Ice on Trees, Power Lines, and Superstructures

As I write this newsletter in mid-December, many locations in the
northeastern U.S. are still recovering from an ice storm that occurred
almost a week ago. This particular storm resulted in massive power outages,
and in some areas the power may not be restored for several more days. So I
thought I would talk about how ice storms develop, and how similar
phenomena can affect boats that may be on the water during the winter

    What is typically referred to as an ice storm is an event where a
significant amount of precipitation in the form of freezing rain occurs.
Freezing rain falls as liquid raindrops, but then freezes on contact with
just about anything. This results in a coating of glaze ice on trees and
power lines, among many other things. If the freezing rain persists long
enough, the coating of ice grows, and becomes heavy enough to weigh down
and perhaps break power lines, and to break off tree limbs and perhaps
entire trees.

    The meteorological situation that leads to an ice storm involves a
warm front. This is a boundary between warmer and colder air masses where
warm air rides up over colder air near the surface. As the warm air is
lifted, it cools leading to condensation and precipitation. When the
temperatures of the respective air masses fall into a certain range, this
precipitation can occur in the form of freezing rain. This occurs when the
warm air being lifted over the colder air has a layer where temperatures
are above freezing, allowing precipitation which develops higher in the
atmosphere as snow to melt into rain when it falls into this warm layer. As
the precipitation continues to fall into the colder (below freezing) air
near the surface, the raindrops will freeze into ice pellets, or sleet, if
the depth of the cold air is enough to allow this process to occur. If the
depth of the cold air is less, and the liquid raindrops do not have time to
freeze before reaching the surface, then they will freeze on contact
leading to the potentially devastating glaze ice.

    Since warm fronts are moving most of the time, and the area where
freezing rain can occur is limited (cold air needed near the surface, but
not too much, which would result in sleet), very often when freezing rain
occurs, it does not last very long and its effects are minimal. However, in
certain cases, like the one last week, a warm front can become stationary
with areas of low pressure propagating along it, and this situation can
lead to prolonged freezing rain in some areas which in turn leads to the
heavy icing.

    With this accumulation of ice on trees and power lines causing all the
trouble chronicled in news reports, it begs the question, what would happen
if this occurred at sea? Similar situations do indeed occur at sea,
although for most recreational boaters, the situation occurs during the
time of year when their boats are in the yard under shrink wrap. For
commercial fishermen, or other mariners who need to operate during the cold
season, though, ice accretion on their vessels is a serious concern. This
phenomena can lead to the failure of antennas and other electronic
equipment, and can make the deck very dangerous for crew members. But the
most serious effect of ice accretion has to do with changes in stability of
the vessel. As weight is accumulated on the vessel above the waterline,
especially over the higher areas, the vessel loses stability and, in
extreme cases, can capsize. In fact, this situation caused the loss of a
fishing vessel and its crew in January 2007, in Nantucket Sound.

    Freezing rain, while it can produce ice accretion on a vessel, is not
the source of the most serious icing on a vessel. Rather, it is freezing
ocean spray which will produce significant, and at times rapid ice
accumulation on all surfaces above the waterline. When a vessel is
operating in air temperatures which are below 28°F (the freezing point of
salt water), and in heavy sea states with strong winds, the amount of water
which is moving across the deck due to spray is much greater than that
produced by precipitation. Thus for vessels operating in the winter, the
combination of cold temperatures and strong winds is more likely to produce
problems due to ice accretion than freezing rain.

    On land or at sea, weather situations that produce a coating of glaze
ice are likely to cause fairly significant problems. Anticipating these
situations and making preparations ahead of time can mitigate the effects
of icing to a degree. In the case of vessels at sea, changing course when
in a heavy sea state (so the vessel is running with the sea) can reduce the
amount of freezing spray, which can make a big difference. Ideally, vessels
will be able to make use of forecast products and avoid areas where heavy
freezing spray is likely.

By Ocean Navigator