CLIMATE OF PHOENIX: PART 1

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 Squaw Peak 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 the White 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



1. January Weather


The Phoenix area generally experiences its coldest weather in January, yet, daytime temperatures still average in the middle sixties. The normal daily maximum is 65.9, and the normal daily minimum 41.2 and the normal mean monthly temperature is 53.6.

The warmest January occurred in 1986 when the mean monthly temperature was 61.4; the coldest was in 1937 with a mean temperature of only 43.2. The highest temperature ever recorded in any January was 88 on the 19th in 1971. The lowest January temperature (and the all-time low for Phoenix) was 16 on January 7, 1913. The warmest night occurred on the 27th in 1988 when the temperature fell no lower than 61. The record cold day for January and for any winter month was January 6, 1913, when the high, low, and mean temperatures were 39, 17, and 28, respectively.

The relative humidity for the month averages about the same as that for December. The low value in the afternoon averages around 33 percent.

The mean hourly surface wind speed is around 5.3 m.p.h., and the prevailing direction is from the east. The peak gust was 60 m.p.h. from the west on January 27, 1983.

Precipitation during the month normally totals 0.67 inches, but it has ranged from 5.22 in 1993 to none in 1912, 1924, and 1972. The greatest amount of precipitation in 24 hours was 1.84 inches which occurred on January 10-11, 1993. There are normally four days with 0.01 inches or more, but January 1993 had fourteen such days.

Snow can occur in January, but it is unusual. Snow in amounts of up to 1 inch has been reported at the official observing station on seven January days since 1896. The heaviest falls of just 1 inch fell in 1933 and 1937. On January 20-21, 1937, amounts up to 4 inches fell in parts of the city and some remained on the ground in shaded areas until the 23rd and 24th.

The mean monthly percentage of possible sunshine is 78 percent. The greatest amount ever recorded was 100 percent in 1924, and the least was 54 percent in 1935.

There are normally 14 clear days, 7 partly cloudy days and 10 cloudy days in the month. The greatest number of clear days was 27 in 1924 and 1925, while the greatest number of cloudy days was 22 in 1957.

2. February Weather


February begins the spring months where warm weather gradually returns to the Desert Southwest. High temperatures slowly rise from a normal of 68 at the beginning of the month to 73 at the end of the month, and nighttime temperatures moderate from 43 to 46. Nevertheless, a nighttime freeze is still a threat in February and into early March. Freezing temperatures can be expected on 3 or 4 nights during February across the valley.

Temperatures in the 90s can occur in February, but such occurrences are unusual. The highest ever was 92 on the 25th in 1921 and on the 27th in 1986, and dropped as low as 24 on the 7th in 1899 and on the 8th in 1933. The coldest February on record was in 1939 with a mean temperature of 48.6 and the warmest was 66.0 in 1991.

The normal rainfall for the month is 0.68 inches, and usually there are four days with 0.01 inches or more of rain. The month can be counted on to have at least one day with a thunderstorm, but in 1931 there were five days with thunderstorms. As much as 4.64 inches of rain have been measured in February back in 1905 and none fell in 1912, 1967, and 1984. There was 0.5 inches of snow on February 2, 1939, and a trace on three other days of the same month. A trace also fell on three consecutive days in February 1985 and a trace in 1994.

There are usually 13 clear days, 7 partly cloudy days, and 9 cloudy days with an expectancy of 80 percent sunshine during the month.


3. March Weather


In March temperatures begin to warm noticeably. The average daily high temperature rises from 73 to 79 during the month and temperatures above 85 are not uncommon in the last days. At the same time, average daily low temperatures rise from 47 to 52. However, it should be remembered that even by the end of the month there is still a 20-percent chance of a 32 degree temperature in the coldest sections of the valley.

The normal mean temperature for the month is 62.2. The warmest March on record occurred in 1972 when the average temperature was 70.6, and the coldest occurred in 1897 with 54.3 degrees. The highest temperature ever recorded on a March day was 100 on the 26th in 1988. The lowest temperature was 25 on the 4th in 1966. Some March days can still be cold, and a high temperature of only 49 was observed on the 2nd in 1915. At the other extreme, the temperature did not fall below 74 on the 28th in 1986. This is warm even for early summer.

Rainfall, during March, averages 0.88 inches. As much as 4.82 inches was measured in 1941, and none was recorded in 1933, 1956, 1959, and 1984. Four days with measurable rain can be expected during the month, but in 1905 there were twelve such days.

