by Yevgeni DEMIN, Cand. Sc. (Tech.), consultant/development engineer, TOO R&D center "Russian Testing of Automatic Facilities"; Viktor KUSHIN, Dr. Sc. (Tech.), laboratory head, Research Institute of Theoretical and Experimental Physics; Dmitry IOSELIANI, leading engineer of the same institute

Sahara, the largest desert of our planet that occupies a fourth of Africa's territory, is often compared to a tumor. The metaphor reflects the sad situation, what with the abnormal soil erosion and weathering-processes producing ever larger tracts of barren sands.

Yes, Sahara keeps expanding, gobbling up ever more territories. A number of untoward factors speed up desert encroachment, such as northeastern trade winds, intensive solar radiation, forest felling, trampling (with pastures devastated by cattle), and overlong droughts, especially in areas at latitude 18N. These droughts entailed disastrous consequences in the 1970s, when all of the cattle and the larger part of farm crops perished.

And yet a retrospective study of archeological evidence shows that Sahara's climate was rather humid about ten thousand years ago. During the Ice Age this land lay in the zone of normal or lower pressure. So the inhabitants could till land, go huning and even fishing! But then change set in: the atmospheric pressure started moving up, and anticyclonic processes in the atmosphere turned into a dominant factor.

Now, speaking of anticyclones: their scenario is as follows. Drawn from the cold layers of the troposphere, the air goes down and warms. The weathermen say that this process is stable in deserts which are a region of enhanced atmospheric pressure. Under like conditions (typical of Sahara and also of dry districts of California, USA, and Mexi-Articles in this rubric reflect the opinion of the authors. -Ed.

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со) as good as no clouds are formed, hence there is scant precipitation. Besides, the absence of shade and the intensive reflection of solar light by sand contribute to soil heating (with the sand surface heated up to 70 - 80C) and thus prohibit monsoon showers. ^*

Consequently, both the geophysical and meteorological factors, bad enough as they are, contribute to further desertification, or desert encroachment. But what about the human factor: can it give a lease of life to Sahara?

Now look. Modern systems of irrigation are largely based on canals and trunk pipelines feeding water from lakes and rivers to mandug pools. It seems the Niger and the Nile could give enough water to Sahara. However, the conventional irrigation techniques won't work here: the water will be absorbed by dry sand, it will evaporate or else fall as dew and go down the sand drain too.

Big fires, meanwhile, can give rise to thick stormclouds and cloudbursts. This phenomenon was noticed long ago. Some tribes in Central America and Equatorial Africa burn grass or bushes to cause precipitation, and they succeed now and then. That is to say, in so doing they induce a cyclone type process: streams of humid air rise up and cool, and then expand with a decrease in the atmospheric pressure. At altitude 10 - 12 km air temperature is always about - 45 ^o C, and that is why the moisture freezes there to descend in the form of icy dust and hail, drawing in its wake air masses. The same amount of air ascends in turn, taking along moisture from the lower layers of the

Shorthand of upward and downward air currents producing clouds in the troposphere.

Outline of supermeteotron: 1- guiding unit; 2-receiver; 3-turning bends; 4- engines.

^* See: Ye. Dyomin "Invisible Assistant of Weather"; Science in Russia, No.l 2003. - Ed.

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atmosphere. This process recurs time and again.

So the meteorological situation in Sahara can be changed by reorienting the descending (downward) air flows upward. Alain Dessin of France built the first setup for this purpose in 1961, and dubbed it a meteotron. In 1966, the Institute of Geology and Geophysics of the Siberian Branch of the USSR Academy of Sciences suggested a much more powerful plant employing the jet fire-thrower technique.

Both units, however, made use of burners polluting the atmosphere due to the incomplete combustion of the fuel. Therefore aircraft turbojet engines were adopted in subsequent models. The results surpassed all expectations-so much so that the ascending (upward) streams of heated air proved to be invisible and had to be stained. To convert a horizontal air flow into a vertical one, every engine was equipped with a turnover crank. A meteotron like that sent the air to an altitude of 1,200m.

Yet experimental tests of this and other plants showed they could only speed up precipitation, and only under favorable meteorological conditions at that.

It looked as if better results could be achieved by increasing the gas jet power. In the latter half of the 1980s, this country's experts came up with an idea of a supermeteotron which had cranks in one common receiver ^* supplied with an extra afterburner. A setup like this can spurt an air stream in a humid atmosphere as high as 5,000 - 6,000 m.

However, ascending air flows are effective only in the initial stage of heap clouds formation. The further growth of heap clouds, as shown by radiolocation studies carried out at the Russian Hydrometeorological Institute (St. Petersburg) depends on horizontal and eddy flows of heat and water vapor from the ambient environment. This means that to arrest desert encroachment and make atmospheric processes over Sahara reversible, a range of combined measures is necessary so as to raise clouds decreasing solar radiation, cool the air and induce precipitation. That's what the authors of the present article suggest in their upgraded supermeteotron model supplied with elements capable of saturating air currents with water and sending them up in a spiral.

Our setup operates like this. It has a gas jet heated to 380 to 950C (depending on the work mode) and injects water which turns into vapor instantly. The upward current draws in the ambient air with spray through clearances (gaps) between plate guides and converts it into a

^* Receiver-here, a gas container equalizing pressure in the process of pulsed in and outflow of gas. - Ed.

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vapor air mixture. Reaching the upper boundary of the troposphere with subzero temperatures, such mixture condenses and implicates the ambient moisture in cloud formation. This is how a cyclonic process is triggered.

One important advantage of our method is that the power of turbojet engines is needed only at the initial, jet formation stage; thereupon the jet is sustained largely through the vapor-water-ice phase transition. Unlike the previous models, the upgraded unit generates powerful moisture-saturated flows (jets) capable of causing precipitation regardless of the presence of natural clouds.

The meteorological situation in Sahara can be reversed by yet another method called cloud seeding. Granules of dry ice (solid carbon dioxide) are scattered into an over- cooled cloud which can otherwise be treated with smoke composed of tiny particles of iodinated silver. All this activates moisture condensation and, consequently, precipitation.

Our country's scientists have pioneered in developing an adequate technology that has been used with much success in the past 30 years in Caucasia, Moldavia and other southern republics of the USSR. Similar studies have been carried out in the United States, Israel, Switzerland and Italy both for experimental and for production purposes. Cloud seeding will work best in combination with other techniques. The technology involving the cloud seeding method comprises these sequential steps. First, super-meteotrons generate moisture-saturated jets of gas rising upward. Reaching the upper layer of the troposphere, these currents condense and produce clouds. Then a volley of rockets is fired for cloud seeding to induce a cloud-burst in the form of rain or hail. The hail is a preferable option because in this case the cooling of the sand layer proceeds much faster. Should the amount of precipitation prove inadequate, the cycle is repeated.

Computer simulation studies have shown this: to activate cyclonic processes over Sahara, we need at least ten supermeteotrons on mobile platforms installed around a district 100 to 120 km in diameter. We have estimated that a cyclone with precipitation over an area of 10,000 to 15,000 sq. km can breach Sahara's anticyclone and clear the way for abundant monsoon showers.

The project we suggest for a cyclone induced by technical means is unique indeed. If realized, it can be a breakthrough in solving the problem of reversibility of atmospheric processes in our planet's desert-lands.

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