Dr. Peter Guest, Research Associate Professor of Meteorology, Naval Post Graduate
School, Monterey, CA & Adapted by Robert Luke, Voluntary Observing Ship (VOS) Program
The most serious form of icing affecting marine operations near the surface (ships,
buoys, platforms) is from sea spray. The information provided in this article is from
a Naval Post Graduate School (NPS) meteorology module that focuses on sea spray vessel
icing, it will focus on what icing is, how icing can affect marine operations, the
causes of vessel icing, how to predict icing situations, and how a vessel can mitigate
Description of sea spray icing
The forward bulkhead of the Research Vessel
Knorr during a scientific cruise in the Labrador Sea in late winter 1997.
Photographed by George Tupper. From Pickart (1997) in Oceanus Magazine.
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. The problem is particularly
dangerous for smaller ships, such as fishing vessels, because they are more likely to be
exposed to sea spray and a relatively smaller amount of ice is required for destabilization.
Ships with relatively large superstructures and low free board are more likely to
experience serious destabilization problems due to icing. On any ship, communication and
navigation equipment, and most any type of deck operation, can be severely impacted or
rendered useless by sea spray icing.
Overland (1990), Fett and Kozo (1992) and Fett et al. (1993) describe a tragic example
of the effects of sea spray icing. During the period January 23-27, 1989, there was heavy
icing in the Bering Sea region that was correctly forecasted to occur. Heeding the forecast,
a fleet of 140 crab fishing vessels remained safe in the lee of the Pribilof Islands. By
January 29, the region of heavy icing had moved to the Gulf of Alaska as forecasted.
Figure 1 shows a trough that extended from a low over the Yukon toward a high
over the Aleutian Islands, creating strong offshore winds over the western portion of the
Gulf of Alaska. The cold air advection during this period is best seen on the 850 hPa chart
from six hours earlier (Figure 2).
Figure 1.-The Fleet Numerical Meteorology and Oceanography Center
(FNMOC) surface pressure chart for 0600 UT, January 29, 1989. Note the region of tight
pressure gradient and high-speed off-shore winds in the western region of the Gulf of Alaska.
Taken from Fett et al. (1993) pages 7-9.
Figure 2.-The Fleet Numerical Meteorology and Oceanography
Center (FNMOC) hPa pressure chart for 0600 UT, January 29, 1989, showing the region of
intense cold air advection over Kodiak Island (the large island to the southwest of Montague
Island) and the surrounding ocean. Just south of Kodiak was where the Vestfjord
was lost four hours later. Taken from Fett et al. (1993), pages 7-10.
One crabbing vessel, the 31 m F/V Vestfjord, attempted to cross the Gulf
of Alaska from the east during this period. The ship had missed the start of the crabbing
season along the ice edge in the Bering Sea and the captain was attempting to make up for
lost time. Whether he was aware of the severe icing forecast and chose to ignore it, or was
just unable to seek shelter, will never be known. The last report came from the ship just
before 1010 UTC, on January 29, approximately 60-70 nmi south of Kodiak Island when the
captain reported heavy icing. By the time of the report, it was probably too late to take
evasive action and the ship was never heard from again, a loss of six lives.
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 and the vessel's characteristics.
The environmental factors which affect sea spray icing are wind speed, air temperature,
water temperature, the freezing temperature of water, relative wind direction, and the sea
and swell wave characteristics (height, period, propagation direction.
The wind speed, air and water temperatures are the most important factors used to determine
the potential for sea spray icing. With the freezing temperature of water being nearly constant,
wind direction can change easily and the sea height values are related to the wind.
Vessel icing can occur when the following environmental factors are present:
High Wind Speed - Usually above 18 kts or 9 m/s but sometimes lower
Low Air Temperature - Below freezing (-1.7°C or 29°F)
Low Water Temperature - Usually below 7°C or 45°F
The first two factors, high wind speed and low air temperatures, are associated with cold
air advection or cold air moving into a region. Cold air advection often occurs after the
passage of a cold front. It is most intense when air formed over continents or ice regions
(i.e. Polar Continental, Arctic and Antarctic air masses) moves over open water in the late
fall, winter or early spring. Long, closely-spaced bands of low level cumuliform clouds called
'cloud streets' are a sure sign that cold air advection is taking place over water. Cloud
streets are very distinctive and easy to spot on satellite images (Figure 3).
Figure 3.-Cloud Streets Infrared satellite image on 0552 UT,
January 29, 1989, showing cloud streets associated with cold air advection in the
Gulf of Alaska. The Aleutian Islands are in the center of the image. This is about
four hours before the Vestfjord sank, as described earlier. The last
reported location of the Vestfjord is indicated by dark cross in the
central right area. Taken from Fett et al. (1993) pages 7-11.
