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 Hemisphere. 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. Origin  Characteristics 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 Northern Hemisphere. 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 submerged. 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 iceberg’s 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 ship’s position must also be determined. Finally, a determination of the ship’s 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. NAV- 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 weather  observation. Q78. Q79. Q50. Q51. REVIEW QUESTIONS Why does it take seawater longer to freeze than freshwater? When does sea ice reach a maximum in the Northern Hemisphere? What is meant by the term "fast ice"? Why should ships avoid operating in ice areas where rafting is occurring? 1-54