THE CRYOSPHERE


In the Introduction, we saw that the cryosphere includes all portions of the Earth where ice and frozen ground exist throughout the year. In this section we are going to examine the major components of the cryosphere, laying down definitions that will be applied in the following section on the remote sensing of the cryosphere. It is necessary to adopt strict definitions of concepts to avoid confusing the various forms that frozen water may take.


Snow

The American Heritage Dictionary defines snow as "frozen precipitation in the form of white or translucent hexagonal ice crystals that fall in soft, white flakes." A geologist defines snow as an "aggregate of delicate skeletal ice crystals formed in the atmosphere by sublimation of water vapor" (Sharp, 1988). For the purpose of this paper, the latter definition will be adopted.

If across all the surface of the Earth, temperatures never supported the fall of snow, there would be no need to consider it as a factor in global change. However, this is not the case. Snowfall occurs from the temperate latitudes to the poles and must be considered a major portion of our source of fresh water. Snow becomes more of a factor to consider in global change because of the fact that there are regions on the Earth where there is permanent snow cover. These regions are not limited to the polar regions but include temperate latitudes, particularly at high elevations. In fact, the snowline is defined as the altitude above which there is permanent snow, regardless of the latitude.


Ice

That American Heritage Dictionary referred to earlier, defines ice as simply "water frozen solid." It's good enough for a dictionary to define a term, but a geologist must characterize a substance, not only by what it is, but by how it formed and how it interacts with its environment. Thus, when reading any literature on the subject of ice, one must be cognizant of the various forms of that it takes.

One must also be aware of the peculiar nature of ice. This frozen state of water, unlike most other frozen states of substances, is actually less dense than the liquid phase. This provides us with the unusual characteristic that it allows for ice to float in water. This is particularly important because it allows the surface of the water body to be covered with ice while the subsurface layers are still in liquid phase.

One mechanism used to distinguish different forms of ice is the temporal dimension. Ice is often referred to by its age. The latest formed ice is referred to as new ice. First year ice refers to all ice that has had no more than one winter's growth. All ice that has survived for more than one summer's melting season is referred to as multi-year ice.

This type of temporal classification can be confusing as it is dependent on the time of the observation and thus one should know the temporal resolution of the imagery to provide detailed analyses of ice observations.

The AGI defines sea ice as "any form of ice originating from the freezing of seawater" (Bates and Jackson, 1984). This definition allows us to differentiate icebergs from sea ice, as icebergs are defined specifically as "a large, massive piece of floating or stranded glacier ice of any shape, broken from the front of a glacier into a body of water." Since glaciers themselves are large masses of ice "formed on land by the compaction and recrystallization of snow, creeping downslope or outward due to the stress of its own weight, and surviving from year to year," we can clearly see the difference between sea ice and icebergs, even though icebergs are found floating in the sea. Also, this avoids the confusion made by mariners, because in common phraseology, all forms of ice in the sea are referred to as sea ice.

Another term that can be misleading is "fast ice." Sometimes confused with new ice, fast ice is defined by the AGI as that sea ice which "forms along and remains attached to the coast or is attached to the bottom in shallow water" (Bates and Jackson, 1984). Fast ice may extend anywhere from a few meters out from the coastline to as much as many kilometers.

Remote sensing scientists must also be familiar with ice terminology that helps discriminate ice features on images by size. This includes "brash ice" which refers to floating ice made up of fragments that are no greater than 2 meters in diameter. There is also the term "floe" which refers to floating ice that is of the order from 20 meters in diameter and up. There is also the ice island which is defined by the AGI as "a large tabular iceberg broken away from an ice shelf and found in the Arctic Ocean, having a thickness of 15 to 50 m and an area between a few thousand square meters and 500 sq. km, or even more" (Bates and Jackson, 1984).

Another ice feature that remote sensing scientists need to be aware of since it is crucial to the merchant mariners is "leads." A lead is defined by the AGI as "a long narrow belt of ocean water through sea ice, navigable by surface vessels" (Bates and Jackson, 1984). The last part of the definition of leads is why the merchants and the US Navy are so interested in leads. In a sense, the leads are the navigable rivers among the sea ice and as one might suspect there are also lakes among the sea ice. These "lakes" among the sea ice are actually called polynyas. They are patches of open water that may form for numerous reasons. Strong offshore winds can prevent the formation of sea ice, but polynyas far from shore are believed to be caused by upwelling of warmer waters from the ocean depths (Stonehouse, 1990).

Remote sensing scientists must be careful to distinguish the polynyas from breathing holes that seals make. One of the differences that appears only in large scale imagery is the fact that the seals continually nibble away at the sea ice to prevent their breathing holes from closing up (Stonehouse, 1990). This leaves conspicuous unattached debris in the vicinity of the breathing.

There are of course other terms that cryologists use, but the ones reviewed here are likely to be of most interest to remote sensing image interpretation.