RAINFALL IN ARIZONA

BY: LOUIS R. JURWITZ

We think of Arizona as the land of sunshine. This is true to a great extent, inasmuch as parts of Arizona and southern California receive the highest percentage of possible sunshine in the United States. However, even in a land of sunshine the life-giving ingredient is rainfall. We are all aware of the fact that civilization could not exist, not only in Arizona but any other place in the world, without rains which replenish soil moisture, keep streams running, fill storage reservoirs, and make possible the existence of our magnificent cities and fertile farm lands in the United States. In Arizona, particularly in the southern desert portions of the state, man is dependent upon stored water, both on the surface in reservoirs, and underground in gravel beds and in large pools that can be tapped by wells. Heat received from the sun is the basic ingredient that is needed to set in motion the patterns that eventually result in the formation of storm systems and precipitation. In a large measure the difference in temperature between land and water masses has a controlling influence on storm patterns, with high level wind circulation patterns distributed around the northern hemisphere acting in response to nature's demand for a state of equilibrium in the atmosphere. Equatorial regions are of course receiving a larger measure of heating than polar regions at certain times of the year, hence the necessity of mixing cold and warm air masses in order to attain a certain measure of equilibrium. In the upper atmosphere, from ten to thirty thousand feet and higher, we find large scale wave patterns wherein ridges and troughs follow each other in patterns around the globe. Alternate areas of circulation from the northwest follow patterns of atmospheric movements from the southwest. Thus, in a large measure, we see one phase of the interchange of cooler air from the north with that of warmer air from the south. These high level circulation patterns are in a large measure connected with surface areas of low and high barometric pressure. We find in the northern hemisphere that two large high pressure cells dominate surface charts; one patterns, with high level wind circulation patterns distributed around the northern hemisphere acting in response to nature's demand for a state of equilibrium in the atmosphere. Equatorial regions are of course receiving a larger measure of heating than polar regions at certain times of the year, hence the necessity of mixing cold and warm air masses in order to attain a certain measure of equilibrium. In the upper atmosphere, from ten to thirty thousand feet and higher, we find large scale wave patterns wherein ridges and troughs follow each other in patterns around the globe. Alternate areas of circulation from the northwest follow patterns of atmospheric movements from the southwest. Thus, in a large measure, we see one phase of the interchange of cooler air from the north with that of warmer air from the south. These high level circulation patterns are in a large measure connected with surface areas of low and high barometric pressure. We find in the northern hemisphere that two large high pressure cells dominate surface charts; one is located in the Pacific Ocean and the other in the Atlantic. The main semi-stationary centers of low pressure are located near the Aleutian Islands and in the Iceland region. These high and low pressure areas are the mechanism which in a great measure accounts for the interchange of air masses from different source regions, thus making it possible for nature to again strive for a semblance of equilibrium in the lower levels of the atmosphere. In the winter time the main source areas for cold air masses are within the confines of large continental areas, mainly in northern Siberia and the northern portion of North America. The source area of warm air masses is located over warm water areas of our principal oceans, for instance, the South Atlantic, the Gulf of Mexico, and the South Pacific Ocean.

We are all aware of simple condensation principles whereby warm air which contains a moderate to large amount of water vapor can no longer contain all of this water vapor when the temperature of the air is lowered, other factors remaining unchanged. This, of course, is one of the main principles in the formation of clouds and precipitation. Due to the difference in density between cold and warm masses of air, we find that the two do not mix to any great extent. One can use as an analogy water and oil, thinking of water in the guise of a cold mass of air and of the oil as a warmer air mass. Should the mass of water and oil collide, we find that the oil will be lifted above the mass of water. This is also true in a large measure when cold and warm air masses collide.

of the main principles in the formation of clouds and precipitation. Due to the difference in density between cold and warm masses of air, we find that the two do not mix to any great extent. One can use as an analogy water and oil, thinking of water in the guise of a cold mass of air and of the oil as a warmer air mass. Should the mass of water and oil collide, we find that the oil will be lifted above the mass of water. This is also true in a large measure when cold and warm air masses collide.