Snow has been observed only four times since 1896. Two-tenths of an inch fell on the 12th in 1917, and a trace fell on the 3rd in 1976, and on the 21st and 27th in 1991.

The month averages 9 cloudy days but has had as many as 15 in 1966. There was only 1 cloudy day in 1917 and 1988. Although not especially a sunny month, March still averages about 84 percent of possible sunshine. In 1988, there was 99 percent sunshine, and in 1935 there was a little as 60 percent.

4. April Weather


Elsewhere in the nation, April is greeted generally as the first month of spring:

"April and May are the keys of the year".
"April showers bring May flowers", etc.

But in Phoenix, spring has been under way for some time before April makes its appearance. The average high temperature rises from 80 at the beginning of the month to 89 by the end. One-hundred degree temperatures are unusual. In 1989 a 105 degree temperature was recorded on the 20th and on the 29th in 1992. The average number of days with temperatures of 100 or higher is less than one. Many years have none at all, but there were nine days with 100 or higher in 1989. Nighttime temperatures rise from an average of 52 on the 1st to 59 on the 30th.

The mean temperature for the month is 69.9, and it has been as warm as 80.1 in 1989 and as cold as 62.4 in 1967.

April's rainfall continues the downward trend toward the May minimum, and the normal is only 0.22 inches. In 1926, 3.36 inches fell, and the last April without any rain was in 1993. Over the years, however, the month averages two days with measurable rain. The most measurable rain days occurred in 1926 with 13.

Winds become a bit more gusty in April with the increase in heating, and gusts on the order of 20 to 24 m.p.h. or higher can be expected on eight days and even gusts of 40 to 44 m.p.h. on one day.

April's sunshine averages 89 percent of possible, reaching a high value of 98 percent in 1954, 1961, 1989, and 1991, and a low of 68 percent in 1926.

Six cloudy days can be expected, but there were as many as twelve in 1959. The last April with no cloudy days occurred in 1920.

5. May Weather


James Russell Lowell said in Under the Willows, "May is a pious fraud of the almanac". In Phoenix this is only too true! In most sections of the Nation, May brings true spring weather; but in the Desert Southwest, it signals the beginning of the long hot summer.

The average date of the first 100-degree temperature is May 14th. Such a temperature reading has been observed only once as late as June 18th in 1913, and 100-degree temperatures are not spring-like!

The average high temperature is 93.6, and the average low temperature is 63.9. The temperature has reached as high as 114 on the 30th in 1910 and has dipped as low as 39 on the 3rd in 1899. Fortunately, these are rare exceptions.

May signals the beginning of the dry season. The month averages only 0.12 inches of rain and is the driest month of the year. The most rain ever recorded in May was 1.31 inches in 1930. The month averages only one day with 0.01 inches or more of rain, and it has had as many as seven such days, but no more. This happened in 1992. In contrast, there were 14 rainy days in February 1905. Thunderstorms occur on the average of once a month, but few can be seen in the distance on about three other days in the month.

Cloudy days are unusual and occur on only about four days. Sunshine on the average reaches the 93 percent level. Never has any May had less than 79 percent of possible sunshine, which was in 1992.

At the same time, May humidities, like June, are generally extremely low in comparison with those of July and August. Perhaps May should be appreciated more than it is because it is nature's way of conditioning residents for the steamy summer season of higher temperatures and humidities.

6. June Weather


June is the 2nd driest and one of the three hottest months of the year. The normal rainfall amounts to only 0.13 inches, but as much as 1.70 inches was measured in 1972. There is usually only one day when 0.01 inches or more of rain falls, and the greatest number of such days was only four. This happened only twice, in 1899 and 1932.

The normal mean temperature for the month is 88.2 and has ranged from 93.8 in 1990 down to 79.0 in 1965. In early June, the normal daily high temperature reaches 100 or higher and stays there until the middle of September. Average nighttime temperatures rise from 68 on the 1st to 78 on the 30th. However, on the 27th in 1990, the temperature did not fall any lower than 93. The month averages three days of 110 degrees or higher, but in 1974 there were eighteen such days. The highest temperature ever recorded at Phoenix was 122 degrees on June 26, 1990.

