more freezing rain a thin layer of ice will be visible on trees, making them glisten in the sun, but
not causing any structural damage.
The structure of trees and their susceptibility to ice loads are discussed in Heinrich (1997)
Hauer et al. (1984), and Burban and Andresen (1994). As the ice load increases, trees with a
greater proportion of their mass in small branches and twigs, decurrent branching patterns, and
large or lopsided crowns start to bend and break. Birch trees bent to the ground are often an early
warning of significant ice loads. Relatively low loads will strip branches off poplars, leaving
only the trunk. Tougher deciduous trees with excurrent branching patterns and fewer twigs with-
stand larger ice loads. Evergreen trees typically suffer the least damage in ice storms. Because
their branches are less exposed than the bare deciduous trees, the ice that accretes initially forms
a protective shell on the exterior needles and twigs. The ice-loaded branches tend to sag, but they
sag onto lower branches that provide additional support. When evergreen branches do break,
they are likely to be in trees with widely spaced branches. Trees of all kinds with limited or dam-
aged root systems may topple under an uneven ice load or be blown over by the wind. The num-
ber of uprooted trees in an ice storm also depends on the state of the ground. Trees are more easi-
ly uprooted in water-saturated, unfrozen soil than in dry or frozen soil.
Any tree damage causes outages in the power distribution system. Distribution lines typically
run along streets to distribute electricity from substations to city blocks, suburban neighbor-
hoods, and rural areas. Trees planted along streets and in yards and growing in woods cut through
by streets will initially bend under the ice load, causing shorts as they lean against the wires run-
ning along the streets and service drops that supply electricity to individual houses. As the ice
load increases, the wires and poles are forced to support more broken branches as well as the
shock load of heavily iced branches and uprooted trees falling on the wires. Compared to these
loads, the ice load on the wire itself is relatively insignificant. These broken trees may pull down
service drops, break wires, connectors, and crossarms, and break or uproot poles. Thus, the num-
ber and duration of outages in the electrical distribution system is heavily dependent on the
amount of tree damage and the proximity of the trees to the wires. Where there are no trees, out-
ages in the distribution system will begin only at relatively high ice loads. The potential of severe
tree damage to distribution lines in ice storms has long been recognized. A conclusion in the Ex-
ecutive Summary of a report for the Department of Energy on an investigation of outages as the
result of ice storms (Commonwealth Association 1979) was that "[t]he majority of the electric
power outages and most utility facility damage...were caused by tree limbs which broke or
sagged into electric utility overhead distribution lines." Their recommendations for reducing the
number of outages include more frequent and thorough tree trimming and better planning in the
planting of new trees near power lines.
Electrical power is transmitted from where it is generated at a hydro or coal plant, for exam-
ple, by high-voltage (typically 69-kV up to 735-kV and higher) transmission lines. The large-
diameter conductors hang by insulator strings from the crossarms of tall lattice towers, mono-
poles, wood H-frames, or other structures, to provide sufficient clearance between the tower and
the conductors, between the ground and the conductors, and between the different phases. Smaller-
diameter static or shield wires at the tops of the towers provide protection against lightning and
complete the electrical circuit. Because of resistance in the wires, heat is generated by the electri-
cal current and the conductor may be warmer than the ambient air temperature. There should be
very little current in the static wires, so they are at the ambient air temperature. Transmission
lines typically run through the trees in a wide right-of-way with the wires above the treetops, so
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