Getting back to the boundary lines between warm and cold air masses, we find that low pressure areas and attendant storm systems form along these frontal boundaries. In other words, between the cold air masses in Siberia and warmer air over the Pacific Ocean we find a boundary line which often results in the formation of storm systems during winter months when temperature contrasts are greatest. Another boundary zone would be between cold air masses from the North American continent and warmer air off the west coast, as well as near the Atlantic Coast during winter.

Storm systems that affect Arizona during the winter months, extending from December through March, normally approach the state from the west. One may divide the storms into two categories: (1) Those approaching Arizona from the northwest, entering the mainland along the WashingtonOregon coast, and (2) the southwest type which approach the west coast from the west or southwest, from the San Francisco Bay area to Lower California. The low pressure areas approaching from the northwest normally result in rather light precipitation, of a showery nature, by the time the effects of the storm reach Arizona. The southwest type, however, brings the best distribution of precipitation over the state, inasmuch as a warm current of fairly moist air often precedes these storms so that periods of precipitation are longer and much more widespread than is the case with the northwest type of storm. Winter precipitation is of utmost importance to the economy of our state. Approximately fifty percent of the annual rainfall in Arizona is received during the period extending from December through March. However, runoff into our reservoir systems from winter storms accounts for approximately eighty-five percent of annual values. This This may seem to be a startling statement, but if we realize the possibilities of winter storms in connection with runoff, we can understand the reason for the high percentage of runoff compared with results of summer storms. Vegetation is dormant, for one thing. Winter storms may be preceded by a fairly heavy snow pack in the high country, which may be melted with the arrival of warm rains, thereby providing what one might term "a double charge" into the river systems of the state. The condition of the soil is an important factor, of course, in the resultant runoff during winter months. Ordinarily, from two to three inches of rainfall is needed to condition the soil before substantial runoff into stream systems can be expected. This is particularly true after a relatively long period without precipitation. Arizona, as we know, lies in what is termed an arid section of the United States. This, of course, implies that the state lies in an area that normally receives relatively small annual precipitation as compared with areas of other climatic structures, such as the Pacific Northwest, the Mississippi Valley or the eastern states. As mentioned previously, storms that enter the mainland from the southwest ordinarily produce the most copious precipitation in Arizona.

OPPOSITE PAGE "STORM NEAR CHINLE" BY WILLIS PETERSON. Chinle is a small village near Canyon de Chelly in the northeastern part of the Navajo Indian reservation in Arizona. The threatening sky added a somberness to the landscape. Here again the components of sky and earth complemented each other to make a dramatic photograph. FIG. 2 GREATEST 24-HOUR PRECIPITATION DEC., JAN., OR FEB.

The Pacific Ocean is a dominant factor in the control of rainfall patterns in the West. Normally a huge area of high pressure extends from the Hawaiian Islands to the mainland and northward to approximately a latitude of thirtyfive to forty-five degrees. This high pressure cell pulsates periodically, shifting its axis from an east-west direction to a north-south direction at times, extending eastward over the Pacific Northwest and Great Basin at times, and retreating from five hundred to a thousand miles off-shore at other times. One might say that it acts as a steering device for storm systems as they move into the western states. If we visualize this high pressure cell overlying the Pacific Northwest and the Great Basin area, we can see that storms forming in the Aleutian Island region which sweep southeastward will meet this barrier of high pressure and its accompanying circulation patterns and be deflected into southern Canada. If the high pressure cell retreats westward so that it is just off-shore, storm tracks will have a tendency to dip farther south, entering the United States over Washington or Oregon. This would result in what has been termed the northwest type of storm system in connection with precipitation over Arizona.

Carrying this picture one step further, if we visualize the high pressure cell as being displaced approximately one thousand miles off-shore, we can see how storm tracks will tend to dip considerably farther south, entering the continent in central California or even farther south before they recurve to the east and northeast. Thus, the pulsation in the location and orientation of this Pacific Ocean high pressure cell has a definite bearing upon rainfall over the West and in Arizona. During the autumn and winter months, beginning about September or October, this high pressure cell retreats off-shore, permitting an orderly procession of storm systems to enter the continental United States. During the early spring and summer months, high pressure often extends inland over the Pacific Northwest and at times covers the Great Basin. Ordinarily this takes place during the latter part of March or April, bringing a halt to winter-type precipitation over Arizona during April, May and June.

