REVIEW QUESTIONS Q76. At approximately what temperature (Fahrenheit) would you expect rime ice to form on a ship, assuming blowing spray is present? Q77.   What elements are included in an ice accretion observation? ICE IN THE SEA LEARNING  OBJECTIVES:  Explain  the importance of sea ice to naval operations. Describe  the  various  sea  ice  classifications,  sea ice sizes, and topography of sea ice sheets. Discuss movement of sea ice and ice of land origin.  Explain the judgments to be made when observing  ice  in  the  sea. Roughly three percent of the world’s water areas are covered in ice. Although small in area, the ice-covered areas of the sea and oceans are important to naval operations because of their proximity to possible hostile forces. Many submarines routinely operate beneath the ice, and surface ships occasionally operate in ice- covered seas or areas frequented by icebergs. The Naval Ice Center in Suitland, Maryland, keeps the Fleet advised   of   the   development,   movement,   and equatorward limit of the ice edge, as well as of the location  and  movement  of  icebergs.  Although  they make extensive use of satellite imagery to detect and track ice, the ice observations from ships operating near the ice provide valuable input to this critical tracking and  forecasting  effort.  Observations  of  ice  seen  floating in the sea are completed as part of each surface weather observation. There are two main types of ice found floating in the sea: sea ice and ice of land origin. SEA ICE Sea ice is ice that forms in the sea. It is, for the most part,  frozen  seawater.  Sea  ice  accounts  for approximately 95% of the ice coverage in the oceans. For seawater to freeze, the temperatures must be colder for a longer period of time. This is due to the salinity of the water and because of the density changes in the water caused by the salinity. We know that pure water freezes at 0°C (32°F) but the freezing point of seawater varies, depending on the salinity (fig. 1-34). Seawater averages 35‰ or 35 parts per thousand by weight salinity. With this salinity, water begins to freeze at -1.9°C (28°F). Freshwater reaches maximum density at 4°C (39°F). In effect, as freshwater ponds and lakes cool, and the surface waters reach 4°C the water sinks and warmer subsurface water rises to replace it. This slows the process of cooling the surface of the pond below 4°C until the entire body of water is cooled to 4°C. After this point, surface waters cooled to less than 4°C are slightly less dense than the water below the surface, and cooling to the freezing point is rapid. Seawater on the other hand, reaches maximum density at the freezing point. When surface seawater is cooled to the freezing point, but before ice can form, the water sinks and is replaced from below by slightly warmer water. The overturn process continues for a long period of  time,  even  in  continued  subfreezing  air  temperatures, until a large column of water can be cooled. Overall, the lower freezing point and greater overturn required makes the freezing process of seawater very slow. The freezing of seawater is further retarded by the mixing action of winds (waves), currents, and tides. Once ice forms, it floats. Ice, even saltwater ice, expands as it freezes, so it is less dense than water at the same temperature. The formation of sea ice usually begins with the onset of autumn, and the first ice usually appears in the mouths of rivers that empty into shallow seas, such as that  off  northem  Siberia.  During  the  increasingly  longer and colder nights of autumn, ice forms along the shorelines (fast ice) and becomes a semipermanent Figure 1-34.—Freezing point and temperature of maximum density versus water salinity. 1-50