Figure 4.-Regions in the North Atlantic (top)
and North Pacific (bottom) where icing generally occurs. Taken from U.S. Navy (1988),
A dangerous coating of ice on the NOAA Ship Miller Freeman in the Bering Sea,
Alaska. Such icing can affect a ship's stability and cause capsizing. Photo from
the NOAA Library Ship Collection. Photo courtesy of NOAA NMAO Pacific Marine Center
The cold advection, and associated serious icing, is most intense when an ice
edge or shore is less than 200 km (108 nmi) upwind. At further distances the air
becomes warmer and icing is less likely to develop. Very close (less than about 5
km or 3 mi, the exact distance is dependent on the vessel) to a shore or ice edge,
waves are not developed and, hence, there is protection from icing even when the
above conditions are met. In the Northern Hemisphere, icing is most likely to occur
in the northern portions of the Atlantic and Pacific Oceans (Figure 4).
It also can occur everywhere in the Arctic Ocean and in the Southern Ocean surrounding
In addition to the environmental factors discussed above, the severity of sea-spray
icing depends on 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's speed, heading (with respect to wind, waves and swell),
length, amount of freeboard, handling, and cold soaking aspect. 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. Overland discusses these factors in detail.
He calculated a threshold significant wave height, h1/3, and associated wind
speed, for a 200 km fetch at which enough sea spray reaches the decks and superstructures
to cause severe icing, assuming cold air and water temperatures are also present
Wind Speeds for Icing to Occur on Various Length Ships
height - h1/3
|Wind Speed at 200 km
(108 nmi) fetch
Note: This is only a rough guide for ships steaming into the wind and waves. The
actual potential for icing depends on the type, load, and handling characteristics
of a particular ship. Any captain or bridge officer who is familiar with a ship
should be well aware of the wind speeds which cause sea spray to reach the deck and
superstructure, and should base their assessment on the potential for icing on this
Another ship factor to consider is cold soaking5. 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.
Prediction of Vessel Sea Spray Icing
Algorithms were developed3,4 that have proven to be useful for predicting
sea spray vessel icing. These algorithms were based primarily on reports from vessels
that were 20 to 75 meters in length.
PPR = Icing Predictor
Va = Wind Speed (m s-1)
Tf = Freezing point of seawater (usually -1.7 °C or -1.8 °C)
Ta = Air Temperature (°C)
Tw = Sea Temperature (°C)
Va(Tf - Ta)
PPR = ----------------------
1 + 0.3 (Tw-Tf)
Table 2 shows the expected icing class and rates for 20 - 75 meter
vessels that are steaming into the wind.
Table 2.-Icing Class and Rate
These icing rates are only a guide. Actual icing rates depend on ship
characteristics, cold soaking and exposure to sea spray.
Using the Algorithm
The following nomograms have been developed for a quick reference. They display
sea spray icing potential class as a function of wind speed and air temperature for a
given sea temperature. These nomograms are slightly different from the ones found in US
Navy5 because they are based on the most recent work by Overland3.
The main difference is that the effect of cold sea water is emphasized more in the nomograms
shown here. Generally, icing is not a problem at sea temperatures greater than 7°C, and
no cases with higher temperatures were considered when the algorithm was derived. Because it
may be possible for icing to occur at these higher sea temperatures, they have been included
These nomograms assume a freezing water temperature of -1.7 °C, which is typical
for sea water. Icing can occur in fresh water lakes such as the Great Lakes. If you want
to use the nomograms for a fresh water case, subtract 1.7 °C (-3.1 °F) from the
water temperature and add 1.7 °C to the air temperature before using the tool, or just
use the algorithm at the top directly and use Tf = 0.
Mitigation and Avoidance of Vessel Sea Spray Icing
Figure 5.-GOES10 2100 UTC on April 6, 2003. Image
courtesy of National Climatic Data Center (NCDC)
Remember sea spray icing is associated with areas of cold air advection and
cold sea temperatures. Obviously, the first and foremost way to avoid icing is to
stay clear of the weather conditions that induce icing. OK, reality sets in and
operational requirements make that idea a fantasy. What then? You can use the
forecast guides to help you navigate the path of least icing. As a mariner, you
should also be aware that Polar Lows (Figure 5) are fast-forming,
intense mesoscale cyclones that are often not predicted accurately. Due to their
high winds and formation in cold air, they can be associated with severe icing.
Be especially aware of the possibility of a Polar Low if you are trying to avoid
an icing situation.