One must remember that this description is primarily of the so-called "normal" regime. There is no assurance that a particular sequence of events described will occur in any one year. Hence, winter precipitation totals in western states will vary from year to year, depending upon the number of major storms that enter the mainland and depending, of course, upon the paths that these storms follow.

To those who have lived in Arizona for one or more summers, the question no doubt arises "What about our summer rains?" Arizona ordinarily enjoys a period of summer rainfall, particularly over the central and eastern portions of the state.

The mechanism involved in the generation of summer rains over Arizona is somewhat different from that described for winter storms. A counterpart of the Pacific high pressure cell is found in the Atlantic Ocean and in meteorological circles is given the name of the "Bermuda High." It normally lies off the coast of Florida, covering a large part of the South Atlantic Ocean. This high pressure cell also has its pulsations similar in many respects to those described with reference to the Pacific cell. During winter months the "Bermuda High" shifts eastward. However, under ordinary circumstances we note that this "Bermuda High," together with its wind circulation patterns in the upper atmosphere, begins a westward movement during the latter part of June. Thus, a large portion of the southeastern states and the Gulf of Mexico is dominated by the circulation patterns of this "Bermuda High" at that time of the year. The circulation of the atmosphere around the southern and western sides of this high pressure system is from the east and southeast. Thus, air masses travel long distances over warm tropical waters in the Caribbean and the Gulf of Mexico and become quite warm and moist. As this "Bermuda High" shifts farther westward, moist tongues of air sweep across the Gulf of Mexico and swing into northern Mexico, thence into New Mexico and eastern Arizona. The stage is thus set for the necessary moisture which must be present for widespread thunderstorm activity.

This moist river of air is exceptionally deep at times, extending to the fifteen and twenty thousand foot levels. Mountain barriers in northern Mexico, New Mexico and Arizona often serve as orographic "triggers" when the mass of air is unstable, releasing heavy showers along main mountain ridges. We are familiar with the marked increase of precipitation with altitude. The occasion of this summertype moist stream of air is a good illustration of how terrain affects precipitation amounts. Mountain ranges normal to the flow of the stream of warm moist air act very similarly to domes of cold air or cold fronts. The stream of moist air is forced to ascend, thus it is cooled adiabatically until the condensation temperature is reached, and precipitation results. Besides the effect of mountain ranges, there are occasions when relatively cool air from the Pacific Ocean may overlie the Great Basin area and portions of Arizona, thus acting as a barrier to the warm moist flow of air. Under these conditions, thunderstorms may occur over any portion of the state, not necessarily near mountain ranges as previously described.

While on the subject of thunderstorms, it may be beneficial to give a brief résumé of the characteristics and behavior of thunderstorms. They are classified according to the physical factors which presumably cause them: One of the main features of a thunderstorm is the strong vertical currents which are present in the main cloud structures. The rising currents are the most violent, sometimes exceeding 60 miles per hour. Compensating downward currents also occur, both within and below the clouds, and in the surrounding air. The rising currents carry warm moist air to higher and colder levels where the air becomes saturated and condenses into the droplets which make up the cloud. These cloud droplets continue to be carried upward and reach altitudes where the temperature is below freezing. Initially the droplets do not readily freeze, but rather become "supercooled," that is, remain liquid even though their temperature is below freezing. Eventually at higher elevations some of the droplets freeze, and when this occurs the ice particles grow rapidly at the expense of the remaining water droplets. This is brought about by the difference in vapor pressure over liquid water and over ice, the value being much less in the case of the ice particles. Thus water evaporates from the droplets and condenses on the ice particles. This process continues until the ice crystal reaches a size so large that it can no longer be sustained in the vertical current. It then begins to fall and gathers additional moisture by collision with drops and other ice crystals. Eventually it melts and falls as a raindrop. Rain can also be produced solely by the collision of cloud droplets if there is a suitable size distribution, and this process may be responsible for some of the rain falling from thunderstorms. The formation of hailstones is not well understood at present, but must necessarily involve strong upward currents which hold the stonestone within the cloud long enough for it to gather a great deal of water through collision with supercooled drops. Alternate lifting and sinking above and below the freezing level has been suggested as the way in which the observed concentric layers of ice are formed. The production of thunderstorm electricity is also not thoroughly understood at present. One theory suggests that it is caused by the breaking of raindrops, while another associates it with the freezing of the supercooled water. Regardless of the cause, a thunderstorm builds up two opposite charges within the cloud. When these charges build up to large values, a spark or discharge occurs, which we call lightning. These discharges can take place either within the cloud itself or from one of the charge centers to the ground. The rate of passage of the different development stages of a thunderstorm is totally dependent upon the duration and intensity of vertical currents which produce the cumulonimbus, or what is commonly known as the thunderstorm cloud. The mechanism described holds true for all four categories of thunderstorms mentioned earlier. Frontal thunderstorms usually occur along a line which demarcates the advancing edge of a mass of cold air. Orographic thunderstorms are very similar, as we mentioned earlier, with mountain ranges acting as permanent "fronts." Thunderstorms occasionally form in regions of falling pressure, where one notices a light pressure trough in isobar patterns. This indicates that air currents are converging horizontally and "stretching" vertically. If the air is originally unstable and quite moist, the lifting which takes place by this stretching process may be sufficient to set off thunderstorm activity.

