November-December 1998, Volume 86, No. 6
The Mystery of Cloud Electrification
How precipitation develops, evolves and is moved by airflow at different levels may explain hurricanes’ lack of lightning
Keywords
lightning, hurricane, cumulus cloud, cumulonimbus, graupel, ice crystals, convection, upshear, downshear, charge separation
Robert A. Black and John Hallett
Poor Captain Nemo met a stroke of bad luck in the Gulf Stream: He and his crew aboard the Nautilus encountered a rare type of hurricane--one with abundant electrical activity. A Midwest squall line can generate lightning over an extended period at a rate of more than a stroke per second; a hurricane, on the other hand, rarely produces a stroke more often than every 10 minutes. Why the difference? Part of the answer has been made clear by recent research, including fly-throughs of both types of storms by instrumented aircraft. But storm clouds still hold many mysteries, making the exploration of cloud electrification a rich and fascinating (and often exciting!) field of atmospheric science.
From a functional perspective, lightning is well understood. In the case of a cumulonimbus cloud, the most common thunderstorm cloud, earth and cloud effectively acquire opposite electrical charges, and the air between them serves as an insulator. When the separation of charge aloft is sufficiently large, an ionized path is formed between the cloud and the ground, and a lightning discharge occurs, with the charge transferred from the lower part of the cloud (usually negative) to neutralize the induced positive charge on the earth below. The more fundamental question, however--why is there a lower-level negative charge in the cloud and a positive charge above?--has been more difficult to answer. Further, why do some clouds achieve the charge separation that produces lightning and others not? And why do some produce lots and others little?
Differences in the lightning productivity of various systems almost certainly lie in their composition and internal motion. In particular, though some have argued that lightning very occasionally may be produced by clouds warmer than 0 degrees Celsius, the presence of ice particles appears to be required for the development of strong electrical fields and lightning. Recent observations suggest further that not just any ice particles will do and that convection in the cloud is fundamental to the formation of the right type.
Understanding how clouds generate lightning is of more than academic interest. Prediction is vitally important for satellite launches, for example, where a single unanticipated stroke can result in the loss of hundreds of millions of dollars, yet unnecessary delays are also costly. Detection of electrical activity may also prove useful in weather forecasting, since it indicates stronger convection and an intensifying system. Finally, because the cirrus clouds produced by strongly convective systems have a profound effect on the earth’s radiation balance, an understanding of how they form may prove fundamental to accurate modeling of global warming.
Clouds from the Ground Up
To understand how a cloud becomes electrically charged, it is first necessary to know what clouds are made of and how they form. The answers to these questions can be gained empirically by sampling cloud particles from aircraft, by simulation in the laboratory or by inference from optical and microwave measurements. A simple thermodynamic approach tells us that cloud droplets form as ascending air expands and cools below its dew point (see Figure 2). Because the atmosphere always contains sufficient concentrations of hygroscopic nuclei (sea salt, for example), air need only become supersaturated by a few tenths of a percent relative humidity over 100 percent to form a cloud. In a typical cloud, droplets of 10 to 20 micrometers in diameter (a human hair is about 100 micrometers in diameter) readily form in updrafts of a few meters per second. These particles fall through the air at a speed of 1 to 5 centimeters per second, negligible compared with the speed of updrafts, so they remain suspended as a cloud.
Drizzle and rain range from 0.1 to 4 millimeters in diameter, respectively. In order for such larger drops to form, particles must collide with each other and coalesce, but collisions are unlikely for very small particles. Because droplets in clouds are so small, they have low inertia and follow the airflow around more rapidly falling larger drops--like small insects being swept around a fast-moving car, with only the large ones striking the windshield. The collision process requires drop sizes greater than about 40 micrometers to start, and drops reach this size by nucleation on particles rarely larger than 1 micrometer. Coalescence after collision is most likely in a particular set of conditions: a relatively clean environment, a sufficient supply of water vapor with cloud-base temperatures above 15 to 20 degrees and a collection of nuclei to give the right proportion of smaller and larger drops. An ocean environment such as Hawaii, where the process was first discovered, is ideal.
The fact that clouds are far from symmetrical about a vertical axis makes the simple picture described above somewhat more complicated. Even a cursory observation of the humble cumulus (cumulus humilus) shows that most clouds have an upwind and a downwind (or, more technically, an upshear and a downshear) side, plus two sides parallel to the wind shear (see Figure 4). This asymmetry results from the fact that wind speed increases (and sometimes changes direction) with altitude. In extreme cases, the cloud top appears to be almost blown off, which leads to the characteristic anvil shape of deeply convecting thunderstorm clouds. The upshear side is composed of new cloud forming in updraft air that only mixes with surrounding air near the cloud top, where the air becomes negatively buoyant by cooling of the evaporating cloud drops and sinks on the downshear side, a region where larger, precipitation-size drops form and fall out of the cloud.
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© American Scientist 1998