Speaking of the Weather

Speaking of the... IN ARIZONA WEATHER MEANS PERFECT CLIMATE -WITH THE SUN AS THE

ARIZONANS, visiting out-of-state points, more especially points in the East and Middlewest perhaps, should be prepared to tell not only about Arizona's many famous scenic wonders, but also about its desirable weather and climate. Most people who ask about the climate presume already to know something about it and will make such comments as "They say it really gets hot out there in Arizona" or, "Is it true that Arizona has bluer skies than anywhere else?" or, "They tell me the air is so pure out there that mountains one hundred miles away look to be only about ten miles distant.

Since climate is defined as the average weather over a long period of years, obviously, before we can clearly visualize the former we should first have at least a fair mental picture of the latter. But since climate really includes the extremes as well as the average of the weather elements affecting any given area, it is likewise perfectly obvious that we can speak authoritatively on climate best by referring to the meteorological records that have been kept from day to day, year after year, decade after decade.

Who have kept these records? For many years this important work was done by the Signal Corps of the Army. On July 1, 1890, however, the U. S. Weather Bureau came into existence, and ever since has continuously made observations of the various weather elements. Consequently, it is now possible for us to talk facts about the climate of our country. Fortunately, in Arizona, Uncle Sam's weather observers, both those in the first-order or regular weather stations and those non-paid, highly civic minded so-called co-operative observers, the latter scattered throughout the state, have preserved this long-standing weather picture.

Therefore, we who would inform outsiders of our climate realize the folly of talking indiscriminately. When it comes to our explaining the whys of the weather, it is not enough to consult these records; we should

By Raymond ... Weather DETERMINING FACTOR OF THE IDEAL LIFE IN THE MOST BLESSED LAND OF ALL.

have some understanding of the science of meteorology. For example, according to the various weather-reporting stations in the state, Arizona has a very high percentage of its possible amount of sunshine, and an extremely low daytime relative humidity. The records in themselves do not enlighten us much as to why this situation exists. Only by peering behind the picture and examining operative forces and natural phenomena can we appreciate the wherefores of the weather. Let us then take a squint at the climatological record for different points over the state and raise the backstage curtain that we may glimpse behind scenes. Doubtless this climatic scene will be more lucid for us if we will briefly recall certain familiar general principles of geography. This terrestrial ball that we call Earth rotates from west to east, is inclined to the plane of its elliptic at an angle of 23½°, and has certain fairly well defined temperature zones, the colder ones being near the poles, and the hottest about midway between. We should remember that this sphere on which we live is moving in an orbit around the sun, giving rise to seasons. Likewise, let us bear in mind that, as a result of the rotation of the earth and the unequal heating of polar and equatorial areas, fairly well defined air currents, or winds, are set up in the earth's atmosphere. We know, too, that the atmosphere, by reason of its weight, exerts, at sea level, a pressure of about fifteen pounds per square inch of surface. Again, there is a continuous interchange of air from equatorial regions to the frigid zones, giving rise to relatively high atmospheric pressure in the high latitudes and relatively low pressure in the lower latitudes. It is equally important that we take cognizance of the fact that in addition to the well known prevailing winds, such as the trades and the westerlies, blowing over the earth's surface, there is a pronounced so-called secondary circulation or the movement of fields of atmospheric pressure. By observing the weather maps for a few

Consecutive days you may get a picture of the behavior of these pressure areas. What is important from the standpoint of Arizona's climate is the track these "highs" and "lows" generally take in the United States. While the latter may enter the continent from almost any Pacific point, the former, especially in winter, invade the United States mostly from the CanadianPacific northwest, or from the Canadian Rockies. After lancing southeastward across the northern and the central part of our country, which they sting with their icy whips, they elbow northeastward. Coming from the higher latitudes these highs, or anticyclones frequently produce cold waves. Arizona, however, is so far to the southwest that seldom does it suffer from the importation of frigid air by these pressure areas. On the other hand a winter "low" on its march eastward from the Pacific will bring sometimes moderate amounts of moisture to this inland Southwest. Fortunately, then, Arizona misses the more adverse effects of these pressure movements, yet occasionally is appreciably benefited by them. This accounts for Arizona's record breaking rainfall this winter. Other climatic factors, most of which have a somewhat more positive effect on Arizona than does the secondary circulation of the atmosphere are the sun, altitude, topography, position and extent of land and water bodies, especially the ocean and the mountains. The sun, especially, is king in Arizona. According to Huntington, Arizona, during the summer time, receives a cooling drift of air, partly moisture laden, from the Gulf of California. As this air reaches higher altitudes and higher latitudes it is further cooled, thus giving some precipitation. While Arizona lies directly in the path of westerlies, the high Sierras and the Coastal ranges, in forcing the air to rise in its eastwardly moving course, receive most of its moisture on the windward of those mountains. Just as the extreme southwestern location of Arizona puts the state out of reach of the most of the cyclones and the anticyclones, so do the great mountain chains to the west act as barriers of wind, and regulators of moisture continental-bound.

