From the time Elijah called upon the Lord for lightning to ignite his altar on Mount Carmel and Samuel appealed for a thunderstorm to smite the Philistines, humanity had dreamt of weather tailored to order. To what extent can modern science make that ancient dream come true?
Meteorologists, cloud physicists, and would-be weather modifiers must, at this time, display considerable humility in answering this question. Yet they now can express modest hope. There is no doubt in my mind that precipitation enhancement and redistribution will contribute to man's welfare in the coming century.
My optimism is based in large part on impending advancements in our ability to measure and monitor atmospheric and cloud properties and to process the vast amount of data which must be used in mathamatical models of clouds in order to predict their formation and evolution. Sphisticated ground and satellite laser and radar systems, now in prototype, will permit us to see better what is happening in and around clouds, as well as to gain new insight into cloud processes and what may happen when we try to influence them. The result will be much better weather prediction on a scale not now possible and acquisition of knowledge to assess the possibility and consequences of intervention.
The larger-scale forces with control atmospheric circulation -- which dictate whether the weather will be cold or warm, whether ther will be widespread clouds and precipitation ro drought -- are beyond deliberate human manipulation. When modern atmospheric scientist talk about weather modification, they generally mean some modest alternation within the framework of what nature provides.
For 70 years, the most vigorously pursued avenues for altering weather have involved changing the processes whereby extremely small cloud droplets evolve into raindrops or snowflakes. These processes are strongly influenced by the size, composition, and number of particles suspended in the atmosphere where clouds form. Sometimes, by introducing specific types of particles into a cloud , the course of events can be altered in a predictable way. This opens the possibility of altering rain or snowfall or influencing the subsequent lifetime, growth, or dissipation of the cloud.
Activity along this line was greatly increased in the late 1940s when Drs. Vincent Schaefer and Bernard Vonnegut, working at General Electric Laboratories, found that the temperature at which ice would form in clouds could be radically altered either by dropping dry ice (solid CO) to cool a small region of the cloud or by introducing silver iodide particles. The presence of ice or water droplets greatly speeds up the growth of drops big enough to fall out as snow or rain. The technique presumes the cloud has already formed, is at a temperature well below freezing, and is still composed of (supercooled) liquid droplets. The rub comes in not being able to tell beforehand in which clouds and on which occasions additional ice production will make an important difference.
In the case of cumulus or convective clouds, there is also the possibility of encouraging vertical growth. Clouds are often in delicate balance with their environment. Very small amounts of energy input can sometimes lead to significant additional growth.
If a cloud containing large amounts of supercooled water is suddently converted to ice under the right atmospheric conditions, the cloud may grow taller due to the buoyancy produced by the heat released when water drops freeze. The potential for producing precipitation may be enhanced by the taller cloud.
Again, the problem is applying this technique in largely the difficulty in assessing cloud properties and the environment to determine susceptibility to treatment, and then executing this treatment promptly. The cloud may be in a treatable state for only a matter of minutes. In effect, unless we have enough data and the physical understanding to predict what the cloud will do naturally, it will be hard to make treatment efficient and even harder to evaluate whether we did anything at all.
Because of the difficulties in monitoring the many complex physical factors that govern any influence of so-called "cloud seeding," most of the field studies in the past 30 years have been conducted and evaluated on a statistical basis, rather than being based on physical experiments in which the sequence of events can be observed step by step. Precipitation in seeded areas was compared to precipitation in unseeded areas, or seeding was alternated with no seeding on a random basis to determine what impact the seeding may have had. The high natural variability of precipitation over an area or over a time period makes these statistical evaluations difficult and usually inconclusive.
What, then, can weather modifiers actually do?
* Supercooled fogs and thin layers of supercooled stratiform clouds can be dissipated as was demonstrated 35 years ago. The latter ability may be useful in some areas where shallow winter clouds deplete sunlight for long periods of time.
* There is strong statistical evidence that winter snowfall can be increased in the mountains of the Western United States. In addition, measurements can determine when "seedable" clouds exist. There are undoubtedly other regions with similar potential.
* Our understanding is inadequate to be sure of the outcome when we seed the cumulus or convective clouds which give most of our rainfall in summer. Experiments in a number of georgraphical locations have been encouraging. But they have not produced enough knowledge to transfer the technique elsewhere or to be sure we are using our tools effectively. However, this technique may one day become valuable for increasing rainfall in arid areas or redistributing it within large cyclonic storms.
* The possibility of promoting large cumulus growth outside the eye wall of a hurricane and increasing the radius of the ring of intense convection and winds, thereby reducing the peak wind speeds, has also been investigated for many years. We are not yet in a position to conclude whether this can be done.
* There is also a strong possibility that seeding techniques can change the trajectory of snowflakes by altering their size. Snow from a cloud fixed in position by a mountain or lake shore would come down some miles from where it would have fallen naturally. Prevention of large hail by freezing the suporcooled water from which it grows or poviding many small competing hailstones to inhibit growth is also a possibility. But this is not yet confirmed to the satisfaction of the scientific community. Attempts to suppress lightning by cloud seeding have not proved successful.
* It should also be noted that cities, major industrial centers, and islands affect the small-scale circulation around them, influencing cloud formation and precipitation in their areas. This means surface characteristics might be altered over many square miles to achieve desired effects. For instance, Dr. James Black of Exxon has suggested coating a patch of coastal desert with asphalt to induce heating and cloud formation downwind and on shore and thus increase rainfall.
We will always be dealing with large-scale atmospheric conditions beyond our control. We will not stop drought, cause rainstorms in the desert, or halt major storms. Our efforts will be directed toward trying to induce some small perturbations and redistributions within what nature provides. And we will be able to achieve some small gains.
The major challenge weather modification presents to society lies in deciding where to increase rainfall when it will mean benefiting some and disturbing others. There will probably be those who will object even to a few hours of sunshine in an otherwise dark February day around the Great Lakes. After all, if the Philistines or the Prophets of Baal had had good attorneys, Elijah and Samuel might have been in deep trouble.