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Public Information Statement
National Weather Service Boston/Norton MA
828 AM EDT Fri Jun 24 2022

...NATIONAL LIGHTNING SAFETY PREPAREDNESS WEEK CONTINUES...

Be Aware!

The National Weather Service (NWS) Boston, MA has declared June
20
through June 24 as Lightning Safety Awareness Week. Each day
during the awareness week will feature information about a
different lightning related topic.

    The Science of A Lightning Strike...

At any given moment there are 1800 thunderstorms in progress
somewhere on the Earth, which amounts to 25 million lightning
flashes each year. Lightning researchers have a better
understanding today of the process that produces lightning, but
there is still much to learn about the role of solar flares in
the upper atmosphere, the Earth's electromagnetic field, and
ice in storms. We know the cloud conditions needed to produce
lightning, but cannot forecast the location or time of the next
stroke of lightning.

Lightning occurs in volcanic eruptions, intense forest fires,
surface nuclear detonations, heavy snowstorms, and large
hurricanes, but it is most often seen in thunderstorms.
Thunderstorms form in air that is moist, unstable, and has a
trigger that causes the air to rise, such as a cold front.
Rising motions within the storm build the cloud to as high as
6 to 10 miles above sea level. Ice forms in the higher parts of
the cloud.

The ice particles vary from small ice crystals to large
hailstones. There are a lot of collisions between the particles
which causes a separation of electrical charges. Positively
charged ice crystals rise to the top of the storm, and
negatively charged particles and hailstones drop to the middle
and lower parts of the storm. Enormous charge differences then
develop.

A moving thunderstorm gathers another pool of positively charged
particles along the ground that travel with the storm. Positively
charged particles rise up taller objects such as trees,
houses, and telephone poles. These particles can even move up
you. Have you ever been under a thunderstorm and had your hair
stand up? if so, you may be the lightning target.

The negatively charged area in the storm sends out a charge
toward the ground called a stepped leader. It is invisible to the
human eye. When it gets close to the ground, it is attracted by
all of these positively charged objects and a channel develops.
You see the electrical transfer in this channel as lightning.
There may be several return strokes of electricity within the
established channel that you will see as flickering lightning.
The lightning channel heats rapidly to 30 thousand degrees or
more and the rapid expansion of heated air causes the thunder.
Since light travels faster than sound in the atmosphere, the
sound is heard after the lightning. If you see lightning and hear
thunder at almost the same time, the lightning is in your
neighborhood.

Not all lightning forms in the negatively charged area low in the
thunderstorm cloud. Some originates in the cirrus anvil at the
top of the storm, where there is a large positive charge. A
strike originating in this area is called a positive flash. It is
particularly dangerous for several reasons. It frequently strikes
ahead of or behind a thunderstorm, away from the rain area,
thus catching people by surprise, like a bolt from the blue.
Positive strikes typically last longer, so fires are more easily
ignited. Also, they usually carry a high peak electrical current
which increases the lightning risk to an individual.

When thunder roars, go indoors.

Be a Force of Nature. When Thunder Roars, Stay Indoors!  Find out
tips for safety indoors and outdoors near and far from home at
our
website: weather.gov/lightning

$$

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