It is well known that lightning is a dangerous and potentially deadly product of thunderstorms that frequently develop over the central plains this time of year. However, what is sometimes overlooked is the fact that lightning can and sometimes does strike a long distance from the actual thunderstorm. Such bolts of lightning are frequently referred to as �bolts from the blue� since they seemingly appear when skies are clear or just slightly cloudy. One such occurrence occurred in the NWS Hastings county warning area on May 7, 2004, when a bolt of cloud to ground lightning hit the ground in northern Osborne County, Kansas - 28 miles to the east of a thunderstorm in northern Rooks County (see Figure 1). Specifically, the thunderstorm which produced the bolt from the blue was located to the northwest of Stockton, with its associated lightning strike making contact with the earth west of the town of Portis.
Bolts from the blue are typically associated with two parts of the thunderstorm � the updraft and the anvil. As a thunderstorm develops, the updraft rises rapidly into the atmosphere, allowing ice to form within the storm clouds. Individual particles of ice begin to collide with one another due to the upward and downward motions inside the thunderstorm. As these collisions take place, an electrical charge (both positive and negative) is created, with the positive charge rising to the top of the storm, and negative charges sinking toward the bottom. As the storm moves along, a positive charge collects on the ground underneath the thunderstorm. When the separation of charges between the positively-charged ground and the negatively-charged base of the thunderstorm becomes too great, a lightning bolt is generated in an attempt to negate the charge separation.
This process holds true for an ordinary bolt of lightning originating underneath a thunderstorm. Bolts of lightning which strike a large distance from a thunderstorm, however, represent a slightly different process. Looking at the May 7 case, the positive charge which rose to the top of the thunderstorm began to spread out in the anvil (as can be seen in Figure 2). Eventually, the charge difference between the positively-charged thunderstorm anvil and the negatively-charged surface of the earth became overwhelmingly large. At this time, a bolt of lightning developed from the top of the thunderstorm to the ground in an effort to minimize the charge difference. What makes the strike so dangerous is the fact that the lightning bolt must travel such a long distance from the top of the cloud to the surface. This means that such bolts must carry an extremely large electrical current if they are to complete the circuit between cloud and ground. These types of strikes are also referred to as �positive lightning strikes�, since they begin from the positively-charged top of the thunderstorm (notice the + sign in Figure 2).
This particular event exemplifies the need to practice lightning safety rules whenever thunderstorms are present in an area, even though skies may appear less than threatening. In this case, skies were only partly cloudy when the lightning bolt stuck, making for quite a deceptive scene. Therefore, anytime thunderstorms are in the vicinity, it is wise to take a break from the day�s work and seek indoor shelter until the storm has moved well past the area.
Page and story by David Lawrence
