BMP - Tree Risk Assessment - 28

Tree failures in normal wind speeds are usually associated with serious,
uncorrected, or unmitigated structural defects or other conditions, alone or
in combination.
When wind speeds are extreme (strong gale force in many areas), even
defect-free trees can fail. When wind speeds reach hurricane force, failure
of defect-free trees can be widespread. Wind speed is variable: winds can
be sustained, but commonly occur in gusts that may exceed the reported
wind speed. Microbursts that produce strong lateral winds can fell even
non-defective mature trees.
Normal and Extreme Wind
Storms can be classified into three broad categories based on frequency of occurrence,
wind speed, and precipitation-normal, extreme, and abnormally extreme. " Normal
weather " is a meteorological term used to describe the weather based on a location's
average temperature, wind, and precipitation for the previous 30 years. Extreme and
abnormally extreme storms are less clearly defined. Normal storms occur multiple
times during the defined time frame. Normal storms may include thunderstorms, snow,
and light accumulations of freezing rain in areas that are subject to those conditions.
Extreme storms occur less frequently within the defined time frame. These events
may include severe thunderstorms, accumulations of freezing rain, and straight-line
winds.
Abnormally extreme storms are unpredictable and occur infrequently. These events
include tornados, hurricanes, heavy freezing rain, or other events.
The Beaufort scale is a commonly used means of categorizing wind speed on a scale
of 1 to 12. Force 9 is a strong gale with a wind velocity of 47-54 mph (76-87 kph
or 21-24 m/s); this is normal in many areas. Tree failures at or below this level are
often associated with defects or conditions that affect a tree's strength or stability.
Force 10 is a storm wind with a velocity of 55-63 mph (88-101 kph, 25-28 m/s); in
many areas these wind speeds are considered extreme.
Force 11 wind (64-72 mph, 102-116 kph, 29-32 m/s), Force 12 hurricane-force
wind (>73 mph, >117 kph, >33 m/s), or higher winds are considered abnormally
extreme in most geographic areas. At these wind velocities, failure of defect-free
trees can be widespread.
It is important for tree risk assessors to be familiar with the weather conditions for
the region in which they are assessing trees. Tree failures are often associated with
forces and additional loading that are associated with weather. To help assessors
calibrate, weather condition guidance is built into the definitions for the ratings of
likelihood of failure.

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