Arctic Region 66.33°N Antarctic Region 66.33°S Snowfall amounts are small due to the lack of water vapor at low temperatures. –Blowing snow is prevalent

Arctic Region

66.33°N

Antarctic Region 66.33°S

• Snowfall amounts are small due to the lack of water vapor at low temperatures.

– Blowing snow is prevalent in the high latitudes and creates the illusion of more snowfall than is actually occurring.

Snow

• Ice Zones

– Seasonal sea-ice zones (SSIZ)

• The SSIZ is delineated by the maximum and minimum extent of the sea ice cover.

Ice Cover

• The Shear Zone

– Shearing motion of the ice pack against the coast results in a broad band of deformed ice that is prevalent in winter and continually opens and closes flaws and leads.

Flaws and Leads

– Sea water freezes at ~28°F (-02°C), sometimes colder due to salinity

– In order to freeze and form ice, water must first be cooled to its freezing point. This cooling implies a loss of heat

– Ice will form first in shallow water in which there are no currents and low salinity

– The greater the depths of high-salinity water, the greater the time of freezing

Formation of Sea Ice

– Heat is removed from the water:• through conduction, from the water through the ice to the air above

• The rate that heat that flows from the water is directly proportional to the temperature difference between the air and the water, and inversely proportional to the thickness of the ice

Formation of Sea Ice– Once a sheet of ice is formed, the thickness increases by freezing of the water on its lower surface


– In very cold temperatures, a thin sheet of ice will grow very quickly at first, (usually 3-4 inches in the first 24 hours)

– The rate slows as the ice thickens

– Often grows horizontally between floes joining them together

– May also grow by accumulation of slush, ice cakes, or ice floes on a windward shore.


The first sign of sea ice formation is frazil ice, which gives water an oily appearance.

Types of Sea IceNew Ice

Recently formed ice composed of ice crystals

NilasThin elastic crust of ice (up to 10cm thickness)

Types of Sea IceYoung Ice

Stage between Nilas and First Year Ice (10-30cm thick)

First Year IceSea ice of not more than one years

growth (30cm or greater)

Old Ice

Ice that has survived at least one summers melt

Types of Sea Ice

Forms of Sea IcePancake IceCircular pieces of ice 30 cm to 3 m in diameter, up to 10 cm in thickness

Brash IceAccumulation of floating ice made up of fragments not more than 2 m across

Forms of Sea IceFloe

Any relatively flat piece of ice 20 m or more across

Ice CakeAny relatively flat piece of ice less than 20 m across

Forms of Sea IceFast Ice

Ice which forms and remains fast along the coast. Fast ice higher than 2m above sea level is called an ice shelf.

• Permanent ice fields exist over Greenland in the Arctic and over the continent of Antarctica in the Southern Hemisphere.

• Permanent ice may be sea ice or landfast ice created from thousands of years snow accumulation and glaciation.

– Icebergs “calve” off the landfast ice and begin their travel into the adjoining ocean following the wind and currents.

– By definition, icebergs are at least seventeen feet proud of the water and fifty feet long. Anything smaller is called a growler or bergy bit (icebergs that have broken up into pieces too small to be considered a threat; they are officially no longer considered to be an iceberg).

Permanent Ice and Icebergs

– The predominant currents control the speed and heading icebergs travel.

– Wind and the Coriolis effect also influences the iceberg’s movement.

• The portion of the iceberg above the water is the “freeboard” and it acts like a sail, the larger the freeboard, the more the wind effects the iceberg’s speed.



• What percentage of an iceberg is above the water?


Answer: 10-20% depending on the density of the ice.


After a while an iceberg will tumble over, due to one or a combination of several factors, including the berg gets top heavy by rubbing water and waves, or significant “calving” from one end of the berg.

It is exactly this unexpected tumbling-over that makes icebergs so dangerous to ships

This is a "fresh calved" iceberg, considering the deep crevasses in the ice.

iceberg_oblique.mov

Iceberg Shapes

Tabular

Non-tabularDomed

Wedge

Iceberg Shapes

Dry-dock

PinnacleBlocky

– Transiting in and around sea ice is very dangerous.

– Only reinforced hulls should attempt a transit through the pack.

– The thickness of sea ice can be deceiving and its hardness can be greater than many metals

– Sea ice leads are often large enough for smaller cargo ships to navigate through.


– Leads may close within hours and trap (beset) ships in the ice pack.

– They are very transient and subject to the wind.


