tion near the Portland airport, about 15 miles
Table 4. Possible effect of elevation on ice load.
south. Some of the differences between ice
Station
Elevation
Ice load
loads at low elevations and on hilltops are proba-
range (ft)
(in.)
bly from the higher wind speeds over hilltops as
well as from the differences between the actual
Burlington, VT
340900
1.1
temperature profile and this assumed gradient.
900
1.2
The range of modeled ice loads shown in
Barre, VT
11601720
0.2
Fig. 16 and Table 4 are consistent with ice loads
1720
1.1
we measured and with most estimated ice loads
Hanover, NH
500780
0.0
that we have been able to document. However, as
7801060
0.8
mentioned in Section 6.1.3, we have a number
10601340
0.9
of reports of much larger ice accretions on com-
1340
1.0
munication towers on mountaintops in New
Concord, NH
340900
0.1
Hampshire and Maine at elevations between 700
9001460
0.5
1460
0.6
and 1600 ft. The photo in Fig. 22a shows ice ac-
creted on 0.25-in.-diameter guys of a tower on
Portland, ME
100660
0.5
Cadillac Mountain (1500 ft) in Acadia National
660
0.9
Park on the coast of Maine. Using the diameter of
Brunswick, ME
80630
0.6
the guy for scale, assuming a round cross section
630
0.7
for the accreted ice, and assuming that this is
Augusta, ME
350
0.8
glaze ice, rather than lower-density rime ice, we
Bangor
190
1.2
estimated an equivalent uniform ice thickness of
between 4 and 5 in. This estimate is
conservative. If the ice is not glaze ice,
or if the accretion cross section is oval,
then the equivalent uniform thickness is
actually less. It is likely that ice that ac-
creted on structures and trees on some
mountaintops was from both freezing
rain falling from clouds that were in the
warm air layer, and in-cloud icing, from
windblown supercooled droplets in low
clouds in the cold air layer. The relative
contributions to the ice load from the
low-level clouds and from the rain can
only be estimated because cloud liquid
water contents are not measured except
at a few sites (at Mount Washington Ob-
servatory in supercooled clouds, for ex-
ample) and by research aircraft. From a
a. ~5 in. on a 0.25-in. guy on the summit of Cadillac
database of measurements from research
Mountain (photo Robert Parker, U.S. Coast Guard).
aircraft we have determined the average
liquid water content of supercooled stra-
Figure 22. Ice accretions with measured or estimated
tus-type clouds to be about 0.15 g/m3
uniform ice thickness.
32
Back to contents page
Previous Page
