continuing education unit
Storm Response
Part 1:Types of Storms and
Their Effects on Trees
By Geoff Kempter
Objectives
• Understand the important role arborists play in responding to
storms
• Describe how loads and stresses added by storms can affect trees
• Discuss some basic information about storms, such as correct
terminology, how storms form and develop, and how rating
scales are applied
• Explain how regional characteristics make some areas more
vulnerable to certain kinds of storms than others
CEUs for this article apply to Certified Arborist, Utility Specialist,
Municipal Specialist, Tree/Worker Climber, and the BCMA
management category.
A
Arborists are intimately tied to the weather, which affects
both daily operations and the trees on which they work.
Storms, as extreme weather events, have an enormous
impact on trees and infrastructure. Knowing how storms
affect trees, how to anticipate and mitigate possible damage ahead of time, and how to effectively respond when
storms strike are important skills for arborists to develop.
In the first of this three-part series of articles, we will
examine how storms affect tree structure, and provide an
overview of the different kinds of storms that often cause
the most damage to trees.
When essential services are interrupted by storms, lives
and livelihoods may be threatened, and utilities, municipalities, and other service providers incur considerable restoration
costs. Moreover, the U.S. Department of Energy estimates
that electric utility service interruptions caused by storms
cost businesses and communities tens of billions of dollars
annually (Campbell 2012). The clearing of trees and branches
after a storm provides faster access for first responders,
line repair crews, and other emergency service personnel.
12 | Arborist•News | www.isa-arbor.com
Whether a storm response consists of one crew dispatched
to remove a single fallen tree, or involves thousands of personnel across an entire region, tree service contractors,
utilities, and their suppliers must be prepared to provide
a swift and well-coordinated response with the necessary
personnel and equipment. It is also important to consider
worst-case scenarios, both locally and in other locations,
and prepare accordingly, and to understand that storm
work poses unique safety challenges that require proactive
measures prior to and during responses (Chisholm 2013).
In most cases, tree crews are among the first to be dispatched to stricken areas. As such, storm response can be
viewed as an opportunity to demonstrate the value and
professional capabilities of all arborists. A successful, large
scale response requires thorough pre-planning, coordination between contractors, utilities, and government authorities, open lines of communication, established chains of
command, and flexibility as workloads and conditions
change. A successful response also allows everyone involved
to work together for a common goal: the restoration of critical services to the communities and customers served by
our industry.
Effects of Storms on Trees
How Branches and Trees Fail
Trees are structures, and like all structures, they have breaking points. As trees grow, they continually adapt to the
load of their own weight as well as added loads brought
on by precipitation and wind. Over time, trees also develop
defects, such as leans, decay, girdling roots, or poor branch
attachments. They can also be damaged in ways that
weaken their structure. Storm conditions increase loads
and stress, which when combined with defects, can lead
to failure (Dunster et al. 2013).
Effects of Rain, Ice, and Snow
Trees must be able to support their own weight against the
constant pull of gravity. In addition, they must support
the accumulated weight of various types of precipitation.
Most trees can tolerate normal conditions, including typical
rainfall, light icing, or snowfall.
However, unusual or extreme
conditions increase the likelihood
of failure. Examples include:
• accumulating ice on
twigs and branches
• heavy downpours on
dense foliage
• wet snow, especially on
trees with leaves on
Distributed across the whole
tree, the added weight leads to
failure if the loads exceed trunk,
branch, or root stress tolerances.
The effects of additional loads
are amplified by the length of a
branch (lever arm), the density
of twigs or foliage, the degree
of lean, and wind.
Effects of Wind
Wind places loads on trees, Utility arborist clears ice-laden branches from utility facilities.
including trunks, branches, and
roots, leading to higher stresses. The varying lengths and
with wind and other added loads, the likelihood of wholesizes of tree branches tend to dampen the overall effects of
tree failure is increased.
wind; however, as wind speeds increase, the loads placed
on stress points within the tree increase exponentially
Storm Types
(Table 1). When the loads exceed trunk, branch, or root
Storms come in many forms, and the damage to trees varies
strength, various types of failures occur.
depending on storm type, location, and intensity. Understanding how storms form, develop, and move is important
Effects of Saturated Soils
in preparing a strategy for response. Though all types of storms
Certain soils can become saturated by heavy rains. Satucan cause severe damage to trees and utility infrastructure,
ration reduces the friction between roots and soil and the
different storms pose unique operational considerations.
holding capacity of roots. When this effect is combined
Table 1. In North America, hurricanes are categorized by intensity according to the Saffir-Simpson Hurricane Wind Scale.
