Q&A
Mapping Lightning
Q. The local TV weather programs
sometimes plot the various
lightning occurrences as they happen
— usually for a three hour period
around the state. I’ve wondered how
the lightning strikes are sensed and
plotted. Maybe this is done visually
from satellite images? Does that work
for lightning below the cloud deck?
Are there sensing stations that give a
bearing to the lightning strike for
stations to triangulate on — either
radio or visual sensors? I’m just a
curious ham operator.
Wally Willhard
Bountiful, UT
the same time, light travels at
186,000 miles per second, whereas
sound travels at the relative snail pace
of one-fifth of a mile in the same time.
Thus, the flash — if not obscured by
clouds — is seen before the thunder is
heard. By counting the seconds
between the flash and the thunder
and dividing by 5, an estimate of the
distance to the strike (in miles) can
be made.
In the NASA lightning sensor, a
low frequency receiver detects the
lightning strike. The leading edge of
the electric-field pulse is used to start
a timer and the leading edge of the
thunder pulse is used to stop the
timer. A microcontroller in each
receiver transmits the time measured
to a processing station, where the
times are converted to distances that
are used to compute the location of
the lightning strike (see Figure 2) to
within 12 inches. However, the NASA
sensors have to be located within a
30 mile radius of the strike to be
accurate — which is no big deal, given
that these receivers cost far less
to make and install than the GAI
direction finders.
You can build a simple, electric-field receiver using the schematic in
A. Satellites are more often used to
follow lightning strikes around the
world and haven’t advanced to the
point where they can accurately map
local areas like Bountiful, UT (but
we’re getting there). For that, a
network of ground sensors is used.
There are two types of sensors
commonly used — magnetic direction
finders and VHF interferometery.
The National Lightning Detection
Network (NLDN), which is operated
by Global Atmospherics, Inc., (GAI)
in Tucson, AZ (a wholly-owned subsidiary of Vaisala) is a network of
more than 130 magnetic direction
finders that covers the entire US —
more than twice the coverage of
existing weather radar networks.
Each direction finder determines
the location of a lightning discharge
using triangulation and is capable of
detecting cloud-to-ground lightning
flashes at distances of up to 250
miles and more. Processed information is transmitted to the Network
Control Center (NCC) where it’s
displayed in the form of a grid map
showing lightning across the US
( www.lightningstorm.com/tux
/jsp/gpg/lex1/mapdisplay_free.
jsp).
Recently, NASA has improved
the resolution of the system by
adding acoustical measurements to
the mix. Although the flash and
resulting thunder occur at essentially
SEPTEMBER 2004
Figure 2
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