If you are caught in a dangerous icing situation, the best thing to do is to
seek immediate shelter in a harbor or downwind of a land mass such as a
coastline, peninsula or island. In these regions, the wind may still be strong,
but the waves will be small, thus minimizing sea spray. If shelter is not available,
then steam downwind to minimize sea spray on the ship's deck and superstructures.
For some ships, it may be better to minimize sea spray by heading into the seas. Do not
wait until the ship has accumulated a large amount of ice. Turning through the trough
can be deadly for a ship that has been destabilized by ice accretion.
"Ice-phobic" (ice fearing) coatings can be applied to repel water and prevent ice
build-up. Ice that does form over the coatings will be easier to remove. U.S.
Navy5 recommends two ice-phobic coatings for prevention of sea spray icing
- Fluorocarbon penetrating coating (FPC): This is used to reduce hull drag
and has been found to be a good ice-phobic coating as well.
- Vellox 140
There are problems with using ice-phobic coatings. They must be constantly reapplied
because they tend to fall off when water and ice brush against them. They should not be
applied to areas where people will be walking because they are very slippery. For these
reasons it is not practical to use them on all susceptible surfaces. Concentrate on
critical areas that are most affected by ice accretion. Ice-phobic coatings alone should
not be relied upon to prevent sea spray icing, but they may help facilitate physical removal.
These are hazardous materials and should used in accordance with published guidelines.
Ice and Snow Removal Methods
Once ice has formed it is necessary to remove it to prevent ship damage and instability.
This requires advance planning. Organize crews to remove ice whenever it is safe to be on
deck. Remove the ice before it reaches dangerous amounts. Usually physical removal is the
most effective method.
One of the following two tools is essential; baseball bats or large wooden mallets.
Stock up before you leave port and bring plenty of extras since they tend to break easily.
The advantage of these heavy wooden tools is that they can remove ice effectively, but
will not damage the ship and equipment as much as metal objects.
The following tools are also useful: steel-bladed ice scrapers, straight bottom shovels,
spades, hoes, picks, brooms, or snow shovels. The ice scraper and straight bottom shovels
are useful for removing thin ice from decks. The spades, hoes, and picks can be used for
thicker ice and the brooms and snow shovels remove snow that can cause problems when it gets
wet and then freezes. Obviously, all these tools must be used with care to prevent damage to
the ship and topside equipment.
Chemicals can be used to remove ice but caution must be used when working with these
items to avoid corrosive damage to your metalwork and to keep your crew safe. The most
economical chemical that could be used is sodium chloride or rock salt.
- Rock Salt (Sodium Chloride) Most economical
- Calcium Chloride Faster acting than rock salt
- Urea Less corrosive than above
- Ethylene Glycol
- Other light de-icers including alcohols
1-3 can be sprinkled on decks to supplement physical removal. 4-6 can be applied with a
garden sprayer to sensitive equipment that might be damaged by a bat or mallet. Follow all
hazardous materials guidelines when using these chemicals.
If available, the following devices are effective for ice and/or snow removal:
- Portable hot air guns Small ice deposits and spot thawing of
- Electric hair driers Same uses as 1.
- Steam lances
Although there are thought processes on the development of vessel icing and
several ways of mitigating this marine phenomenon have been discussed, there is
no sure way of avoiding icing conditions due to the harsh nature of the environment
and the ship's routes to "get the job done." Many factors have to be considered and
it is usually the seasoned experience of the captain that truly understands their ship
and the situation encountered as to what needs to be done. As it has been said before,
"knowledge is power."
1. Fett, R.W., and T.L. Kozo, 1992: Navy Tactical Applications Guide, Volume 8,
Part 2, Arctic - East Siberian/Chukchi/Beaufort Seas Weather Analysis and Forecast
Applications, NRL/PU/7541--92-0005, Naval Research Laboratory, Monterey, California,
2. Fett, R.W., R.E. Englebretson and D.C. Perryman, 1993: Forecasters handbook for
the Bering Sea, Aleutian Islands and Gulf of Alaska, NRL/PU/7541--93-0006, Naval Research
Laboratory, Monterey, California, pp. 302.
3. Overland, J.E., 1990: Prediction of vessel icing for near-freezing sea temperatures,
Weather and Climate, pp. 5, 62-77.
4. Overland, J.E., C.H. Pease, R.W. Preisendorfer and A.L. Comiskey, 1986: Prediction
of vessel icing. Journal of Climate and Applied Meteorology, pp. 25, 1793-1806.
5. U.S. Navy, 1988: Cold Weather Handbook for Surface Ships, OPNAV P-03C-01-89, Chief
of Naval Operations, Washington DC.
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