Other areas of the world have differing systems for classifying the intensity of tropical cyclones (adapted from NOAA).
Category
Sustained winds
mph km/hKnots
Types of damage due to hurricane winds
1
74–95 119–153 64–82 2
96–110 154–177
83–95
3 (major)
111–129 178–208 96–112 4 (major)
130–156 209–251
113–136
5 (major)
157+
252+
137+
Some damage. Some tree branches snapped and some trees toppled. Extensive damage
to power lines and poles, causing power outages lasting a few to several days.
Extensive damage. Many tree branches snapped and many trees uprooted, blocking
numerous roads. Near-total power loss expected with outages lasting from several days
to weeks.
Devastating damage. Many trees snapped or uprooted, blocking numerous roads.
Electricity and water unavailable for several days to weeks.
Catastrophic damage. Most trees snapped or uprooted and utility poles downed.
Residential areas isolated. Power outages last weeks to possibly months. Most of the
area uninhabitable for weeks or months.
Catastrophic damage. Most trees snapped or uprooted and utility poles downed.
Residential areas isolated. Power outages last weeks to possibly months. Most of the
area uninhabitable for weeks or months.
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june 2013 | 13
continuing education unit
National governments issue watches and warnings for
potentially affected areas as storms become threatening, or
the probability of development increases. In general, a watch
means that conditions are favorable for storms to occur,
while a warning indicates that a storm is imminent or occurring, and that immediate action is required (NOAA 2013).
the center. They are often the underlying cause of various
other kinds of storms and are usually accompanied by
one or more form of precipitation (American Meteorological Society 2013). A cyclone is not the same as a tornado,
which is a local phenomenon, although tornados often occur
as a result of cyclones (Australian Government Bureau of
Meteorology 2013).
Tropical cyclones develop over warm ocean waters and
feature strong winds and heavy rains. Development of tropical cyclones peaks during late summer and early autumn
when waters are warmest. The most powerful tropical
cyclones are known as hurricanes in the Atlantic and eastern Pacific, typhoons in the northwestern Pacific, and simply
tropical cyclones in the South Pacific and Indian Oceans
(Australian Government Bureau of Meteorology 2013).
The development of tropical cyclones is carefully monitored by both meteorologists and those who may be affected
by a strike. Because their movements can be forecasted,
those in or near the forecasted storm track usually have
several days to prepare prior to landfall.
Extratropical cyclones, as the name implies, occur in
higher latitudes, and often mark the boundary between
different air masses that characterize changes in weather.
They are frequently associated with severe weather, including high winds, heavy rain, snow, thunderstorms, and
tornados. Extratropical cyclones are often larger than tropical cyclones, but tend to have lower sustained wind velocities. Tropical cyclones often become extratropical as they
move into higher latitudes. Superstorm Sandy, which
struck the eastern coast of North America in 2012, was
making such a transition as it made landfall. Sandy was
unusual—and especially devastating—in that it combined high winds and storm surge typical of a hurricane
with the large size and diverse conditions, including heavy
mountain snows, more typical of an extratropical cyclone
(Voiland 2012).
Cyclones
“Cyclone” refers to any large area of circulating, low atmospheric pressure, including tropical systems such as hurricanes and typhoons, and extratropical cyclones, which shape
the weather in higher latitudes. Cyclones rotate clockwise
in the southern hemisphere and counterclockwise in the
northern hemisphere, with winds spiraling inward toward
digitalglobe
Thunderstorms
Large extratropical cyclone off the west coast of North America.
14 | Arborist•News | www.isa-arbor.com
Thunderstorms form when moist, unstable air rises into
the atmosphere. They feature heavy rain, lightning, thunder, sometimes hail, and are often accompanied by strong
winds (Scientific American 1999). Thunderstorm intensity often varies regionally; for example, in North America,
thunderstorms are most common along the Gulf Coast
and in Florida. However, a higher frequency of severe thunderstorms occurs in the central United States. Straight-line
winds in severe thunderstorms have been measured at over
120 mph (193 km/h) (Johns et al. 2012). The likelihood
of thunderstorms impacting any given area is often forecast days in advance; however, the precise path and intensity of individual storms is generally not known until hours
or less prior to strike.
A derecho is a widespread and long-lived complex of
thunderstorms that travels more than 240 miles (386 km),
NOAA
with wind speeds greater than 58 mph (93 km/h). Distances of more than 700 miles (1126.5 km) and wind
gusts over 100 mph (161 km/h) are not uncommon. For
many inland areas, derechos are among the most destructive
storms in terms of tree damage, utility service interruptions,
and cost of repairs (Johns et al. 2012).