This combination of low latitudes plus the diminution of moisture in the atmosphere gives Old Sol an all-season, year-around ticket in this Arizona weather picture. Even in winter, when an occasional stray atmospheric depression brings unusual precipitation to the state, Old Sol manages somehow to keep in the saddle, and permits Jupiter Pluvius to ride with him only if the latter will ride behind. Old Sol insists on holding the reins, however, and from a corner of his big bright eye keeps tab on the rains. Visitors to Arizona, for the first time at least, usually get a very pleasant surprise when they note the extremely verdant arboreal aspect of the state's desert areas. They expect to see vast stretches of sand, practically devoid of vegetation, as is the case in true deserts. Perhaps the most plausible explanation for this condition is that, while in the majority of the deserts of the world, rain, if any, falls during but one season, usually winter, in Arizona, as hinted above, there are two seasons affording moderate moisture, summer and winter. As a result of this profusion of sunshine and of its intensity in summer, Arizona air, becoming super-heated at the ground surface, ascends by convection. In this process of heat-transference, since the molecules in the medium itself migrate, quantities of air, after reaching higher altitudes, where atmospheric pressure is less than at the surface, condense. This adiabatic cooling of an air mass gives birth to the cumulus cloud. Furthermore, if as usually happens on summer afternoons in Arizona, this uplift of air continues for a few hours, the up-push is so pronounced as to induce the formation of the cumulo-nimbus, or thunderhead cloud. Obviously, since convection is most active at that time of day when the sun-heating is greatest, summer thundershowers in Arizona, rendering more tolerable the high afternoon temperature, and leaving the atmosphere charged with freshness and buoyancy that make breathing itself a joy, do much more than just dress the desert in a vestment of verdure. Thus, while it is the secondary low that is responsible for most of Arizona's winter precipitation, it is the low induced by the sun, or the so-called "thermal" low which is responsible for those blessed summer showers. Obviously, however, if it were not for the oceans, which supply the moisture, and for winds which waft it, the sun could not harness it. Should you visit Arizona, you will hardly fail to note the mountain-valley breezes. The basis for this phenomenon is much the same as for the land-sea winds. In Arizona it would be difficult, climatically speaking, to over-emphasize the effects of topography. The plateaus of almost horizontal strata 5,000 to 7,000 feet high are sharply separat-ed from the lower basin country by the great Mogollon Escarpment, a line of southward facing cliffs extending from west-northwest to east-southeast, almost across the state, dividing it almost in the center. This south-facing slope, situated to receive more insolation than are the more horizontal valleys threading southward, is warmed more than the valley areas during the daytime. This induces an up-valley drift in the atmosphere. At night, however, the more pronounced loss of heat by radiation from the escarpment gives rise to a down-valley breeze. This condition brings cool, refreshing air to resi-dents in the but-recently sun-heated semi-desert sections. If your sleeping porch is on the escarpment side of your house your sleep should be sound and delightful. In Arizona, therefore, with mountains and val-leys constituting so much of the area of the state, the night air, even in summertime, is a lung tonic. In the winter this Mogollon backbone formation catches the slanting rays of the sun, reflecting welcome warmth to people residing along its southern base. Thus both latitudinally and topographically much of Arizona is ideally situated and adapted for affording her inhabitants healthful, invigorating air, day and night, summer and winter.

We mentioned briefly the decidedly brac-ing quality of the atmosphere immediately following a summer shower in Arizona. Perhaps if we emphasize the "why" for this condition, the statement will have more meaning for us. Again, those mountain barriers in California enter the picture. Also the remoteness of Arizona from large urban or factory centers which pollute the atmosphere with smoke and dust. Relatively few mechanical impurities are thus allow-ed to invade the atmosphere of Arizona. Thus, even prior to the showers, the Arizona air is exceptionally pure; however, since every raindrop has as its nucleus a dust particle, it is only logical that the air after the shower is virtually devoid of dust. Certainly, this dust-free condition of the atmosphere in Arizona makes possible its extreme clarity. Furthermore, this enhanc-ed visibility definitely accentuates the azure of Arizona skies. Arizona skies are bluer; you can see farther in her air than else-where; her sunsets are more gorgeous.

For the same reason more and brighter stars are visible in the Arizona night sky; Lady Moon sheds a richer radiance over this land Father Sun nurtures so well.

Having stated that the factor of relativehumidity is of utmost importance in the make-up of Arizona weather, perhaps it would be in order here to discuss this subject somewhat more fully. Relative humidity is defined as the amount of moisture present in the atmosphere at any given time and place as compared with the amount the air there and then is capable of holding at the same temperature. Of course, the atmosphere behaves in accordance with Charles' law, which states that at a constant pressure the volume of a given quantity of any gas varies directly as the temperature to which the gas is subjected. Thus, the higher the temperature, the greater the moisture-holding capacity of the air. It doesn't require an Einstein then to understand why the noon-time relative humidity of Phoenix or Yuma or of any other Arizona point is usually much less than it is at an early morning hour. The same amount of moisture, by reason of the increase in temperature, has been disseminated throughout a much larger volume of air, hence the moisture content per unit-volume is necessarily correspondingly less.

Probably a word as to the method of determining the relative humidity will lend meaning to our saying that sensible temperatures are not determined by high readings of the thermometer alone. The U. S. Weather Bureau determines it by means of either the psychrometer or the hygrograph. The former is merely two like-thermometers (Turn to Page 38)