A thunderstorm is one of nature's most spectaculardemonstrations. We all realize that a tremendous amount of energy is released during the life cycle of an average thun-derstorm. It is estimated that approximately forty-four thousand flashes of lightning occur each day over the world. Each flash is equivalent to about twenty-five hundred horse-power applied continuously for twenty-four hours, which illustrates in a startling manner the tremendous forces that are released by these phenomena.

demonstrations. We all realize that a tremendous amount of energy is released during the life cycle of an average thun-derstorm. It is estimated that approximately forty-four thousand flashes of lightning occur each day over the world. Each flash is equivalent to about twenty-five hundred horse-power applied continuously for twenty-four hours, which illustrates in a startling manner the tremendous forces that are released by these phenomena.

It is hoped that this brief discussion of thunderstorms will give you a better conception of what is taking place within innocent looking cumulus clouds that are observed as they they develop into towering clouds and even-tually into full-fledged thunderstorms.

Shifting our attention back to the moist rivers of air that occasionally flow over Arizona, we notice that there seems to be a periodical intrusion of moist air into Arizona in cycles ranging from one to two weeks in length. Ordinarily the injection of this moist air into Arizona will last for three or four days, when thunderstorm activity reaches its maximum. Even after the flow of moist air is cut off, the transition which takes place as the air becomes drier and more stable will last for another three or four days, with diminishing thunderstorm activity.

The orientation of mountain ranges is a vital factor in the frequency of thunderstorm occurrence. For example, the Mogollon Rim country will produce an area of maximum thunderstorm activity with southerly currrents of warm moist air. With southeasterly currents the flanks of main mountain ranges such as the Bradshaws and Pinal Mountains, and portions of the Rim section oriented in a more northeast and southwest direction, will have areas of maxi-mum thunderstorm activity.

Just a few words in connection with tropical storms or hurricanes, as we more commonly call this type of storm. While Arizona has never been subjected to the impact of a full scale hurricane, the state has received some of its largest rainfall totals as a result of hurricane activity. Normally, hurricanes lose intensity very rapidly after entering a land mass. However, the tremendous currents of tropical moist air that were involved in the hurricane are injected into the general circulation patterns of the atmosphere. Hurricanes which may produce rainfall patterns over Arizona may be those which form off the west coast of Mexico and enter the mainland north of Mazatlan, or hurricanes which sweep across the Gulf of Mexico and enter the mainland on the east coast of Mexico. An example of the latter type was the extremely heavy rainfall that occurred in the central portion of Arizona between August 26-30, 1951. The hurricane that was involved in this particular case swept across the Gulf of Mexico and entered the mainland a short distance north of Tampico on August 22nd. The stream of exceptionally moist tropical air, that was involved in the hurricane after it broke up upon entering the mainland, was injected into the main circulation patterns of the upper atmosphere, which at this time were from the southeast. Thus, the river of moist air reached southern Arizona by August 26th, when a few isolated heavy thundershowers occurred. The main trajectory of the moist air entered Arizona in the vicinity of Organ Pipe Cactus National Monument and proceeded north-northeastward on August 27th and 28th, striking the main ramparts of the Bradshaw Mountains, where the heaviest concentration of rain occurred. Crown King, for instance, received a five-day total of 13.56 inches. The moist air currents were caught up in a westerlytrajectory and resulted in another high intensity area over the Mazatzal Mountains. Sunflower had a total of 12.11 inches for the period. By this time most of the moisture was depleted from the air and only moderate to light rains fell over northern and eastern portions of the state. Approximately twenty-five new 24-hour rainfall records were established during this storm. The greatest 24-hour total occurred near Bagdad, with 5.38 inches recorded; Castle Hot Springs received 5.32, and Crown King 5.31 for the three highest one-day totals recorded during the storm period.