A tornado, also known as twister, is a violently rotating
air column extending from a cloud to the earth (Edwards
2013). In some areas, the term cyclone is incorrectly used to
identify a tornado. Tornados form in certain types of
thunderstorms and in tropical cyclones making landfall.
A funnel cloud is a forming tornado; technically, it is not
a tornado until it reaches the ground.
Winds in tornados may exceed 200 mph (322 km/h)
and can cause extreme damage where they strike; however,
damage is usually restricted to a relatively narrow corridor.
The Enhanced Fujita Scale (EF Scale) is used to characterize tornados according to their estimated strength, from
EF-0 to EF-5 (Table 2). EF ratings are based on observed
damage following the storm, as it is not possible to measure
the actual wind speeds in individual tornados.
Map showing severe thunderstorm watch frequency in the continental United
States.
Three-second gust
mphkm/h
0
1
2
3
4
5
65–85105–137
86–110138–177
111–135178–214
136–165215–266
166–200267–322
200+322+
Tornados occur in temperate
regions around the world, but are
most common in North America
east of the Rocky Mountains, from
southern Canada to the Gulf Coast,
and eastward to the Atlantic Ocean,
with the highest frequency from
the south-central Great Plains eastward to the Appalachian Mountains. Some tornados are isolated
events, but when conditions are
favorable, widespread outbreaks of
tornados are possible. Forecasters
are capable of identifying conditions favorable for the development
of tornados, monitoring development, and for issuing warnings
when necessary; however, they are
unable to provide advance accurate forecasts of the timing, strength,
and location of tornados.
Tornado in Dimmitt, Texas, in 1995.
G. Carbin, NOAA Strom Prediction Center
EF Rating
StormRespArt
Table 2. The Enhanced Fujita Scale.
Composite radar image showing path of derecho across the eastern U.S., June 29-30, 2012.
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continuing education unit
Coniferous trees in snowy areas naturally shed snow, or have
adapted to heavy snow loads, but wet snow can cause severe
damage to conifers that are not well-adapted to snow loads.
Other High-wind Phenomena
Local terrain, climate, and seasons can combine to create
unique local wind events. Terms such as “Chinook” (Rocky
Mountains of U.S. and Canada), “Santa Ana” (California
and Mexico), “Scirocco” (Mediterranean), “Nor’easter” (eastern coast of U.S. and Canada), “Nor’wester” (New Zealand),
and “Southerly Buster” (Australia) are just a sampling of
names that refer to unique wind phenomena in various parts
of the world. They often occur seasonally and come from
a specific direction. The known or common wind history
for a region can be useful for increasing success in selecting,
placing, and managing trees in any region of the world, and
for planning appropriate storm responses when necessary.
Conclusion
Storms are natural occurrences, affecting trees and people
all over the world. Arborists are increasingly requested to
respond to storms that have occurred hundreds or even
thousands of miles (or kilometers) away from their home
territories. As trees and urban forests are increasingly recognized as valuable community assets, an understanding
of the basics of different storm phenomena and how they
affect trees becomes more important. In addition, our growing knowledge of tree risk and tree failure patterns allows
for better assessment and mitigation of risk both before
and after storms strike.
As much as any profession, arboriculture is intimately
tied to weather. When storms strike, the specialized skills
of all arborists are in short supply. As such, arborists have
a responsibility, both to their clients and to the public at
large, to respond quickly, safely, and effectively.
Conifers in snowy areas are adapted to snow loads.
Winter Storms
Ice storms develop when rain from relatively warm air falls
through cold air near the ground that is below freezing
temperatures. Under such conditions, raindrops become
supercooled [below 0°C, (32°F)] but remain in a liquid
state until they hit something and turn to ice. Trees and
utility infrastructure, having lots of surface area, can accumulate many times their weight in ice and can be severely
damaged in heavy ice storms.
Heavy wet snows (generally defined as lower than a
10:1 snow-to-water equivalent) occur when temperatures
are near freezing. In addition to being heavier than drier
snows, these snows tend to cling to trees and infrastructure. They cause the most damage to trees in spring and
autumn, when leaves are present on deciduous trees.
16 | Arborist•News | www.isa-arbor.com
Part two of this three-part series will discuss planning, preparation, and responses to storms, both large and small. Part three
will examine unique operational considerations presented by
storm response, including safety, reporting requirements, media
relations, preserving damaged trees, and returning home safely.
Literature Cited
Campbell, Richard J. 2012. Weather-Related Power
Outages and Electric System Reliability, Congressional
Research Service, 7-5700. <www.crs.gov>
Chisholm, Mark. 2013. Trees and Storms: A Recipe for
Hazards, Learn Storm Safety. STIHL Corporation.
