3. EQUIVALENT UNIFORM
ICE THICKNESS
The ice that accretes on trees
and structures in freezing rain
varies tremendously in cross-sec-
tional shape. The shape depends
on the rate of rainfall; wind
speed; air temperature; humidity;
the shape, size, color, orientation,
exposure, and temperature of the
substrate; and on any internal
heating. On horizontal surfaces,
like streets and sidewalks, the ice
will form a uniform layer, per-
haps with thicker ice in low ar-
Figure 7. Ice on picnic table off Route 11, in northeastern
eas where the precipitation ponds
New York, January 8, 1998 (photo Mulherin).
(Fig. 7). On vertical surfaces, like
street signs, some ice will freeze
directly to the windward surface,
but much of the precipitation will
freeze as icicles as it runs down
the sign and begins to drip off
(Fig. 8). On branches, guys, ca-
bles, wires, conductors, angles,
and other two-dimensional com-
ponents, a wide variety of cross-
sectional ice shapes have been
observed (Fig. 9). Because of
this, one cannot compare report-
ed ice thicknesses, both with
each other and with the uniform
Figure 8. Ice on a sign near Dannemora, New York, Janu-
ice thicknesses in the codes, stan-
ary 8, 1998 (photo Mulherin).
dards, guidelines, and maps that
were discussed in Section 2.
We have found that the accreted ice load on two-dimensional objects (branches, wires, angle
sections) is best described in terms of the equivalent uniform ice thickness. This can be calculated
from either the weight or the volume of ice in a sample. If the ice sample on a wire, for example,
is weighed, then
d2
d
m
t=- +
+
(1)
4 πρi L
2
where t = equivalent uniform ice thickness (cm)
d = diameter of branch (cm)
7
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