Despite the increasing heat, the air is very dry with the relative humidity even slightly lower than that of May and the lowest of the year. Afternoon readings on the average dip as low as 11 percent. Sunshine is at its maximum and averages 94 percent. June 1916, 1917, 1928, and 1939 all had 100 percent sunshine. The lowest ever recorded was 78 percent in 1931. There are usually only two cloudy days, and the most that has ever been observed was six in 1956.

Toward the end of June, more thunderstorms become visible in the distance along the mountains heralding the arrival of the annual Arizona monsoon, that hot and humid period of midsummer.

7. July Weather


July is the month in which the Arizona monsoon usually arrives from the subtropical latitudes. This monsoon features an inflow of a deep blanket of moisture along with the usual summer high temperatures. A useful definition of a "monsoon day" for the Phoenix area is any day during which the average of the hourly dew point temperatures equals or exceeds 55 degrees F. Over the period of record, the average date of the first day of this event is July 7th.

The monsoon season generally begins in early July and extends through the middle of September; however, it began as early as June 16th in 1925 and as late as July 25th in 1987. The monsoon is not necessarily a permanent feature but may come and go, giving residents brief respites from the muggy weather. On the average there are about 20 monsoon days in July, but there were as many as 31 days in 1984 and as few as seven days in 1987.

Increased thunderstorm activity accompanies the arrival of the monsoon. Thunderstorms are visible on the average during 25 days of the month. On six or seven of these days, the storms are close enough for thunder to be heard at Sky Harbor International Airport. Dust storms associated with these thunderstorms are rather common.

With the increase in thunderstorm activity comes an increase in rainfall. The average for the month is 0.83 inches, but 6.47 inches fell in 1911. The month averages four days with 0.01 inches or more of rain but has had as many as thirteen days in 1896. The probability of a trace or more of rain on any particular day rises from 21 percent on the first of the month to a peak of 54 percent during the last ten days as the monsoon influence intensifies. July is the windiest month of the year with an average velocity of 7.1 m.p.h.

July's average temperature of 93.5 degrees is the highest of the year, and the month features an average high temperature of 105.9 and an average low of 81.0. The temperature has reached as high as 121 on the 28th in 1995. The lowest ever recorded was 63 on the 4th and 5th in 1912. There are usually 26 days with 100 or higher and five days with 110 or higher during the month. In 1989, there were 16 days with 110 or higher. Nighttime temperatures generally fall to around 80, however, it did not fall below 93 on the 20th in 1989.

8. August Weather


By August the summer heat begins to moderate slightly, but humidities are higher than in July. This makes many residents feel that it is much hotter than it really is. Actually, the average high temperature is 103.7 or 2.2 degrees lower than in July.
August still averages 23 days with temperatures of 100 or higher and 2 days of 110 or higher. It has had as many as 31 days of 100 or higher in 1973 and 1975 and in several earlier years. There were as few as 9 such days in 1955. In 1995, there were 10 consecutive days of 110 or higher.

Rainfall in August averages 0.96 inches, the second highest of any month of the year. The most rain ever recorded in August was 5.33 in 1951 and the least was a trace in 1973 and 1975. July is the only other month that always has had some rain.

Thunderstorms and "dusters" are most frequent in August with an average of 7 thunderstorm days, and thunderstorms clouds are usually visible on 24 days of the month.

Sunshine averages 85 percent during the month, but there are usually 4 cloudy days. In 1957 there were 10 cloudy days, and in 1953 and several other years there were none.

9. September Weather


September usually signals the end of the monsoon season. Although the long sustained periods of high humidity that occur principally in July and August may have ended, periods of high humidity do still occur in September. Twelve days in the month can usually be classified as monsoon days.

The average monthly temperature is 85.6 or 7.9 degrees lower than that of July. On the 1st of the month the average high is 102 and the low 77; by the 30th the average high is down to 94 and the low 68. Readings of 110 or higher are uncommon, but there were four such days in 1945 and 1982, and a reading of 116 was observed on the 1st in 1950. The hot weather is not completely over as there usually are on the average 13 days with 100 or higher during the month. The average date of the last 100 degree temperature is the 27th. The lowest temperature ever recorded was 47 on the 20th and 21st in 1965.

Rainfall averages 0.86 inches. The most ever recorded was 5.41 inches in 1939, and the least was none in 1953, 1957, 1968, 1973, and 1988. The month averages three days with 0.01 inches or more, but in 1939 there were nine such days.

There are usually four days with thunderstorms and nine other days when thunderstorms are visible over the surrounding mountains.

