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Lightning severely damages and kills thousands of trees each year. Many of
these trees line community streets and surround homes and schools. Lightning
strikes to ground occur 9 million times a day—6,200 times a
minute—100 times per second.
Figure 1 shows how many ground strikes per
year occur across the United States. This type of map is available for most
places around the world. Many of these ground strikes involve trees.
What Is Lightning?
Lightning comes in a number of forms. The lightning that damages trees is a
cloud-to-ground charge exchange. Ground-striking lightning balances atmospheric
electrical charges with the ground. Lightning follows a path with the least
resistance to electrical flow. The massive charge exchanges of lightning are
produced by interactions among water vapor, water droplets, and ice in the
lowest few miles of the atmosphere. Lightning generators are massive storm
clouds within which electrical charges become separated and concentrated.
Storms generate large updrafts and pull moisture-laden air up to high,
cold altitudes. Various types and sizes of precipitation form. The electrical
charges that generate lightning are formed when tiny ice crystals and larger
wet ice particles (graupel) collide. Millions of ice collisions per minute
within storm updrafts cause negative charges to be concentrated in the graupel
and positive charges to be concentrated in tiny ice crystals. These tiny,
charged ice crystals are blown to the top of the storm cloud while the larger
particles fall toward the middle and base of the storm cloud. Figure 2 shows a
simple view of where charges are located within a storm cloud.
Cloud Leader
Ground-striking lightning begins as fingers of negative charge continually push
downward from the lower portion of a cloud. The lower part of a storm cloud has
tremendously concentrated numbers of negative charges. The negative fingers of
charge, which quickly push out and pull back into the cloud, are called "cloud
leaders." Cloud leaders are constantly stretching downward through the air,
following the pathway of least electrical resistance. Precipitation columns,
recent lightning paths, and even cosmic ray paths help determine the zigzag
path of cloud leaders moving downward. The jagged form of lightning comes from
the stairstep pathway followed by cloud leaders dangling below a cloud. Cloud
leader movements are fast. Cloud leaders reach out for the ground at 450,000
miles per hour (Figure 3).
Ground Streamers
Because positive and negative charges attract, a wave of positive charge swells
up below the storm base. This wave of positive charge follows beneath the storm
along the ground and mounts up toward the storm base over and through any
object connected to the ground. Where the ground wave peaks on tall objects
(such as the tops of trees), the positive charges form "ground streamers."
Ground streamers are like short flags of positive charge flowing from tall
topographic features and structures (Figure 3).
Isolated, tall trees have the potential for strong ground streamer strength and
easily serve as a conduit of charge exchange. As cloud leaders approach close
to the ground (within 100 to 400 feet of the ground or tall objects), they make
a connection with ground streamers over the tops of tall structures. As the
opposite charge paths connect, the pathway is open for a rapid, massive charge
exchange. The associated light discharge is called lightning (Figure 3).
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International Society of Arboriculture
