Scientists have long wanted to manipulate the weather. The most common idea is that clouds can be "seeded" to produce rain in areas of drought, but an equally important goal is to clear fog around airports and other areas where visibility is impaired.
In the last several years, my team at the University of Utah has developed a new seeding technique that can clear supercooled cloud and fog on a scale several times larger than a commercial airport. The method, which can be applied from a vehicle on the ground, has been used to clear airports, cities and roads, and has recently helped NATO peace-keeping operations at Bosnian airports.
The technique works with supercooled clouds and fogs, which consist of small water droplets at temperatures below freezing. Such fogs often form when the natural mechanism for ice formation is ineffective, and they often occur in mountain valleys and plains during cold seasons. In this situation, the supercooled droplets can be transformed into snow by introducing “seed crystals” into the fog.
Seeding causes a large number of small supercooled droplets at high vapour pressure to evaporate and recondense onto a smaller number of seed crystals at a lower vapour pressure. When the seeding works correctly, the grown crystals will fall out as snow and rain, leaving a clear space behind. For the technique to work, the seed material must cause large numbers of ice crystals to form.
Research into cloud seeding began in 1946, when Vincent Schaefer of General Electric noticed that a small piece of solid carbon dioxide (dry ice) can generate a large number of ice crystals in a laboratory-generated supercooled fog. He then went up in a small aircraft and dropped about 1.5 kg of crushed dry ice into supercooled stratocumulus clouds over western Massachusetts, and found that snow crystals did indeed fall out from the cloud. In the following year Bernard Vonnegut of General Electric found that particles of silver iodide can also generate large numbers of ice crystals if the cloud is cold enough.
With these reports, the world started to anticipate the day when people would be able to control the weather. In reality, however, a statistical approach was taken to investigate the technique further – much like the evaluation of a new medicine – which meant that there was no real investigation of the cause and effect of cloud seeding. The conditions needed to be fixed to prove statistically that seeding was effective, but the conditions should be varied to find the optimum method. Projects often proved a useless result, or failed to produce an effect that was physically convincing.
This problem was recognized in the 1970s. Emphasis then shifted to a scientific study of cloud processes, but the engineering of the seeding technique remained unexplored. At Utah we have focused on clarifying the fundamental processes involved in cloud seeding and on developing a practical seeding technique that makes best use of the feedback mechanisms involved.
This work has shown that the seeding reaction begins with ice formation or nucleation, and that this can occur through two possible mechanisms. In homogeneous nucleation, typically observed when seeding with dry ice, strong cooling of sublimating dry-ice pellets forces water vapour to condense in the surrounding air and the droplets freeze immediately. Heterogeneous nucleation is more complicated, and requires the help of foreign particles such as silver iodide.
Today, silver iodide is the seeding agent of choice because it can be released in dry or warm air at low altitudes, while dry ice pellets have to be dropped from high altitudes. Silver iodide also produces more ice crystals at lower temperatures. I have also observed the same behaviour with organic ice nucleants such as metaldehyde.
Experiments have shown that seeding with dry ice and liquid carbon dioxide can lead to ice nucleation that is almost independent of temperature. This is an advantage when forming ice crystals, since the heat generated in the phase change from supercooled liquid to solid causes the air containing the crystals to rise, and the maximum growth rate is sustained because the number of crystals produced does not depend on temperature.
An effective way to convert a large cloud mass to snow and rain is to ensure that the seeded particles that rise to the top of the cloud on a thermal fall back into the cloud. For this to happen the crystals must grow, which in turn suggests that they must co-exist with the supercooled droplets for a significant amount of time.
Since the supercooled droplets disappear as the crystals grow, more of the supercooled cloud is entrained into the thermal through eddies generated at its base. In the case of dry ice or silver iodide, a vertical ice plume is generated: the buoyant energy associated with the thermal is converted into an accelerating updraft with a low resistance force, similar to cigarette smoke. However, the crystal growth does not last long enough for crystals to fall back through the rising ice plume, and so this technique can only treat small volumes.
In contrast, the technique that we have developed ensures that the thermal lies horizontally. In this case the buoyant energy is converted effectively into turbulent energy, which means that the rising thermal has a slow updraft velocity. To maximize the interaction time of the rising thermal with the supercooled cloud, the seed crystals are generated at the bottom of the supercooled zone, just above the 0 °C isotherm. The seeding then induces a slow but large dynamic effect that lifts the underlying layer of moist air and increases the cloud volume that can be modified. It therefore makes most effective use of the energy created in the phase change.
Using this technique, we have sprayed liquid carbon dioxide – a homogeneous ice nucleant – directly into supercooled stratus clouds over the Great Salt Lake. This generated an optical effect known as undersun and then completely cleared the cloud over an area a few kilometres wide.
We have also used ground-based vehicles to seed supercooled fogs at night. This leads to the formation of light “pillars” – vertical streaks of light – above and below street lights and other light sources, which indicates that the ice crystals are orientated horizontally. The seeding cleared areas of up 100 km2 and caused about 1 cm of snow to fall on the ground.
Seeding of supercooled fog early in the morning often leads to other optical effects called parhelia, which are mock images of the Sun again produced by horizontally orientated ice crystals. The liquid carbon dioxide used in the seeding is either an industrial by-product or comes from natural gas, and does not alter the amount of carbon dioxide in the atmosphere.
The technique is already being used to clear supercooled cloud and fog, and it is most effective in targeting the effect at a particular region. For cloud seeding, it is more effective to inject the seed crystals from the air, and we anticipate that radar will be used to guide unmanned aircraft to the correct seeding position. Fog can be cleared from the ground, and it should be possible to develop a simple device that could be used to clear fog as and when it occurs.
