When lightning strikes during a hurricane, Dr. John Hallett--a professor in DRI's Atmospheric Sciences Center and recipient of the 1998 Nevada Regents Researcher Award--is not far behind. As one of the world's leading scientists in the study of ice crystals and cloud
negative charge in the cloud to the ground. |
From a Nevada and southwestern U.S. perspective, thunderstorms take on a different importance. These storms are much smaller than hurricanes but are organized in a way such that water droplets cooled below freezing--supercooled water--can reach higher, colder levels and give lightning discharges to the ground as updrafts and downdrafts develop. Such storms are sometimes responsible for lightning-induced fires. To understand these impacts, we need to have a grounding in some of the principles of cloud formation and electrification, what Hallett calls "a rich, fascinating, and often exciting field of atmospheric science." Lightning occurs when sufficient opposite electrical charge becomes separated and moved into regions by air flow to give strong local electric fields. Discharges occur between a lower-level negative charge in the cloud and the ground, between positive and negative charges within the cloud, and between an upper-level positive charge in the cloud and the ground. The latter carries the most current, increasing the risk of fire in the dry Southwest. Lightning discharges, called sprites, also occur from the tops of these storms into the stratosphere.
Hallett will use a new, high-energy physics facility--the z-pinch machine--at the University of Nevada, Reno (UNR) to further study these phenomena. This unique device, developed for fusion-energy research, generates pulsed electrical discharges with two trillion watts of power. The output from the machine will be fine tuned to simulate natural lightning in ice and water clouds produced in the laboratory. Dr. Bruno Bauer of the UNR Physics Department was instrumental in bringing the z-pinch machine to Nevada from Los Alamos National Laboratory in New Mexico.
The idea that lightning is caused by opposite charges sounds simple, but much remains to be understood about the nature and distribution of electrical charges in clouds. Why, for instance, do clouds usually--but not always--have a lower-level negative charge and a positive charge above? Also, why do some clouds achieve sufficient charge separation to cause lightning while others do not? A line of thunderstorms developed along a cold front, for example, can produce lightning at a rate greater than one stroke each second, while hurricanes may only produce a stroke every ten minutes.
When we look at a cloud from the ground, we usually see one large, relatively uniform region; but clouds are anything but uniform and static. They are characterized by varied composition and differences in internal motion. Ice crystals within clouds are required for the development of strong electrical fields and lightning. Recent research involving the use of instruments on NOAA P-3 Orion aircraft that fly through storm clouds suggests to Hallett and Black that "not just any mix of ice particles and cloud droplets will do and that convective motions in the cloud are fundamental to the formation of the right conditions for charge separation."
What is convection? It is movement resulting from differences in density and the action of gravity. In the atmosphere, heated air rises because it is less dense while cooled air sinks. Clouds are often formed under conditions with different winds at different levels. The upper part of a cloud, for example, can be displaced or even blown off by strong winds aloft, a process called shear.
Temperature is another factor affecting cloud dynamics, the formation of ice particles, and electrical charge. At certain temperatures, very small soil particles in the atmosphere trigger the formation of ice crystals in clouds. As these become larger and fall at a certain rate, they can build into a form of soft hail known as graupel. Experiments by Hallett and Clive Saunders of the University of Manchester Institute of Science and Technology suggest that graupel moving through a cloud is a potential source of electrical charge separation.
The surface of a growing graupel particle is complex. The particle is covered by droplets in various stages of freezing, with the graupel growing in some places and evaporating in others. This highly complex process contributes to the transfer of electrical charge as the ice particles impact and bounce; but even to Hallett, the exact nature of this process is still unclear. Recent observations made from aircraft indicate that the effect of wind speed at different elevations also plays a role in electrical charge separation in clouds.
What interests Hallett most is the cloud region at the transition between water droplets cooled below freezing and ice crystals. In this region, upward-moving graupel changes to a combination of ice crystals and graupel and then to downward-moving snow. Ice crystals are produced from the growing graupel (by the Hallett-Mossop process discovered in a laboratory study during Hallett's sabbatical leave in Australia in 1973) and produce separation of charge.
In addition, a minimum amount of time and sufficient cloud volume are both necessary for this process to result in a charge separation capable of producing lightning. Clouds that are too small, for instance, won't work. They must be deep enough to cover the range of temperatures required for charge separation and big enough to separate a sufficient quantity of charge for a lightning strike.
and the eye in the center. Photograph courtesy of the Hurricane Research Division of NOAA. |
Hurricanes are generally very good at forming rain but do not generate the same pattern of water and ice movement of a typical thunderstorm. As a result, the charge separation is not very great; and hurricanes produce no or very limited lightning. On rare occasions, they do so; and this results from a local, strong convective cell that rotates around the eye. Why is all of this important? Weather forecasting, for one. Electrical activity is an indicator of convection in storms, and stronger convection means that the storm is intensifying and possibly changing direction of motion. Hallett and Black also point out that "prediction of lightning is vitally important for satellite launches...where a single, unanticipated stroke can result in the loss of hundreds of millions of dollars." Human safety is also an issue since the presence of supercooled water in the atmosphere can lead to the formation of ice on aircraft. The risk of wildfire in the West is related to relatively small clouds with cool bases that occur over both the desert and mountainous areas of desert regions and sometimes have the ability to produce lightning strikes to the ground.
Another important implication of this research is the effect of cloud formation on global warming. Hallett says, "Water vapor probably has as much or more of an effect on the Earth's radiation budget than carbon dioxide, the best known greenhouse gas." This is in part because water vapor in the atmosphere usually but not always acts in much the same way as carbon dioxide: both allow more solar energy to pass through than outgoing infrared radiation to escape. The net effect is that the Earth gets warmer. Yet the cirrus clouds formed aloft may have the opposite effect.
Determining the exact impact of the effect of water vapor is very complex, particularly because water vapor is not distributed evenly throughout the atmosphere. But our ability to predict the level of water vapor that moves into the upper atmosphere is related to lightning, which is a strong indicator of the forces contributing to the formation of the high-level cirrus clouds that affect radiation balance. Much remains to be learned regarding cloud electrification and the impacts of associated atmospheric processes. Whether on the ground in the laboratory, designing measuring instruments, or flying into a hurricane in pursuit of more information, Dr. John Hallett will be in the forefront of this search.
Roger Kreidberg
This story summarizes an article that first appeared in the
November-December 1998 issue of American Scientist, and the topic will be
developed further in an article by Black and Hallett in the Journal of
the Atmospheric Sciences. This story may not be reprinted without
permission.
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