<www.learnstormsafety.com/safety101.html>
Dunster, Julian, et al. 2013. Tree Risk Assessment Manual.
International Society of Arboriculture. Champaign,
Illinois, U.S.
NOAA. 2013. Weather Ready Nation. <www.nws.noaa.
gov/com/weatherreadynation>
u
NOAA
Composite showing all tropical cyclone tracks, 1950–2005.
Areas Vulnerable to Tropical Cyclones
In the North Atlantic, hurricanes most often affect the Caribbean islands, Central America, Mexico, the Gulf and eastern
coasts of the United States, and eastern Canada. In the eastern Pacific, hurricanes are relatively common on the west coast
of Mexico, and less so in the Hawaiian Islands. In the western Pacific, the islands and coastal regions north of 10° latitude
as far north as Russia are susceptible to typhoons.
The northern Indian Ocean is a breeding ground for tropical cyclones, which affect the southern coast of Asia, including
the Indian subcontinent.
In the Southern Hemisphere, the South Pacific Islands, the northern portions of Australia, New Zealand, Madagascar, and
the east coast of Africa are vulnerable to tropical cyclones. Tropical cyclones do not occur off the west coast of South America, and occur only rarely in the South Atlantic Ocean.
Tropical cyclones depend on favorable atmospheric conditions and warm open water to gain and maintain strength. After
making landfall, they lose strength at a rate that depends on overall intensity, terrain, and forward speed. For example, in
2005, Hurricane Katrina made landfall and moved quickly inland over relatively flat terrain in the southern United States.
Hurricane force winds extended 200 miles (322 km) inland and tropical storm force winds were felt 400 miles (644 km)
from the coast. On the other hand, as storms encounter mountainous terrain, their strength can be quickly reduced.
Damage to infrastructure can be from both wind and flooding. Low-lying areas, such as barrier islands and river mouths,
can be severely affected by the combination of storm surge, heavy surf, and high winds. These areas are often heavily developed, resulting in costly damage. Wetlands are known to cushion the impact of tropical cyclones; however, wetlands are
often damaged or removed by development, leaving some areas even more vulnerable. Steep terrain, while reducing the
intensity of storms, also increases the rate of runoff from associated heavy rains, and often contributes to the incidence of
flash floods.
Tropical cyclones have been battering the world’s coasts for eons—long before human beings and their assets were present
to be affected. With a clear understanding that storm strikes are inevitable in vulnerable areas, strategies increasingly focus
on making infrastructure more resilient, and preparing for swifter response and recovery.
june 2013 | 17
continuing education unit
American Meteorological Society. 2013. Glossary of
Meteorological Terms. <http://glossary.ametsoc.org/
wiki/Cyclone>
Australian Government Bureau of Meteorology. Tornado,
twister hurricane, tropical cyclone, typhoon – what’s
the difference? <www.bom.gov.au/index.php>
Voiland, Adam. Hurricane Sandy: NASA’s Earth Observatory. <www.nasa.gov/mission_pages/hurricanes/
archives/2012/h2012_Sandy.html>
Scientific American. 1999. Science Desk Reference. John
Wiley and Sons, New York, New York, U.S.
Johns, Robert H., Jeffry S. Evans, and Stephen F. Corfidi
et al. 2012. About Derechos: NOAA Storm Prediction
Center. <www.spc.noaa.gov/misc/AbtDerechos/
derechofacts.htm#strength>
Edwards, Roger. 2013. The Online Tornado FAQ.
Storm Prediction Center: NOAA Storm Prediction
Center. <www.spc.noaa.gov/faq/tornado>
Geoff Kempter is Technical Services Manager for the
Asplundh Tree Expert Co., where he spent seven years as an
assistant storm coordinator. He has been an ISA Certified
Arborist since 1991, and recently received the TRAQ
qualification from ISA. He also serves on the ISA Board
of Directors and the ASC A300 Accredited Standards
Committee, and is addicted to weather-related websites.
CEU Questions
Now you can take this quiz, and others, online for free! Visit the ISA website for more information (www.isa-arbor.com/education/
onlineLearning/ arbNewsQuizzes.aspx). If you need a username and password, send us an e-mail ([email protected]).
To receive continuing education unit (CEU) credit (1 CEU) for home study of this article, after you have read it, darken the appropriate circles
on the answer form of the insert card in this issue of Arborist News. (A photocopy of the answer form is not acceptable.) A passing score for this
test requires 16 correct answers.
Next, complete the registration information, including your certification number, on the answer form and send it to ISA, P.O. Box 3129,
Champaign, IL 61826-3129. Answer forms for this test, Storm Response Part 1: Types of Storms and Their Effects on Trees, may be sent for
the next 12 months.
