Lightning. It avoids the ocean, but likes Florida. It’s likely to
strike in the Himalayas and even more so in central Africa. And
lightning almost never strikes the North or South Poles.

These are just a few of the things NASA scientists at the National
Space Science and Technology Center (NSSTC) in Huntsville, Ala., have
learned using satellites to monitor worldwide lightning.

“For the first time, we’ve been able to map the global distribution of
lightning, noting its variation as a function of latitude, longitude
and time of year,” said Hugh Christian, a scientist from NASA’s
Marshall Space Flight Center in Huntsville, Ala. and project leader
for the lightning team at the NSSTC’s Global Hydrology and Climate
Center.

This new perspective on lightning is possible thanks to two
satellite-based detectors: the Optical Transient Detector (OTD) and
the Lightning Imaging Sensor (LIS).

“These are two optical sensors that we’ve flown in lower Earth orbit,”
said Christian, whose team developed the sensors. “The Optical
Transient Detector was launched in 1995 and we got five good years out
of it, compared to the two years expected, before it stopped
transmitting data. The Lightning Imaging Sensor was launched on the
Tropical Rainfall Measuring Mission satellite in 1997, and it’s still
going strong.”

“Basically, these optical sensors use high-speed cameras to look for
changes in the tops of clouds, changes your eyes can’t see,” he
explained. By analyzing a narrow wavelength band around 777 nanometers
— which is in the near-infrared region of the spectrum — they can
spot brief lightning flashes even under daytime conditions.

Before the Optical Transient Detector and Lightning Imaging Sensor,
only approximate global lightning patterns were known. Ground-based
lightning detectors employing radio-frequency sensors provide
high-quality local measurements. But because such sensors have a
limited range, oceans and low-population areas had been poorly
sampled.

The development of space-based optical detectors was a major advance,
giving researchers their first complete picture of planet-wide
lightning activity. The new maps show that Florida, for example, is
one place where the rate of strikes is unusually high.

Dennis Boccippio, an atmospheric scientist with the NSSTC lightning
team, explained why: “Florida experiences two sea breezes: one from
the East Coast and one from the West Coast.” The “push” between these
two breezes forces ground air upward and triggers thunderstorms.

Within thunderclouds, turbulence spawned by updrafts causes tiny ice
crystals and water droplets, called “hydrometeors,” to bump around and
collide. For reasons not fully understood, positive electric charge
accumulates on smaller particles — that is, on hydrometeors smaller
than about 100 micrometers — while negative charges grow on the
larger ones. Winds and gravity separate the charged hydrometeors and
produce an enormous electrical potential within the storm.

“Lightning is one of the mechanisms to relax this build-up,” said
Boccippio.

Another lightning hot spot is in the Himalayas, where the extreme
local topography forces the convergence of air masses from the Indian
Ocean.

And where does lightning strike most frequently? Central Africa.
“There you get thunderstorms all year ’round,” Christian said. “It’s a
result of weather patterns, air flow from the Atlantic Ocean, and
enhancement by mountainous areas.”

The satellite data also track patterns of lightning intensity over
time. In the Northern Hemisphere, for example, most lightning happens
during the summer months. But in equatorial regions, lightning appears
more often during the fall and spring.

Meanwhile, areas such as the Arctic and Antarctic have very few
thunderstorms and, therefore, almost no lightning at all.

“Oceanic areas also experience a dearth of lightning,” Christian said.
“People living on some of the islands in the Pacific don’t describe
much lightning in their language.” The ocean surface doesn’t warm up
as much as land does during the day because of water’s higher heat
capacity. Heating of low-lying air is crucial for storm formation, so
the oceans don’t experience as many thunderstorms.

According to Boccippio these global patterns probably aren’t much
influenced by human activity. Some people have suggested that
buildings and metal communications towers increase the overall
frequency of lightning strikes. But, “lightning that does make it to
the ground is pretty much creating its own channels,” Boccippio said.

“The likelihood that we are changing the amount of cloud-to-ground
strikes with construction of towers is very slim.” He cautions,
however, that this has not been verified experimentally.

To answer such questions, a new lightning detector — the Lightning
Mapper Sensor or “LMS” — is on the drawing board at the National
Space Science and Technology Center. The proposed instrument would
circle our planet in a geostationary orbit over the United States,
detecting all forms of lightning with a high spatial resolution and
detection efficiency.

The LMS, or something like it, could provide valuable — even
life-saving — data to weather forecasters. “The same updrafts that
drive severe weather often cause a spike in the lightning rate at the
onset of a storm,” explained Boccippio. So, measuring the rate of
lightning flashes in real time might offer a way to identify
potentially deadly storms before they become deadly.