.

THE MYTH OF INCREASING MOISTURE LEVELS IN PHOENIX

By Robert C. Balling, Jr., and Sandra W. Brazel
Office of Climatology, Arizona State University

Is Phoenix becoming more humid? Many local residents believe that irrigated landscaping, swimming pools, and lakes and canals in new housing developments around the city are forcing moisture levels noticeably upward. However, many scientists have shown that cities usually act to decrease moisture levels in the atmosphere. This is caused by (a) paved surfaces that store little moisture and force rapid runoff following a rain event and (b) increased temperature in the “urban heat island”.

Despite local interest in atmosphere moisture trends in the valley, surprisingly little scientific research has directly addressed this issue.

We decided to examine the Phoenix, Arizona, weather records from 1896-1984 to see if there has been a change in the humidity of the Phoenix urban area. We chose relative humidity and dew point temperatures for statistical analysis. The dew point temperature is a better indicator of the amount of moisture in the air, which is the major contributor to human discomfort.

Since Arizona has a distinct two season rainfall pattern (a monsoon season, July through September and a winter season, December through April), we chose the months of May, June, October, and November for analysis. These transition months should be the least affected by large-scale weather disturbances since they are in between the precipitation seasons. Thus any urban effect on humidity should be clearly evident.

We chose six different relatively sophisticated statistical techniques to analyze the time series patterns in the atmospheric moisture data. These techniques basically search for “climatic signals” that may be contained in the “noisy” variance patterns in our data. These statistical procedures allow us to make conclusions regarding any trends, cycles, or discontinuities in the moisture records.

The results for the dew points were somewhat surprising. In May, October, and November, our statistical procedures indicated that the variations in the data were random; however, some form of non-random variation appeared to exist in the June dew points. Our analyses showed that trend was not the source of non-random inter-annual variation in June (or any other month). The systematic variations in June were found to be related in several significant cycles in the data. One cycle showed a maximum occurring in 1943, and a minimum 1898. This important cycle shows that we are presently heading towards another minimum projected for 1987. Another cyclical pattern showed maxima in 1917 and 1962, and minima in 1939 and 1984. Clearly dew points are not rising in Phoenix.

Given the steady or falling dew points, and assuming the highly probable occurrence of some urban heat island effects (higher temperatures in the city), the relative humidity values should display decreasing levels, again contrary to popular opinion. All of our statistics from each month indicated a strong downward trend in the relative humidity levels. The levels display a peak in the 1920s and a pronounced minimum in the 1970s and 1980s. So we have concluded that while increases in irrigated and sprinkled areas and open water surfaces may have occurred in the growing Phoenix area, many other effects of urbanization have apparently produced an overriding, counteracting impact on the atmospheric moisture levels.

AVERAGE TEMPERATURE and RELATIVE HUMIDITY BY FIVE-YEAR PERIODS
1896-1995

5-Year Interval Temperature Relative Humidity
1896-1900 69.8 38
1901-1905 70.1 39
1906-1910 69.7 44
1911-1915 68.9 44
1916-1920 68.9 48
1921-1925 70.2 44
1926-1930 70.9 41
1931-1935 71.8 40
1956-1960 71.3 40
1936-1940 71.9 47
1941-1945 70.5 41
1946-1950 71.3 43
1951-1955 71.0 41
1961-1965 69.6 38
1966-1970 70.7 40
1971-1975 71.8 35
1976-1980 73.5 46
1981-1985 74.3 39
1986-1990 75.8 34
1991-1995 74.6 37

These values of relative humidity are averages of the five years. The yearly averages are based on the averages of the twelve months. The monthly averages are based on daily values taken at 5 a.m. and 5 p.m.

These data also show high values in the 1910s and 1920s and low values in the 1970s and 1980s. This is in good agreement with the above research project.

It again points out that with urbanization, more buildings of all kinds, more paved surfaces and the heat island effect, the relative humidity decreases.