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STORMPLANET - reference & education Most of the following was taken out of NOAA Technical Memorandum ERL NSSL -102 by Holle and Lopez. As the name implies, it is a bit technical. Lightning discharges can be classified into two types: Cloud to ground (CG) discharges. These discharges have at least one channel connecting the cloud to the ground [CG; or cloud to water (CW)]. Cloud discharges that have NO channel to ground. These cloud discharges are, in turn, classified as in-cloud (IC), cloud to air (CA), and cloud to cloud (CC). This discussion will highlight the CG types of flashes. A CG lightning discharge is typically initiated inside the thundercloud. It is first apparent when a faint negatively charged channel, the stepped leader, emerges from the base of the cloud. Under the influences of the electric field established between the cloud and the ground, the leader propagates towards the ground in a series of luminous steps of about 1 microsecond (1*10 -6s, or .000001 second) in duration and 30 meters (Krider 1998) in length, with a pause between steps of about 50 microseconds. The stepped leader reaches the ground in tens of milliseconds (1*10 -3s, or .001 second) depending on the tortuosity of its path. when the stepped leader channel approaches the ground, it has about 5 Coulombs of negative charge on it and carries a very strong electric potential with respect to ground of about -10*8 volts. The strong electric field between the leader and the ground causes upward moving charges, or streamers, from objects on the ground. When one of these streamers contacts the tip of the leader, 50 to 100 meters above the surface, the following occurs: The leader channel is connected to the potential to ground Charge starts flowing to ground Current wave propagates as a bright pulse up the channel This discharge process is called the return stroke. and takes less than 100 microseconds. The charge deposited on the leader flows down the channel behind the wave front producing a current at the ground that has an average peal value of about 30 kilo amperes. It takes about 1 microsecond for the current to reach its peak value, and about 50 microseconds to decay to half that value. As the leader charge flows down the channel to the ground, electric and magnetic field changes are produced that propagate outwards from all the segments of the channel involved in the current flow. These field changes have rapid variations that follow the channel of the stepped leader. The field changes have electrostatic, inductive and radiative components, and each of the components has fluctuations of different frequencies that have different attenuation characteristics as the fields propagate from the lightning channel. Therefore, the shapes of the field changes are strong functions of the radial distance from the channel. The detailed structure of the first several microseconds of the electric and magnetic field changes produced by the return stroke is of fundamental importance in cloud to ground lightning detection systems. After the current has ceased to flow down the stepped leader channel, there is a pause of about 20 to 50 milliseconds. After that, another leader can propagate down the already established but faint lightning channel. This leader is not stepped, but rather continuous and is called a dart leader. On the other hand, no dart leader might occur and the flash may end. A dart leader is produced when additional charge is made available to the top of the decaying channel in less than about 100 milliseconds by the breakdown mechanism known as K and J processes. The dart leader deposits about one coulomb of charge along the channel and carries cloud potential to the vicinity of the ground. Again, a return stroke is produced. The peak amplitude of the current flowing in subsequent return strokes is usually, but not always, smaller than that of the first return stroke. As a consequence, the induced field changes are also usually smaller in amplitude and have a shorter duration than those of the first return stroke. dart leaders and the return strokes subsequent to the first are normally not branched. the combinations of the leader and the return stroke is known as a stroke. All strokes that use essentially the same channel to ground constitute a single cloud-to-ground flash. A flash might be made up of one to a few tens of strokes. Lightning discharges to ground can also be initiated by downward moving leaders that are positively charged. The resulting return stroke effectively lowers positive charge from the cloud to the ground. The combination of the leader and the return stroke is then called a positive stroke. Usually, there are no subsequent leaders down the existing channel, so that only the one stroke makes up the positive flash. Generally, positive flashes constitute only a few percent of all CG flashes. The peak current of their return strokes, however, can be larger than the peak current of the negative return strokes, and, thus, can cause greater damage than negative flashes. A large percent of forest fires and damage to power lines is probably caused by positive flashes. There is an extremely small percentage of flashes that are initiated from the tops of buildings and towers, as well as those triggered by rockets attached to ground by wire. Their leaders move up to the cloud. and their channels branch upward. THUNDER Thunder is caused by the extreme heat associated with the lightning flash. In less than a second, the air is heated to 15,000 to 60,000 F. When the air is heated to this temperature, it rapidly expands. When lightning strikes very close by, the sound will be a loud bang, crack or snap. The duration of the thunder associated with a nearby lightning strike will be very short. Lightning which strikes farther away will rumble for a longer period of time as the sound arrives at different times due to the length of the lightning flash (typically many miles long). Thunder can typically be heard up to 10 miles
away. During heavy rain and wind this distance will be less, but on quiet
nights, when the storm is many miles away, thunder can be heard at longer
distances. Lightning may strike anywhere, and getting the dramatic image on film is relatively easy compared to other nature photography situations. If you've been a photographer for a while, you've probably heard the old photographer's secret of success: "f/8 and be there." This is absolutely true when photographing lightning with one big exception. The f/8 is especially good, but you don't want to be exactly there. Five miles away is close enough. So how do you photograph lightning? If you have a place in mind that has a good vista, watch the weather reports and "be there" while the storm is passing. The following procedure assumes a tripod, ISO 100 film
speed, and a dark night sky: Shoot at Night To photograph lightning during daylight is next to impossible. There's too much light to slow down the shutter speed enough to capture lightning even under the dark clouds of a heavy storm. Lightning takes its own picture when and where it wants, and no matter how fast your reflexes are, it's gone before the shutter opens. If an image happens, it is either an accident, or the photographer was lucky enough to have a very long sequence of lightning hits at the same place. Almost all daylight situations can be very frustrating. The darker the sky, the longer the shutter is allowed to stay open at f/8. Therefore, your chances of getting a good photograph of lightning are best on a very dark night. In this situation you may leave the shutter open indefinitely and get multiple bolts in the same frame. The intensity of the storm, your distance from it, and your film
speed are variables. However, assuming the storm is approximately five miles
away and you are using ISO 100 film, f/8 is the best aperture. Lightning will
make an exposure at almost any aperture, but if you try to lengthen the exposure
time by closing the aperture more, or by using neutral density filters, you risk
getting images of the lightning bolts that are thinner than desirable. If the
storm is more than five miles away, or filtered by smog, you might try opening
the aperture to f/5.6. If it is closer than three to five miles try f/11, but
think of your safety first. |
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