THUNDERSTORM

THUNDERSTORM ARIZONA SKY... BIG GAME HUNT

THE GAME-A THUNDERSTORM OPEN SEASON-JULY THROUGH SEPT. WHERE-ARIZONA-ANYWHERE SOUTH OF THE MOGOLLON RIM Equipped with a couple of 'high powered' cameras, the 'rifle' loaded with black and white sheet film, and the 'side arm' with color film, let's start out early some July day with our equipment, the necessary canteen of water, a great deal of patience and sufficient 'ammo' in search of what is one of Arizona's greatest spectacles, the desert thunderstorm.

If further analogy is permitted, we may say that a thunderstorm, like all game, is also born of dam and sire. However, here the dam is a great stream of incoming moist air, and the sire is a mountain range which sets off the motion giving birth to the storm.

Most of the mechanics of the three types of thunderstorms were adequately discussed in a previous article, entitled "Rainfall in Arizona" (see ARIZONA HIGHWAYS, July, 1954). Here, however, we are solely concerned with the orographic or mountain storms, so let us set off on our big hunt to the mountains where 'last season we saw some big ones.' On the day of the hunt, the morning breaks bright and clear, which makes the granite peaks appear at hand rather than miles away as they actually are. For several days now, moist, warm, convectionally unstable air has been moving northward and westward from over the warm waters of the Gulf of Mexico and spreading out over Arizona's valleys and foothills until the river is now wide and deep. It is this air in which the thunderstorm will be given birth and nurtured and will grow, phase by phase, until it reaches its prime, releasing its pent-up fury in wind and rain and lightning. It is is mid-morning by the time we have driven our four-wheeled burro as far back into the hills as it will go, and strapped on our hunting gear. The temperature in the lower levels of air which cover the broad valley behind us has begun to rise as the day progresses, and these lower levels of air are becoming less dense with the warming. As a result, a faint breeze is beginning to stir as heavier air flows down from the higher slopes, buoying up the warmer air in the valley. This advection and convection of air will continue to grow in intensity and soon now-about 11:00 A.M.-little white fair-weather clouds -cumulus lumilus-will appear above the mountain crests (photo #I) and slowly build valleyward. On a typical day, this valley cumulus will heighten and broaden a little and then remain with little further activity. But not so for our 'big game on the mountains.' Here, the combination of uplift of convection coupled with the orographic uplift of the air as it strikes the mountain and rises, causes the cumuli to grow as the condensation level of the air parcel is reached and the air which was latently unstable at sunrise has now reached absolute instability. If this instability is of sufficient magnitude, the thunderstorm-to-be is sired.

By early afternoon we have spotted our 'game,' so here we check and load our equipment and settle down for the big shoot as we patiently wait for the storm to come of age.

Unlike thunderstorms elsewhere, the desert thunderstorm, due to the drying effect of the desert, is, on most occasions, of a high-level type. For this reason, more diurnal heating and subsequent convection and advection is required to set the system in motion, reaching its peak activity usually after sunset to midnight, or even later. In addition, one theory proposes that the 'after sunset or nocturnal thunderstorm' is brought about in part by the radiational cooling of the air at upper levels of the cloud. For after the sun sets, the clouds and the air above it begin to cool rapidly, and as air cools it becomes more dense, thus falling, and warm energy-laden air at lower levels is forced upward adding to the growth of the storm.

Well, by now our fair-weather cumuli along the mountains have grown to considerable size and a great billowing flat-based cumulus congestus has formed with its typical cauliflower structure, showing evidence of the great motion and turbulence now taking place within the cloud. It is sometimes observed that the towers of cumulus congestus break through minor inversions in the upper atmosphere, in which case a veil or pileus forms around and above the towers. This is brought about by the turbulence which has transported so much moisture up to and under the base of the high level temperature inversion that when this air is lifted by the ascending towers, it becomes saturated. The cloud is then called cumulus pileus.

As the cloud grows further it forms into layers, the top of the cloud changing into ice or snowy crystals. It has then developed into a cumulo-nimbus-a shower or thunder cloud. When this occurs, the cloud begins to lose its cauliflower structure, the protuberances become less pronounced and the crevices gradually disappear. Watching for this transition, we often see it take place just at sunset, and few sights are as beautiful as the shadows taking over and obscuring the earth's details with a veil of purple, while yet the great crests of the cloud are bathed in orange and red and gold (photo #II) of the sun, now setting beyond the mountains to the west. With the coming of night, the upper portion of the cloud is changing into a tangled web with only slight traces of convective elements, and during this process precipitation may be released in the form of virga if the air below is very dry, or rain if complete evaporation does not take place. With continuing change the cloud's uppermost reaches have fanned out into a great anvil shape which may cover a great deal of the sky far from the heart of the storm. This cloud is called cirrus densus and may reach levels of over 20,000 feet, representing the greatest height to which convective or orographic activity reaches in the atmosphere. Now the wind is freshening and lightning is beginning to play through the cloud and from the cloud to ground. As the storm approaches, the wind first blows toward the storm and is characterized by its burden of dust and warmth as it comes from the desert, often obscuring the setting sun (photo #III). As the storm moves overhead, the wind reverses direction and blows out from the storm in a forward direction. This air, through which rain has just fallen or is falling, smells fresh and clean, often bearing the moist odor of creosote bush. Rather than warm, it may often be as much as 20°F cooler, for it has been cooled by the evaporation of rain which has passed through it. As the forward half of the storm passes, the precipitation becomes less intense and gives way to a more continuous rain of smaller and smaller drops, gradually decreasing in intensity until it ceases. By now the sun has set and only the more prominent features of the cumulo-nimbus are apparent in the upper light of the sky. If we have been fortunate enough to have kept our 'powder dry' by choosing a storm that has moved around us rather than over us, we probably will be able to do some shooting of the lightning which so often accompanies our thunderstorms. Here, some inspiring shots can be made by merely setting our cameras on tripods, aiming in the direction of the greatest activity and with the shutter on "Time" making an exposure of several minutes depending upon the frequency of the strikes. On the night of at of July 3, 1954, shortly after 10:00 P.M., one such three-minute vigil produced a photograph with over 20 strikes on it (photo #IV). Just barely noticeable is Squaw Peak and Camelback Mountain silhouetted against the lighted sky. The camera set-up was about 30 miles to the west, looking eastward across the Valley of the Sun. Well, the storm that had its beginning as a little puff of white against the azure blue hours ago has reached the zenith of its fury and with each drop of falling rain and each stroke of lightning it is slowly dissipating; this often in less than an hour. This is clearly seen on a moonlit night when cast-off fragments of the storm-that-was fan out and drift across the face of the sky, until by hours before the new sunrise the sky is again clear and void of all traces of the thrashing, roaring 'animal' that charged and spent itself the night before.