The rainfall patterns change in September from the evening thundershower pattern of the monsoon season to a more generally distributed pattern more typical of the winter months.

There are usually three cloudy days, which next to June's two days, make it one of the least cloudy months of the year. Sunshine averages 89 percent of possible.

10. October Weather


During the month of October, weather is generated more from storm activity over the Pacific Ocean than from the subtropical moisture. High pressure systems that bring Indian Summer to the eastern sections of the Nation pass across the Great Basin area to the north, and dry, cold fronts often pass southward across Arizona, drying out and cooling the air.

The average temperature for the month is 74.5 or 19.0 degrees cooler than July. Daytime high temperatures usually begin the month at 94 but cool off to 82 by Halloween. Nighttime low temperatures drop from 67 to 54. These are the largest changes in normal high and low temperatures that occur during any month of the year. Record temperature extremes for the month range from 107 in 1980 to 34 in 1971. October does average one day each year with at least 100 or higher. The latest in the season that a temperature of 100 or higher has ever occurred was October 20, 1921.

Rainfall amounts to only 0.65 inches on the average. Rain falls more evenly during the 24 hours in contrast to the summer months when it is concentrated during the night.

The most rain that ever fell since records began was 4.40 inches in 1972. There is usually one thunderstorm day during the month.

The month averages four cloudy days. It averages 88 percent of possible sunshine and has never had less than 65 percent.

On the whole, October is a magnificent month with enough of the summer warmth to make outdoor living the most enjoyable of the year and yet with enough coolness to make it invigorating.

11. November Weather


With November usually comes the first 32 degree temperatures or below in the valley. Although the average date of such occurrence is December 12th at Sky Harbor International Airport, it usually occurs by:

November 21 in Buckeye
22 in Tempe
23 in Litchfield Park
...............24 in Mesa
...............25 in Deer Valley

The average temperature for the month is 61.9. Daytime high temperatures at the beginning of the month usually are about 81 and nighttime lows are normally 54. By the end of the month these temperatures have dropped to 70 and 44, respectively. The highest reading was a 96 on the 1st and 2nd in 1924, and the lowest was 27 on the 23rd in 1931.

By November the area is definitely under the influence of weather systems of more northern latitudes, and rainfall averages 0.66 inches. The most rainfall ever recorded was 3.61 inches in 1905, and the last time that no rain fell during the month was in 1980.

Snow has been observed only once since 1896. One-tenth of an inch was measured on the 28th in 1919.

Sunshine averages 83 percent of possible. However, there has been as much as 98 percent reported in 1948 and 1956 and as little as 62 percent in 1965 and 1982. On the average there are 18 clear days, 6 partly cloudy days, and 6 cloudy days.

The average wind velocity for the month is 5.3 m.p.h., and the strongest peak gust ever recorded was 60 m.p.h. on the 30th in 1982.

Unlike other sections of the Nation, this month is not the melancholy time that precedes the depressing winter months, but rather it is an invigorating month of sparkling days and cool nights.

12. December Weather


By December, freezing temperatures in the valley are rather common, and freezing temperatures can be expected somewhere in the area on fifteen to twenty days of the month.

High daytime temperatures on the first of the month are about 69 and taper off slowly to 65 by the end of the month. Nighttime temperatures drop from 44 to 40. The highest temperature ever recorded was 87 on the 10th in 1950, and the lowest was 22 on the 31st in 1900 and on the 26th in 1911.

Total rainfall for the month averages 1.00 inch and is the highest of any month of the year. There are usually four days with 0.01 inches or more. Pacific storm systems move a little farther south, bringing more moisture to replenish the water supply with snows in the mountains. The most rain recorded was 3.98 inches in 1967, and the least was none in 1900, 1901, 1917, 1958, 1973, and 1981.

A trace of snow has been reported on eight December days since 1896; 0.1 inches fell on the 11th in 1985, and 0.2 inches on both the 21st and 22nd in 1990.

Sunshine now averages 77 percent with nine cloudy days. There was 98 percent of possible observed in 1958 and as little as 47 percent in 1914.