If you do not pass the quiz, ISA will send you a retake answer sheet. You may take the quiz as often as necessary to pass. If you pass, you will not be
notified; rather, you will see the credit on your CEU report (available online). Processing CEUs takes four to six weeks.
CEUs for this article apply to Certified Arborist, Utility Specialist, Municipal Specialist, Tree Worker/Climber and Aerial Lift, and the
BCMA management category.
1.After a strong storm hits an area
a. some arboricultural safety rules
are suspended
b. fallen trees and branches must be
removed to provide access to stricken
areas
c. trees must not be removed because
they provide essential services
d. trees that remain standing have
proven that they are not hazardous
2.For arborists, storm work is
a. nothing more than a distraction
from scheduled work
b. an opportunity to hike prices
c. impossible to prepare for because
every storm is different
d. an opportunity to demonstrate
professional capabilities
3.Leans, decay, girdling roots, and
poor branch attachments
a. are defects that, if one or more are
present, require tree removal
b. are definite indicators of future
failures
c. seldom lead to tree or branch failure
d. are defects that develop over time
18 | Arborist•News | www.isa-arbor.com
4.Storm conditions
a. increase loads and stress in trees
b. make trees more brittle
c. do not damage structurally stable
and healthy trees
d. predominantly result in whole-tree
failure
7.Saturated soils
a. are unhealthy due to high fat content
b. reduce friction between roots and
soil
c. do not affect the holding capacity
of roots
d. result in failure at the trunk flare
5.The accumulated weight of rain,
snow, and ice
a. is minor in comparison to the
weight of the tree
b. is added to the load of the tree’s
own weight
c. can only cause branch failure
d. places the same amount of force
on key stress points
8.Generally, a government may issue a
watch for storms when
a. conditions are favorable for a
storm to occur
b. a storm is striking a nearby area
c. immediate action is required
d. storms have dissipated, but may
redevelop
6.The load added by wind
a. is not a concern for inland areas
b. increases in a linear fashion as
wind speed increases
c. will strip leaves before trees and
branches fail
d. is dampened by the varying
lengths and sizes of branches
9.Strategies for storm response
a. are pretty much similar for all
types of storms
b. vary depending on what kinds of
trees have failed
c. are dependent upon knowing how
storms form, develop and move
d. are determined by government
agencies
10.A cyclone
a. is another name for a tornado
b. is like a tornado only much larger
c. is a large area of circulating, low
atmospheric pressure
d. only occurs in the tropics
11.The type of storm known as a hurricane in North America is called a(n)
a. typhoon in the northwestern Pacific
b. monsoon in India
c. extratropical cyclone in Australia
d. typhoon in the eastern Pacific
12.The Saffir-Simpson Scale is
a. used to rate tornado intensity
b. only used when storms are over
warm ocean waters
c. not useful in planning response to
storms
d. one of several methods used to
rate tropical cyclones around the
world
13.Tropical cyclones strengthen and
develop
a. only near land with low coastlines
b. over warm, open water with
favorable atmospheric conditions
c. at the equator, then move north
or south from there
d. over flat, low-lying land, with
devastating consequence
14.Extratropical cyclones
a. have stronger winds than tropical
cyclones
b. never form from tropical cyclones
c. are large systems that are
associated with many kinds of
severe weather
d. are always formed from colliding
warm air masses
15.The Enhanced Fujita scale
a. characterizes the relative intensity
of typhoons
b. measures the wind speed of tornados
c. weighs the impact of hurricanes
d. characterizes tornados according
to their estimated strength
16.Thunderstorms
a. are most intense in southern climates
b. can generate straight-line winds in
excess of 120 mph (193 kph)
c. cannot be forecasted because they
develop quickly
d. cannot spawn tornados if it is
raining
18.An ice storm develops when
a. rain freezes before it lands
b. raindrops are supercooled when
falling through air below 32°F (0°C)
c. supercooled water forms from
rapidly melting snowflakes
d. cold fronts collide with warm fronts
19.Heavy, wet snows
a. are more likely to damage
evergreen trees than other kinds
of trees
b. do not damage evergreen trees
because they are well-adapted
c. are especially damaging to
deciduous trees with leaves on
d. are associated with extratropical
cyclones making landfall
20.Chinook, Sanata Ana, Scirocco, and
Nor’wester are names of
a. cars
b. fish
c. winds
d. derechos
17.A widespread and long-lived complex
of thunderstorms that travels a long
distance is called a
a. derecho
b. southerly buster
c. straight-line wind
d. linear vortex
I n t r od u c t i o n
t o
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