– Sea ice ahead may be recognized by “ice blink”, which occurs when ice/snow reflects light off low clouds or refracts in the lower atmosphere.

– This creates a white glare on the horizon to approximately 15 elevation above the horizon.


Ice BlinkIce Blink

• Strong offshore winds in the vicinity of mountains cause adiabatic warming of the descending air and can induce stratiform cloudiness and deep fog layers over open water despite high wind speeds.

• Channeling through the east-west oriented mountain chains in the high latitudes may induce strong low-level surface winds over the Arctic and Antarctic ice packs.

Wind

• Rapid developing low-pressure systems along the southwestern coast of Greenland and along the coast of Antarctica commonly produce winds greater than 60 knots, due to the strong pressure gradients.

• The temperature and humidity gradient in the layer of air immediately covering the ice is strong enough to ensure 8-10 knots of surface wind above the ice during most of the year.

Wind

Fog

• Steam fog is sometimes referred to as “arctic sea smoke” because of its prevalence over leads and polynyas in the sea ice.

• Occurs when the sea temperature is greater than 16F warmer than the air temperature.

• In winter, Arctic snowfall is light, primarily due to the low moisture content of the cold air.

• Occurs frequently (approximately 160-170 days annually, but is very light and of short duration).

• Maximum (90%) occurs in the late summer (July through October in Northern Hemisphere and November through February in the Southern Hemisphere) during the most rapid ice thaw.

Precipitation

Fata MorganaLooming Mirage

Sea Spray Icing

Sea spray icing is a serious hazard for marine operations in high latitude regions. Many ships and lives have been lost when ships sank, or became disabled, after the accretion of ice on decks and superstructures.

Large amounts of ice can raise the center of mass on a ship enough to result in a catastrophic loss of stability. Capsizing, extreme rolling and/or pitching, and topside flooding can occur as a result of the loss of stability and extra weight from the ice burden.

Sea spray icing is particularly dangerous for smaller ships, such as fishing vessels, because they are more likely to be exposed to sea spray and a smaller (relative) amount of ice is required for destabilization.

Sea Spray Icing

• Sea spray icing occurs when cold, wave-generated spray comes in contact with exposed surfaces and the air temperature is below freezing. There are two general

factors to be considered:

– Environmental Factors

– Vessel Characteristics

• Environmental Factors

The environmental factors which affect sea spray icing are: 1. Wind Speed2. Air Temperature3. Water Temperature4. Freezing Temperature of Water5. Wind Direction, Relative to the Ship6. Swell and Wave Characteristics

Wave Size Wave Length Wave Propagation Direction

Factors 1-3 are the most important to consider when determining the potential for sea spray icing. Factor 4 is nearly constant, Factor 5 can be changed by altering the ship heading, and Factor 6 is closely related to the wind.

Sea Spray Icing

Vessel Characteristics

Icing can only occur when there is a source of water for wetting the deck, superstructure and other exposed parts of a ship. Some ship factors to consider are:

– Ship Speed– Ship Heading (with respect to wind, waves and swell)– Ship Length– Ship Freeboard– Ship Handling– Ship Cold Soaking

In general, for the same environmental conditions there will be more sea spray reaching the ship deck, superstructure etc. when the ship is traveling faster, into the wind and waves, and for smaller ships and ships with less freeboard.

Sea Spray Icing

• Another ship factor to consider is “cold soaking” (US Navy, 1989).

– When a ship has been in cold temperatures for a long time (2-3 weeks for most vessels) the body of the ship will remain cold even if the air temperature is warmer. In this situation, icing may be more severe than expected given the current environmental conditions.

Sea Spray Icing

– Small ships• Added weight reduces freeboard and therefore reduces

stability of the vessel.• Ice forming high on masts, rigging and superstructure

produces a large heeling lever and the vessel may become top heavy and capsize.

– Large ships• Accumulation of ice on decks, deck equipment and super

structures, impairs their overall efficiency and maneuverability.

• Ice accumulation on antennas may render radio and radar systems inoperable

Icing

Ship generated spray will be greatly reduced in calmer water.

In very high latitudes, ships should not seek shelter in the lee of the ice edge.

• Provides negligible shelter from the wind.• Coldest air and sea temperatures are located in this region.• Provides most severe conditions for icing.

Reduce the amount of Icing

Documents

Arctic Region 66.33°N Antarctic Region 66.33°S Snowfall amounts are small due to the lack of water vapor at low temperatures. –Blowing snow is prevalent