FIG. 3 GREATEST 24-HOUR PRECIPITATION JULY, AUG., OR SEPT.

A search of weather records in Arizona indicates that a majority of record rainfall totals of high intensity storms were connected with tropical air that was once a part of a hurricane before its arrival over Arizona.

Figure 1 illustrates the average annual precipitation distribution in Arizona. In most cases these averages were based on thirty years of rainfall records extending from 1921 through 1950. As can readily be seen, high annual totals are limited to elevated sections of Arizona. If the twenty-inch isohyetals were superimposed on a topographic chart, they would in a large measure cover the highest elevations of the state. This illustrates the tremendous effects that mountainous sections of Arizona have in the rainfall picture. The chart also illustrates quite graphically the relatively small sections of the state that must contribute the major portion of runoff in connection with storage of water in reservoirs and underground channels. The average annual rainfall totals vary from 3.12 at the Yuma Valley substation of the Weather Bureau to 25.25 at Bright Angel Ranger Station on the North Rim of the Grand Canyon. The average annual rainfall for the entire state is 13.90 inches. Monthly average precipitation for Arizona is distributed as follows: Many months have seen absolutely no rainfall over the State of Arizona, with the highest monthly precipitation ever recorded in the state standing at 16.95 inches at Crown King in August, 1951. Maximum recorded point rainfall data in Arizona are quite limited. However, an illustration of what might be expected is reflected by the record for Phoenix, Arizona, which is as follows:

Figures 2 and 3 contain the distribution of maximum 24-hour rainfall ever recorded at each station in Arizona during winter and summer seasons. One must bear in mind the limitation of these two charts. The length of record at individual stations varies considerably. Hence, the values indicated should not be considered as limiting values, but merely as an index of what can be expected in the way of high intensity rainfall patterns in Arizona.

Figure 2 contains the greatest 24-hour precipitation amounts recorded during any one day in December, January and February. Hence, it reflects the distribution of winter high intensity rainfall patterns over the state. One might trace the four-inch or greater rainfall areas to obtain some idea of the very small section of Arizona which is subject to precipitation of that magnitude during winter months. An inspection of Figure 3, which contains the same information for the summer months of July, August and September, illustrates the high intensity nature of summer rainfall in Arizona. The four-inch isohyetal line encompasses an area many times larger than that depicted in Figure 2. It also is a graphic illustration of the tremendous effects that mountain ranges have on the distribution of high intensity summer rainfall patterns in the state. For instance, the areas receiving five or more inches of rainfall on any one day are restricted to the Bradshaw Mountains, Mazatzal Mountains, the Pinal Mountain range, and the rugged mountainous terrain north of the Bill Williams River. However, the fourinch isohyetal envelops most of the principal mountain ranges in Arizona. Highest daily values were in most cases connected with tropical moist air masses that were the remnants of decaying hurricanes, as described earlier.

The location of records which show five or more inches within the 24-hour period in Arizona are as follows:

As mentioned earlier, these are merely an index of the intensities that can be expected in Arizona, inasmuch as the length of records at stations varies greatly.

The Weather Bureau maintains climatological records at each of its Section Centers distributed over the United States. The Section Center for Arizona is located at Phoenix and it is the repository of all climatological data for the state. Hence, if a need develops wherein climatological records should be consulted, one can call on the Section Center.

This discourse on the cause and effects of storms that visit Arizona has been but a very brief digest of the subject. Many books and papers have been written on single phases of the description that was given.

Should your interest in meteorology be aroused to some extent, or if a clearer picture has been created of forces and patterns which are involved in the production of weather in Arizona, my efforts have been rewarded. High intensity rainfall creates a real problem for engineers. It is hoped that the data presented can in a measure help to solve some of the problems arising in construction and maintenance.