The structure of an iceberg, and to some extent the
appearance, depends upon the ice that produced the
berg. Pinnacled bergs come from glaciers that plow
across uneven ground on their way to tidewater, but
may also develop as fragments that break off of shelf
ice. Tabular bergs are produced as portions of sheet ice
separate and float free, but may also be produced as
large portions of shelf ice break free. Pinnacled bergs
are more common in the Northern Hemisphere, while
tabular bergs are more common in the Southern
Icebergs are simply called "icebergs" when they are
large or massive. Smaller pieces of ice are called "bergy
bits" and "growlers."
Like icebergs, bergy bits and
growlers originate from glaciers or shelf ice. They may
also form as larger icebergs disintegrate. A bergy bit is a
medium-size fragment of glacial ice, or about the size of
a small cottage. A growler is a small fragment of ice
about the size of a truck. It is usually of glacial origin,
and generally greenish.
Icebergs originating in Greenland average 70
meters in height and 280 to 450 meters in length when
first formed. The largest ones may exceed 400 feet in
height and several miles in length. The tabular bergs of
Antarctica average 30 to 40 meters in height, but their
horizontal dimensions greatly surpass the bergs of the
For example, one iceberg
observed near Scott Island in 1956 measured 60 miles
by 208 miles.
The portion of an iceberg that is visible above the
water is dependent upon the type of the berg and the
density differences between the seawater and the ice.
The type of berg (pinnacled or tabular) determines the
height of the ice above the water. In the case of the
tabular berg, the depth below the surface is about 7
times the height above the water line. In the case of the
pinnacled berg, the depth below the surface averages
about 5 times that above the water line.
With regard to density, seawater with a temperature
of -1°C and a salinity of 35% produces a density
condition that allows nearly 90 percent of the ice to be
Pinnacled icebergs often have rams (protrusions of
ice beneath the surface). These rams can be a great
hazard to vessels that might pass close by bergs of this
type. The S.S. Titanic sank in the North Atlantic with a
great loss of life after striking this type of iceberg.
Movement of Icebergs
While the general direction of the drift of icebergs
over a long period of time is known, it may not be
possible to predict the drift of an individual berg at a
given place and time. Bergs lying close together have
been observed to move in different directions. The
reason for this is that icebergs move under the influence
of the prevailing current at the icebergs submerged
depth. The subsurface currents often opposes the
existing winds and near-surface currents.
OBSERVING ICE IN THE SEA
When making an observation of ice in the sea, either
sea ice or icebergs, you must determine the type of ice
present. If the ice is sea ice, you must determine the size
of the area covered by ice, the arrangement of ice, and
the stage of development. If icebergs are present, you
must determine the number of bergs present. If the ship
is approaching an ice field, the direction of the ice edge
relative to the ships position must also be determined.
Finally, a determination of the ships ability to operate in
the ice and the trend of any changes in the ice conditions
must be made. Although these determinations sound
complex, they are relatively easy.
METOCCOMINST 3144.1 provides a detailed list of
the determinations that must be made. Additional
information on ice is available in the Ice Observation
Handbook, published by the Naval Ice Center, Suitland.
In this section, we have discussed the various
procedures used to observe the different elements in
surface weather observations and surface aviation
weather observations. We have also introduced many
technical terms used to discuss and accurately describe
the weather elements. Later, we will cover the
observation recording methods and the specific code
forms used to report surface weather observations.
However, before we discuss that information, we must
discuss some calculations that the observer is routinely
asked to determine from data measured during the
Why does it take seawater longer to freeze than
When does sea ice reach a maximum in the
What is meant by the term "fast ice"?
Why should ships avoid operating in ice areas
where rafting is occurring?