THE MONSOON SEASON

Monsoon season

Arizona's summer rains draw their power from engines of the atmosphere half a world away. Even the word monsoon evokes mystery...

For many weeks, sometimes months, sunshine reigns unchallenged. Then an omen appears. Darkening clouds cloak the mountaintops. The huge cauliflowershaped masses of moisture balloon higher each afternoon. Swirling and seething, the thunderheads roil up eight miles or more, lightning bolts streaking from them like flung javelins of angered deities. The thunderbird flaps its great wings, and the fresh fragrance of imminent desert downpours tantalizes the nostrils. Nature is ready to explode onto Arizona another monsoon season. When nature thunders, mortals listen. Well they might. For about eight weeks, intermittent but torrential summer rains will pummel the Southwest. Phoenix, on

The Omen

Moisture-laden air streams into Arizona from the Gulf of Mexico in summer, creating spectacular, often localized wind and rainstorms nearly every afternoon and evening during July and August.

which less than eight inches of precipitation descends in most years, has been drenched with almost a fourth of that in a single August hour.

Sheets of water sweep the thirsty soil. Streets flood, and usually dry streambeds brim with runoff. Winds whip sand into abrasive blankets, obscuring vision. Yet the episodes ordinarily are brief. Within hours, the freshened desert glistens beneath a starlit night.

Fortunately, too, the heavens will have vented only a small fraction of their potential. A single severe thunderstorm, several miles across, may pack the force of ten nuclear bombs, or the energy equivalent of burning 7000 tons of coal.

Navajo Indians call these boisterous storms of summer "male rains," contrasting them with the more gentle "female rains" of winter. The latter, arriving infrequently from the Pacific during the period from November to March, make up more than half the rainfall over Arizona. But the inland Southwest's summer rains definitely differ. The Indians are right.

Straightforward as the consequences of the monsoon storms may be, the causes are complex. They are powered from half a world away by such machines as the circulation of the global atmosphere and the earth's rotation. Even the word for them evokes mystery.

Monsoon arises from the Arabic mau-sim, for season. The term originally characterized only the weather pattern of the Arabian Sea, where winds flow from the southwest for half of each year, then from the northeast the remaining six months.

But today the World Meteorological Organization, the science's arbiter on nomenclature, attests to the legitimacy of the term in Arizona usage. By the WMO definition, any continental-scale climate ruled by prevailing winds that seasonally reverse directions is a monsoon climate.

Admittedly, when most people think of monsoons, the awesome invasions of southern Asia come to mind. Withering winds that surge south from the Gobi Desert brush that subcontinent from January until June. Then their direction changes, almost to the opposite compass point.

The winds, wet and chill now, flow north across the Indian Ocean and the Bay of Ben-gal. As the currents cross the warmer land, rising hot air-the sort of thermals that often buffet airplane passengers-will shove them higher into the atmosphere to freeze and form raindrops. And that is the standard script for making monsoons.

At least part of this phenomenon was perceived first in 1686 by an English scientist who never had heard of "male rains." From his own seafaring, though, he had learned much about the effects of sun-light. So he explained the movement of

TheBuildup

(FROM FAR LEFT) Air heated on the desert floor rises and expands, sweeping the moisture in the upper atmosphere into dense thunderbeads that can rise 40,000 feet into the summer sky. DICK CANBY (ABOVE) Typical billowing cumulonimbus clouds of the monsoon season - huge, dense, and seemingly firm enough to bounce on. P. A. NISBET

The Storm

Jagged lances of lightning stab the Tse Tonte Mesa on the Navajo Indian Reservation. GARY LADD (INSET) A summer storm electrifies Pinnacle Peak, north of Scottsdale. JIM MARSHALL.

The monsoons by "the Action of the Sun's beams upon the Air and Water as he passes every day over the oceans."

Although his concept continues to this day to be a classic of atmospheric research, the Englishman became most famous as the first person to plot a comet's orbit. Sir Edmund Halley did not live to witness the next return of the comet named for him. But he would have fathomed both the destruction and the deliverance that mon soons rain on Asia.

The moisture they deliver is boggling. On the Indo-Chinese border, some areas average 436 inches of precipitation a year! Two months of relentless rains in 1972 caused the deaths of more than 500 Filipi nos. Last year, similar storms killed eighty people in the Philippines and another ninety-five in India. Still, were it not for the monsoons, the region-one of the poorest and most populous in the world -could not raise enough rice to sustain its millions of people.

Arizona needs the moist winds less critically, and its storms rage with far less feroc ity, of course. Without them, however, the desert Southwest would appear dramatically different.

Desert temperatures soar during May and June, and rainfall each of those months in central Arizona averages less than a fifth of an inch. Indeed, the first 100-degree day can be expected around May 16. Phoenix has recorded ten Mays and twenty-two Junes with no rain at all. Afternoons in June, the relative humidity drops to about twelve percent. Wind and the ceaseless sunshine rob the landscape of what little moisture remains. Only a few large streams still run. Spring wild flowers wilt. Grasses become brown. "The strong afternoon heating in the desert causes the air to expand and move upslope toward the mountains," John Ten harkel, a National Weather Service meteor ologist, points out. That results "in brisk southwest winds which further increase the high evaporation rate and add to the fire hazard in the forests and woodlands."

Should such drought persist through the summer, obviously a grim toll would be taken on the wild plants and animals. Few could withstand the desiccation.

But for most of Arizona, by July, relief is on the way. As as the giant saguaro cacti stand guard, the monsoons will come. True, very little of the rain will reach as far west as extreme southwestern Arizona around Yuma or across the Colo rado River into southeastern California. As a result, that is one of the most barren swaths of North America. Only Death Valley is more arid.

