Presentation on theme: "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."— Presentation transcript:
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
Formation of Sea Ice –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.
Formation of Sea Ice The first sign of sea ice formation is frazil ice, which gives water an oily appearance.
Types of Sea Ice New Ice Recently formed ice composed of ice crystals Nilas Thin elastic crust of ice (up to 10cm thickness)
Types of Sea Ice Young Ice Stage between Nilas and First Year Ice (10-30cm thick) First Year Ice Sea 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 Ice Pancake Ice Circular pieces of ice 30 cm to 3 m in diameter, up to 10 cm in thickness Brash Ice Accumulation of floating ice made up of fragments not more than 2 m across
Forms of Sea Ice Floe Any relatively flat piece of ice 20 m or more across Ice Cake Any relatively flat piece of ice less than 20 m across
Forms of Sea Ice Fast 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 icebergs 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 icebergs speed. Permanent Ice and Icebergs
What percentage of an iceberg is above the water?
Permanent Ice and Icebergs Answer: 10-20% depending on the density of the ice.
Permanent Ice and Icebergs 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.
–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. Permanent Ice and Icebergs
– Leads may close within hours and trap (beset) ships in the ice pack. – They are very transient and subject to the wind. Permanent Ice and Icebergs
– 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. Permanent Ice and Icebergs Ice 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 16 F 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
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 Speed 2.Air Temperature 3.Water Temperature 4.Freezing Temperature of Water 5.Wind Direction, Relative to the Ship 6.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