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 and Atlantic 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:

StationCountyData Began

Fort DefianceApacheDecember 1, 1851
Camp CrittendenSanta CruzDecember 1856
Fort MohaveMohaveJune 1859
Fort GrantGrahamDecember 1, 1860
Camp GoodwinGrahamAugust 1864
Fort Whipple (Prescott)YavapaiJanuary 1865
Fort McDowellMaricopaSeptember 1, 1866
Camp WallenCochiseNovember 1866
Camp Date CreekYavapaiJanuary 1867
Fort BowieCochiseAugust 1, 1867
Camp Willow GroveMohaveNovember 1867
Camp RenoGilaFebruary 1, 1868
Fort Verde (Camp Verde)YavapaiFebruary 1, 1868
Camp HualapaiYavapaiDecember 1869
Fort YumaYumaJanuary 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 at Sky 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 AIRPORT1128 Feet
ELEVATION OF IVORY TIP OF BAROMETER1109.31
ELEVATION OF STATION PRESSURE1107
ELEVATION OF GROUND AT HYGROTHERMOMETER1110
ELEVATION OF GROUND AT WIND VANE AND ANEMOMETER1110
ELEVATION OF CLIMATOLOGICAL STATION1107
ELEVATION OF GROUND AT OFFICE1106
ELEVATION OF HYGROTHERMOMETERAbove Ground, 5
ELEVATION OF WIND VANE AND ANEMOMETERAbove Ground, 33
ELEVATION OF SUNSHINE SWITCHAbove Ground, 7
ELEVATION OF PYRANOMETERAbove Ground, 6
ELEVATION OF RAIN GAGEAbove Ground, 5


Acceleration of Gravity at Phoenix: 979.428 cm/sec2

Boiling Point of Water at Phoenix: 210oF
Temperature Conversion F to C (insert here)
Precipitation Conversion I to M (Insert Here)
Pressure conversion I to M (insert here)

IV. TEMPERATURE


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

MonthMAXIMUMMINIMUMMEAN

January 65.9 41.253.6
February 70.7 44.757.7
March 75.5 48.862.2
April 84.5 55.369.9
May 93.6 63.978.8
June103.5 72.988.2
July105.9 81.093.5
August103.7 79.291.5
September 98.3 72.885.6
October 88.1 60.874.5
November 74.9 48.961.9
December 66.2 41.854.1

Annual 85.9 59.372.6

HIGHEST MEAN AND LOWEST MEAN BY MONTHS AND YEAR OF OCCURRENCE 1896-1995

MonthHIGHESTYEARLOWESTYEAR

January 61.41986 43.21937
February66.01991 48.61939
March70.61972 54.31897
April80.11989 62.41967
May87.01984 69.01917
June93.81990 79.01965
July97.41989 85.41912
August95.81981 84.61918
September91.01983 78.91900, 1912
October82.41988 66.21916
November66.61995 55.01922
December61.31980 46.61911

Annual77.01989 67.81964

HIGHEST AND LOWEST MEAN MAXIMUM AND HIGHEST AND LOWEST MEAN MINIMUM
BY MONTHS AND YEAR OF OCCURRENCE - 1896-1995
MEAN MAXIMUMMEAN MINIMUM
HIGHESTYEARLOWESTYEARHIGHEST YEARLOWESTYEAR
January 74.2198653.5194949.8199331.91937
February 78.7199160.5193953.2199133.41964
March 86.7197267.31897
1958		57.2198640.31917
April 94.2198975.1190065.9198947.51967
May101.2198483.7191772.8198454.21917
June108.6197496.0196780.8198160.61965
July109.8198997.7191285.0198973.11912
1913
August107.3196297.0195584.7198171.31900
September103.8197993.6190080.6198363.61912
1965
October 95.8195280.7191970.3198850.81923

November 82.3194968.5193154.6199537.31916

December 74.9198059.2190948.21977
1991		32.61916
Annual 89.8198978.8195364.1198953.11917

 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
1896-1995

HIGHESTLOWEST
MAXIMUMDAYYEARMINIMUMDAYYEAR
January 8819197116 71913
February 9225
27		1921
1986		24 7
8		1899
1933
March10026198825 41966
April10520
29		1989
1992		35101922
May11430191039 31899
June12226199049 41908
July12128199563 4
5		1912
1912
August116 1
4		1972
1975		58201917
September116119504722
20
21		1895
1965
1965
October1071
2		1980
1980		34301971
November961
2		1924
1924		27231931
December 871019502231
26		1900
1911
Annual122JUN 26199016JAN 71913
Temperature

