statistically the change is relatively small. In a typical area of
central Florida the median stroke current is about 25kA, where in
central Colorado it is about 20kA. In the highest current range
(200-300kA) there are actually more strokes in Co .03% vs .02% in Fl.
Note, the database my software uses only covers 10 years so there is
still probably a fair amount of uncertainty in it.
It IS highly variable and in addition to the lower air density in
CO the humidity is typically much lower so there are probably
too many variables to make any simplistic statements.
A note on observing single storms. I have also seen the phenomena
mentioned where lightning seems to stop before it reaches my station and
then starts again on the other side. However in examining the
distribution of strokes for that time period it showed that the storm
approaching me was dying and the strokes were spaced out well before
they could have 'seen' my tower farm, and then a new (or rejuvenated)
cell started striking again on the other side. In my case I am near the
high point on a long wide n-s oriented ridge so it is not uncommon for
cells to be steered or disrupted by the uplift on one side... and I have
fairly frequently seen cells develop quickly on the eastern downslope
even when there was nothing to the west. A good analysis of strokes to
an area as small as a ham station would take many years of data,
probably much more than is available in the current u.s. dataset. Even
for detailed studies of power lines the minimum size data block we
normally use is 10 minutes on a side (about 10mi) and we average the
data over a 10 year period.
The W0ZV station and my station in CO were on flat prairie with
an absolute minimum number of trees and even relatively few
power lines because of the low population density. The cessation
of lightning as the cell passed overhear was dramatic. Smaller
storm cells would have frequent strikes as they approached until they
got within 2500 ft or so. Lightning would then STOP until the cell
passed by and would then start back up at about the same
distance on the other side. I saw this time and time again. While
the cell was overhead there would be no lightning but my guy wire
insulators would crackle almost continuously. W4ZV has confirmed
this in previous posts and K0RF and I have discussed this many times.
K0RF's site is much different in that it is on the prairie, but the foothills
are much closer. In addition, K0RF's site is on top of a small knob
that sticks up a couple of hundred feet above the surrounding land,
making his towers effectively much higher than their height.
I have NOT been a "student" of lightning in recent years--only trying
to reduce damage. Before I put up my big towers in CO I was hit
a number of times in the first two years that I was there. Hitting the
power pole a couple of times as well as a small vertical and another
short tower. So I took strikes that did some damage on average of
twice a year. Once I put up the big towers and properly grounded
them I had no more strikes. Except for a strike to the barn which
was 500 ft from the house where a 175 ft tower was located and the
roof line of the barn is actually BELOW that of grade level at the
house. But the barn was outside of any cone of protection from
the tower.
With a dozen years at the CO location the correlation between
properly grounded towers and a significant reduction of strikes
was very strong. To the extent that I worried about lightning damage
until I got the big tower up and then I never had any more strikes
to the house and tower area, and the one lone strike to the barn.
My interest in lightning was in the late 1970s and involved applying
my radio astronomy background to the interferometric tracking
of the stepped-leader at low VHF frequencies. By making positional
measurements every few micro-seconds it was possible to watch
the stepped leader move from one position to the next and "brew"
(pause) until it made the next jump. Previous research involved
"time-of-arrival" measurements that were not very good (translation=
lousy) but the interferometric measurements were beautiful.
Warwick, J. W., C. O. Hayenga, and J. W. Brosnahan, 1979: Inter-
ferometric directions of lightning sources at 34 MHz. J. Geophys.
Res., 84, 245--2468.
--John W0UN
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