Campbell CS110, Инструкция по эксплуатации, Страница: 27 / 86

Instruction Manual
17
The user is responsible for determining if a CS110 site is representative of the
CS110 2 Metre CM10 Tripod Site, and if not, for determining the
appropriate site correction.
The atmospheric electric field at the Earth’s surface during fair weather conditions
is on the order of –100 V/m; the negative sign indicating that the electrostatic
force on a positive charge is directed downward to the Earth’s surface
[McGorman and Rust],[Rakov and Uman]. Ballpark site corrections are sometimes
computed in fair weather conditions by assuming a -100 V/m fair weather field. The
accuracy of a fair weather site correction is questionable because local conditions
may result in a fair weather field significantly different (>100%) from –100 V/m.
Also, the unknown electric field offset may be significant when calibrating at –100
V/m. This offset can be measured by covering the stator with a clean Zero Electric
Field Cover (PN: 010346. Fair weather field site correction is not recommended for
lightning warning applications because of the relatively poor accuracy in determining
C
site
.
6. Lightning Warning
Lightning warning devices fall into two classes: lightning detectors and electric
field monitors. Stand-alone lightning detectors provide warning based on nearby
discharges, but give no warning until a detectable discharge occurs. Electric field
monitors measure the atmospheric electric field, indicating the presence of nearby
electrified clouds capable of producing lightning discharges. Consequently,
electric field monitors can give warning at the beginning of storms prior to
hazardous discharges. Both lightning detectors and electric field monitors are
employed in high-risk applications.
Lightning safety guidelines based on human observations exist and should
not be ignored simply because of the presence of sensitive electronic
instrumentation.
The NOAA 30/30 rule suggests seeking shelter if thunder is
heard within 30 seconds of a lightning flash (approximately 6 miles), and
remaining in a sheltered area for 30 minutes after the last lightning or thunder
before resuming outdoor activities [NOAA].
It should be noted that
no method of lightning warning completely eliminates
the risks associated with lightning
. As mentioned, lightning detectors give no
warning until a detectable discharge has occurred. Atmospheric electric field
yields warning prior to the “first strike” for storms developing overhead, along
with some indication of the end of a thunderstorm. Yet there are occurrences of
cloud-to-ground lightning discharges striking the ground several miles away from
the electrified cloud where the discharge initiated [NOAA]. Electric field
monitors may give no practical warning in these instances because the electric
field in the vicinity of the strike point may not indicate hazardous levels until
milliseconds before the strike. Consequently,
while lightning warning systems
can greatly reduce the probability of death or injury from lightning
discharges, they cannot reduce this probability to zero.