LOWEST MAXIMUM AND HIGHEST MINIMUM
BY MONTHS AND DAY AND YEAR OF OCCURRENCE
1896-1995

LOWESTHIGHEST
MAXIMUMDAYYEARMINIMUMDAYYEAR
January39 6
21		1913
1937		61271988
February46 6
8		1899
1903		65211996
March49 2191574281986
April52 1194977271987
May54 1191586291983
June68 2189993271990
July79 1191193201989
August7327195192101995
September6623189590 3
3		1982
1983
October5630195982 11987
November4028191971 11988
December3610189859 11949
Annual36 DEC 10189893 JUN 27
JUL 20		1990
1989

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

GREATESTLEAST
RANGEYEARRANGEYEAR
January691971331995
February611921311993
March671966351980
April621949441931
May661910401981
June631929381986
July501905331959
1981
August511918
1962		311955
September581945351981
October 661917331983
November671931381986
December581911351992
Annual961990771984
1991
1992

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

HOTTESTYearCOOLESTYear
93.4198184.11912
93.3198984.21913
92.7198884.41923
92.1199484.61965
92.0197785.11964

HOTTEST THREE
SUCCESSIVE MONTHS		HOTTEST TWO
SUCCESSIVE MONTHS
Temp(Combination)Temp(Combination)
94.8June, July, August 198195.6July, August 1989
94.4June, July, August 198895.5July, August 1981
94.4June, July, August 198995.1July, August 1988
93.9June, July, August 198594.8July, August 1991
93.8June, July, August 199494.7July, August 1985
93.5June, July, August 197794.6July, August 1977
93.2 June, July, August 198694.6
94.6
94.6		July, August 1994
July August 1995
July, August 1995

HOTTEST MONTH
TemperatureMonthYear
97.4July1989
96.2July1988
95.8August1981
95.6July1980
95.5July1983
95.3August1994

WARMEST AND COLDEST WINTERS 1896-FEBRUARY 1995
(December, January, February Combination)
WARMESTYearCOLDESTYear
60.61980-198149.31963-1964
59.41985-198649.81948-1949
58.61991-199250.01916-1917
58.51990-199152.21945-1946

COLDEST TWO
SUCCESSIVE MONTHS		COLDEST MONTH
48.0January, February196443.2January 1937
48.2December, January1936-193744.6January 1949
48.3December, January1948-194946.6December 1911
48.4December, January1931-193247.0January 1932
48.7January, February194947.1December 1916

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

HIGHEST HOURLY DEW POINTS


Dew Point (F)MonthDayYear
79July191957
78August91977
78August201978
77August11951
76July101899
76August101913
76August41943
76July311945
76July171953
76July221966
76August191966

HIGHEST DAILY AVERAGE DEW POINTS

Dew Point (F)MonthDayYear
74August41943
73August191966

HIGHEST MONTHLY AVERAGE DEW POINTS

Dew Point (F)MonthYear
68August1955
67August1943

RECORD LOW DEW POINTS IN DEGREES AND DATES OF OCCURRENCE

1896-June 1995

LOWEST HOURLY DEW POINTS


Dew Point (F)MonthDayYear
-23November281976
-22December211977
-15January291970
-14February21972
-13Februrary31972
-13April131974
-12March101977
-11February41972

LOWEST DAILY AVERAGE DEW POINTS

Dew Point (F)MonthDayYear
-10December211977
-5February31972

LOWEST MONTHLY AVERAGE DEW POINTS

Dew Point (F)MonthYear
20February1972
21March1977

RECORD LOW HOURLY HUMIDITIES IN PERCENT AND DATES OF OCCURRENCE

1896-June 1995
Relative Humidity (%)MonthDayYear
2May81904
2May161907
2May131976
2December211977
2April211979
2June41982

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.
TEMPERATUREDEW POINT HUMIDITY HEAT INDEX
1007545112
1017443112
1026935110
1036934110
1046832111
1056932113
1066729112
1076627112
1086828116
1096626116
1106222113
1116322116
1125918113
1135918115
1145716114
1155615115
1165615117
1175614117
1185614118

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 IntervalTemperatureRelative Humidity
1896-190069.838
1901-190570.139
1906-191069.744
1911-191568.944
1916-192068.948
1921-192570.244
1926-193070.941
1931-193571.840
1956-196071.340
1936-194071.947
1941-194570.541
1946-195071.343
1951-195571.041
1961-196569.638
1966-197070.740
1971-197571.835
1976-198073.546
1981-198574.339
1986-199075.834
1991-199574.637
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.