For most of Arizona, the monsoons should rumble in-the Indians used to say-"about a week after locusts begin to Sing at night" or when "the clouds look like many sheep."

The National Weather Service, more precisely if less poetically, expects the onset around July 8, although the season has started as early as June 16 and as late as July 24.

A monsoon day, by the Weather Service book, occurs when dew points (the temperature at which moisture can condense in the air) average fifty-five degrees. The higher the dew poin point, the more water vapor there is in the atmosphere. Scientists consider the dew point a more accurate sign of mugginess than relative humidity, which indicates by percentage the amount of moisture in the air in relation to how much the air could contain. Because warm air can hold more water vapor than cool air, humidity varies as temperatures rise and fall.

The two measurements are made with much the same technique. A wet-bulb thermometer with a moistened cotton wick is placed before a fan or twirled in the air. The amount of liquid water that evaporates from the wick is related to how much gaseous water vapor is present in the air. The drier the air, the lower the wet-bulb temperature. One can then combine this wet-bulb finding with the temperature taken with an ordinary dry-bulb thermometer and use a standard chart to calculate both relative humidity and dew point. When air temperature and dew point match, the relative humidity is 100 percent.

As many as ninety-nine fifty-five-degree dew point days have been reported in Phoenix in a year, seventy-three of them consecutive. Both were records reached in 1984. A string of thirty-three consecutive days is normal. The total number of monsoon days averages fifty-seven; those usually are punctuated with days of crystal clarity.

Days of that sort can deceive, for thunderstorms often offer little warning. Once I attended a party on what began as a brilliantly clear summer night. Foolishly I left my car windows open. The festivities were so noisy few people were aware that outside a downpour had come and gone. When I returned to my car, the floorboard was ankle-deep in water.

If John Tenharkel had been at the party, he probably would have closed his windows. Tenharkel submits that Arizona has not the familiar four seasons but in fact five. Besides winter, spring, and autumn, the state can claim the "dry monsoon" of May and June, then the summer's "wet monsoon."

Unlike the tropics, where there are two seasons, wet and dry, most parts of the United States and Europe define their four seasons by astronomical events, specifically-

The Rains

After the sound and fury of roaring winds, lightning, and thunder, the cooling rains soothe the summer earth. (BELOW) The Verde Valley, central Arizona. (INSET) Padre Bay on Lake Powell. BOTH BY JERRY JACKA

ARIZONA'S TWO MONSOON SEASONS

DRY MONSOONS, occurring from April to June, are caused by high altitude winds pushing moist air east from the Pacific Ocean. Some moisture is lost as rain when the air rises over the mountains on its way to Arizona. Afternoon heating on the desert causes the air to rise to between 20,000 and 30,000 feet where it cools by expansion, producing "fair weather cumulus clouds." Rain from these clouds has a long way to fall and usually evaporates before reaching the ground.

WET MONSOONS in July, August, and September are caused by warm moisture-laden high altitude winds from the Gulf of Mexico and Gulf of California. The convection-heated air rises rapidly from the desert, building huge dense cumulonimbus clouds reaching 40,000 feet. When the cooling air can no longer hold the moisture, thunderstorms begin.

Typically the equinoxes and the winter and summer solstices. These happen to correspond fairly well with the weather. But Arizona, Tenharkel proposes, shares some characteristics with both the two-season tropics and the four-season higher lati tudes. It is "the change" from dry to wet monsoon that makes us different, he says: "Arizona lies roughly halfway between the fringes of the tropics and the belt of strong westerly winds, commonly called the jet stream or storm track, which marks the zone of four seasons. Arizona has some aspects of both types of climate." What sets the stage for the dry-monsoon season? Because of our planet's rotation, the shifting slant of the sun shoves that west-to-east storm track-the westerlies -farther and farther northward in the spring. In most years, the jet stream retreats so far north by mid-May that cold fronts seldom reach Arizona. While such fronts produce rain and cooling in the north, they bestow on Arizona only winds and evaporation that draw moisture from the ground and from vegetation. Over these weeks, air streams into the state from the southwest, from the Pacific across northern Baja California, but that air is too frigid to blot up moisture from the sea. Signs arise in May and June, to be sure, that things will not stay the same. "Fair weather cumulus clouds," or dry-season thunderheads, occasionally billow above the mountains. But rain-making requires a complicated and precise set of conditions and ingredients. An average raindrop needs about a million cloud particles. Therefore the fair-weather fleece seldom grows enough to produce precipitation. When it does so in this sere season, most of the moisture evaporates before it can cascade onto the surface. The wringing out of the water creates the dark drapes or fingers-the virga-that descend from clouds of middle height and falsely suggest that rain is falling on the horizon. Although virga (Latin for branch or rod) may drop within a few hundred feet of the surface, none actually reaches the ground. The old Navajo called the phenomenon "the hairs of clouds," and the motif has decorated their jewelry for centuries. But by June, the atmosphere already is brewing a transformation. A worldwide girdle of high-pressure cells called the subtropical high slowly slips toward the north and west, driven again by the angle of sunlight. And over the Atlantic, far to the east, increasing atmospheric pressure fuels the growth of the Bermuda High, a huge, clockwise whirlpool of air. As the high widens into ever more enormous circles, it swirls westward, hurtling its powerful breath up into the Great Plains and triggering tornadoes. Ultimately the Bermuda High shoulders air from the Gulf of Mexico and the Gulf of California into the Southwest. Being warm and wet, those winds are unwieldy and buoyant and tend to rise when they are warmed from below. Again John Tenharkel: "The rising air expands and cools and releases its moisture to form clouds. Condensation of water vapor into cloud droplets releases heat, which makes the rising air warmer and still more buoyant, causing it to rise higher and condense out still more water." At last, Arizona's season of monsoon rains has arrived.