|Sea Ice Seasonal Period Temperatures|
|Seasonal Period||Ice Surface Temperature in Fahrenheit||Time of Year|
|1||4 to 14||WINFLY to late November|
|2||14 to 23||Late November to mid-December|
|3||23 to 27||Mid-December to early January|
|4||27 to 28.5||Early January to Febuary|
Weather can, and generally will, turn bad while you're out on the sea ice. It's important to continually monitor "Herbie Alley" and Mt. Discovery for signs of approaching bad weather. Watch for a lowering ceiling of clouds against Mt. Discovery and the eventual disappearance of Mt. Discovery. Fog blowing in from open water in McMurdo Sound or blowing snow can disorient you just as quickly as any Condition I storm.
More importantly, poor weather conditions will obscure surface definition, making it impossible to detect cracks.
16.3b Sea Ice Cracks
Cracks are fissures or fractures in the sea ice produced by the stresses of wind, wave, and tidal action and thermal forces. There are four types of cracks generally found in McMurdo Sound.
-Tidal Cracks occur in fast ice when the tidal action moves the sea ice above or below the level at which it is shorebound.
-Straight-Edge Cracks (see diagram below) form as tension is released in the ice sheet. The ice thickness at the edge of this crack will be the same as the surrounding ice thickness. These cracks typically form quickly, and the crack shows either open water or a thin layer of ice covered with snow.
*[See figure ³SEAICE4²]
-Spreading Cracks form as forces acting on the ice sheet cause the ice to crack and spread apart over time. These cracks tend to form slowly, and can stay active for quite some time. The center of actively spreading cracks may be open water or thin ice. The safe edges of these cracks are difficult to judge without getting out of your vehicle and profiling the crack. See the diagram on the following page.
*[See figures ³SEAICE5² and ³SEAICE6²]
Pressure Ridges (see the photo below) form when ice is broken by pressure and thrusts up into a chaotic pattern of elevations and depressions. Use caution when crossing pressure ridges, as the uneven chunks of ice can be hazardous to your footing. Occasionally, pressure ridges may pull apart, forming a combination Spreading/Pressure Ridge.
* [See figure ³SEAICE7²]
16.3c Locations of Cracks
You can generally expect to find cracks anywhere on the sea ice. However, certain sites historically produce a series of cracks year after year. Typically, cracks will form around any coastline, island, grounded iceberg, or glacier jutting out into the sea ice. These cracks tend to radiate out from the land, similar to the spokes of a wheel. For example, Cape Armitage, Hut Point, Knob Point, Turtle Rock, Erebus Ice Tongue, the Delbridge Islands, and Cape Evans are all areas where you can expect to find cracks. See the map of cracks on the following page.
* [See figure ICETONG7²]
16.3d Thin Ice
Thin ice is a hazard encountered soon after McMurdo Sound has frozen over in the winter. It can also pose a problem at the center of active or newly formed cracks, especially if the thin ice has been covered with snow. Thin ice can form around shoals, where water currents may cause the sea ice to erode from below. As the air and sea temperature starts to rise, the sea ice becomes progressively weaker and starts to thin, both from the top and bottom. Ice temperature monitoring stations will be established at various locations and monitored on a regular basis.
16.3e Melt Pools
Melt pools (see the photo below) are areas on the sea ice that have subsurface melting. This usually occurs later in the season, typically in late November through December. An ice lens usually forms over the melt water, giving the impression that it's solid. Always drive around melt pools. If it's impossible to drive around a melt pool, get out of your vehicle and drill the underlying ice to determine its thickness.
* [See figure ³SEAICE8²]
16.3f The Ice Edge
The ice edge can be dangerous because there is always the possibility of breakouts, at any time in the season. Travel to the ice edge should be undertaken during periods of good weather and calm sea conditions. Also, you need to be alert to the possibility of attacks by leopard seals (and possibly killer whales). It's a good idea to be roped up and belayed by a team member when working at the ice edge.
16.4 How To Profile A Crack
To profile a crack, start by probing aggressively with an ice axe across the crack. Then determine where the safe edges of the crack are, by sliding the pick of the axe over the surface. Drill test holes approximately 15 inches apart starting in the center of the crack to determine its effective width. (See the photo below.) The effective width should be no more than 1/3rd of the length of your vehicle's track.
* [See figure ³SEAICE9²]
On to Chapter 17: Glacier travel and crevasse rescue.