Description of the Atmospheric Electric Field
Mission Instruments field mills have numerous applications, but the most prominent is their use in determining the likelihood of a nearby lightning strike occurring. Lightning is caused by electrical charge separation in the atmosphere. The intensity of this charge separation can be determined by measuring the electric field, which is accomplished with a field mill. Thus, monitoring of the electric field can warn people of a potentially dangerous situation, before the first lightning occurs. Following is a description of the process.
On a clear day, when the atmosphere is clear of storm clouds, the primary source of electric charge creating an electric field on the surface of the earth is the ionosphere. This can be thought of as a large dome-shaped electrode high above the earth, which produces positive charges which contrast to the relatively negatively charged earth. This scenario creates what is termed a "fair weather" electric field due to the positive charge overhead. When this "fair" field is measured by the EFS 1001 field mill, it can be seen to produce an output of from 50 to about 200 Volts per meter ("V/m"). This value varies, depending upon conditions in the atmosphere, and is also altered by "local effects". Such effects are caused by anything which can carry electrical charge, including but not limited to atmospheric space charge, dust, smoke, litter, etc. Usually, though, the field stays between -50 and -200 V/m during fair or non-stormy weather.
When thunderclouds form, however, processes within their vicinity cause the formation of negative electric charges (the opposite of the ionosphere) at the cloud base. As the charge builds, it creates a "foul weather" electric field which grows and then begins to cancel out the "fair weather" field. As it builds further, it becomes many times greater than the fair weather field. It is this "foul weather" electric field which intensifies to the point that the air can no longer insulate the opposite charges. Finally, the positive and negative charges are drawn together suddenly via any convenient "weak spot" which occurs in the atmosphere. This is the energetic discharge we call lightning.
Foul weather electric fields can reach values of well over 10,000 volts per meter at the ground during a storm.
It is the separation of positive and negative electric charges into large groups which creates the lightning hazard. These groups, of opposite polarity, are naturally attracted to each other but held apart by the atmosphere's insulating properties. As these groups grow during the formation of a storm, the force of their attraction can exceed the atmosphere's ability to keep them separated. Lightning is the sudden, intense electrical recombination of these groups which occurs when this point is reached. The local electric field varies in proportion to the strength of these groups and their distance from the measuring device, so its measurement gives an idea as to the likelihood of lightning occurring. A strong electric field indicates that the situation is conducive to the formation of lightning.
This is an over-simplified explanation of what is actually a very complex process. It is clear, however, that determining the local electric field can play a major role in determining the likelihood of lightning occurring in a particular area of the earth.
Military and other government agencies have determined that electric fields above 2000 Volts per meter create the greatest lightning threat. Many operations centers have requirements to cease and secure certain operations when the electric field reaches this number in an effort to reduce damage or injuries caused by lightning. A high electric field reading does not ensure that lightning will occur, but only that conditions are conducive to its occurring.
Experiments have shown that due to the relatively large size of thunderclouds, the electric field does not show tremendous variation over short distances. If the electric field has reached a value of 2000 Volts per meter at one location (a dangerous level), it will be reasonable to assume that the level is dangerous for several miles, at least, in any direction. Likewise, if the level is below 500 Volts per meter (a relatively non-hazardous level), it can be assumed that the hazard is low for at least several miles.
Although predicting an actual lightning strike is difficult if not impossible no matter what technique is used, monitoring the local electric field along with some interpretation and experience can be one of the best ways to determine how likely it is for lightning to strike an area.
The electric field variations during a typical thunderstorm are shown in Figure 1. Before 2240 hours, the field is low and positive (fair weather polarity), but small "bumps" indicate distant lightning. The local threat is probably still low at this time. At 2240, the field "crosses zero", and begins to climb. This is when one should prepare to take cover. Around 2250 the field exceeds 2kV/m, and the threat should be considered serious. The small "bumps" due to the distant lightning are often not present, particularly if the buildup is directly overhead and lightning has not yet begun. What is important here is the average level of the field.
As a storm builds, the physical arrangement of the charged bodies (clouds) combined with the various effects they have on the local atmosphere can create a variety of electric field build-up patterns. Also, when lightning strikes, either between the cloud charge pockets or between these pockets and the earth, it will cause a large change in the electric field as seen by a field mill. These changes, often fast-occurring and short-lived, can take the field back and forth between fair and foul polarities many times during a storm as can beseen between 2300 and 2330. No lightning events occur after about 2341. This is typical of end-of-storm behavior, although a significant threat still exists. Lightning often does occur during this phase of the storm. For optimum safety, one should wait until the high fields have subsided for perhaps 20 minutes or more. Sometimes a storm can "restart", or a second storm can move in as another is moving out. The ALB 101 monitor is designed to keep the warning active during this time to ensure that the threat has passed.
More comprehensive descriptions of the atmospheric electric fields at ground level in and near thunderstorms can be found in numerous texts and scientific publications related to atmospheric electricity.