LO2-4 Flashcards
What is the cause of wind waves?
Wind.
What is the cause of swell waves?
Distant weather systems.
What is the cause of rogue waves?
A combination of local winds and swell when when in phase with one another/converging oceans.
(twice the height of the significant wave height.)
What temperature does seawater freeze at?
-2 degree/c
How is the significant wave height determined?
1/3 of the average wave heigh (many waves will actually be bigger)
What is water sky?
A dull neutral-coloured sky near the horizon caused by the reflection of the colour of the sea and so indicating open water when seen over an ice-covered sea
What does a Nomogram graph display?
The level of ice creation at different sea water temperatures.
What is the maximum travel distance of icebergs in the north Atlantic?
400 miles south east of Cape Farewell during April.
What do the three layers of the EGG code mean?
- Top row is how many tenths of that highlighted area is covered in ice.
- First row down is the division of that coverage into thick, medium and thin ice.
- Second row down is indicating what type of ice has formed at each thickness.
What is new ice?
- Recently formed ice which includes Frazil, grease, slush and Shuga ice.
- These types of ice are all composed of ice crystals which are only weak when frozen together and have a definite form only while they are afloat.
What is Nilas ice?
A thin elastic crust of ice up to 10 cm thick which is formed on the waters surface.
What is young ice?
- Formed during the transition of Nilas and first year ice (10-30cm thickness)
- Grey ice: less elastic than Nilas and breaks on swell and usually rafts under pressure (10-15cm thick).
- Grey-White Ice: Under pressure it is likely to ridge than raft.
What is first year ice?
Sea ice of not more than one winters growth, developing form young ice.
What is old ice?
Sea ice which has survived at least one summers melt.
What is frazil ice
Fine spicules plates of ice which are suspended in water.
What is grease ice?
A soupy layer on the waters surface which reflects little light giving the water a matte appearance.
What is Light Nilas?
More than 5cm thick and light in colour.
What is pancake ice?
Circular pieces of ice 30cm to 3m wide and 10cm thick.
What is grease ice?
- A later stage of freezing than frazil ice where crystals have coagulated to form a soupy layer on the surface.
- 10-15cm thick, less elastic than Nilas and breaks on swell. (usually rafts under pressure).
What is first year white ice?
- Sea ice of not more than one winters growth developing form young ice (30cm or greater).
- stage 1: 30-50cm
- Stage 2: 50-70cm
- Medium first year: 70-120cm
- Thick: 120+ cm
What is old ice?
- Ice that has survived at least one summers melt.
- Generally smoother than first-year ice and may be subdivided.
- Second year ice: Old ice which has survived only one summer’s melt. Thicker than first-year ice, it stands higher out of the water.
- Multi-year ice: Old ice which has survived at least two summer’s melt. Hummocks are smoother than on second-year ice and the ice is almost salt-free. Where bare, this ice is usually blue in colour.
What is a Polar front?
- Formed by the meeting of two very different air masses winds can be highly variable, blowing at times with great speed and intensity.
- When the cold air flowing out of the polar regions and the warmer air moving in the path of the westerlies meet, they do not mix well. The denser, heavier cold air pushes the warm air upward, forcing it to rise rapidly. The line along which these two wind systems meet is the polar front.
- The weather that results from the meeting of the cold polar air and the warmer air from the subtropics can be very changeable and at times stormy. Weather is very much influenced by the development of waves to the west that form into depressions that drive eastward and the intervening ridges of high pressure.
- (The front migrates north and south frequently as families of depressions).
What are general characteristics of a TRS?
- Organised convection such as thunderstorm activity.
- Winds at low levels circulating anti-clockwise (Northern Hemisphere)
- The system can reach up to 10km in height.
- Isobars are close to being true circles.
- Very steep pressure gradients.
What is required for the formation of a TRS?
- A source of warm, moist air derived from tropical oceans with sea surface temperatures normally in the region of 26 degree C +.
- Winds near the ocean surface blowing from different directions converging and causing air to rise and storm clouds to form.
- Winds which do not vary greatly with height (known as low wind shear). This allows the storm clouds to rise vertically to high levels.
- Sufficient distance from the equator for Coriolis to promote circulation.
What is a 250 hPa upper surface chart used for?
- This chart is primarily used to locate the jet stream.
- 250 hPa occurs at 39000 ft.
What are the indications of a TRS?
- The waves generated by the storm travel at a great speed away from the storm centre. This swell will arrive will in advanced of the clouds, pressure changes and other indicators.
- A change in the intensity or direction of the trade winds.
- Irregularity in semi diurnal range of barometric pressure.
- An oppressive atmosphere with increasing humidity.
- An ugly lurid sky at sunrise and sunset.
What actions should be taken when navigating through a TRS in the Northern hemisphere?
Dangerous semi-circle - Put wind on starboard bow and make best speed. Alter course to stbd as the wind veers.
Navigable semi-circle and in the path - Put wind on starboard quarter and make best speed. Alter course to port as the wind backs.
What are the reasons for differential heating?
- The land has a lower specific heat capacity than water.
- The land is a poor conductor of heat.
- The depth of penetration of solar radiation. Insolation can only effect the top few centimetres of soil.
- Turbulence. Turbulence only affects water and will cause water which has been warmed at the surface to be mixed downwards though a layer which may be as much as 100 meters deep.
What is the associated weather of a South West monsoon?
- Bay of Bengal: Winds are moderate to fresh, force 4 to 5, Weather is generally cloudy and unsettled with less cloud and rainfall near the eastern coasts of India and Sri Lanka.
- Arabian Sea: Winds are SW, over the most of the sea winds are force 4-5, but in the western part the average is force 6 which may reach force 8, weather is cloudy and unsettled with heavy rainfall on the western coasts of India and Burma.
- South China Sea: Winds are force 3-4, fog may occur on the coast of China due to the warm moist air travelling over water previously cooled by the NE monsoon winds, Away from the land weather is generally fair about half of the sky is covered and rainfall is significant on exposed coasts.
What is the associated weather of the North East monsoon?
- Bay of Bengal: Prevailing winds are a North Easterly Force 4, weather is generally fine and cloud amounts are small, the south west of the bay is cloudy, Considerable rainfall on the eastern coast of southern India and the eastern coast of Shri Lanka, visibility generally good apart form the northern part of the bay it may be reduced by sea smoke.
- Arabian Sea: Prevailing winds are North Easterly force 4, weather is generally fine clear with cloudiness and rainfall increasing towards the South and East, visibility generally good but it may be reduced by dust and haze in the North and East.
- South China Sea: Prevailing winds are North to North East and reach force 5-6 at the height of the season, weather is generally fine and clear except near the coasts of South China and Vietnam where spells of overcast drizzly weather with poor visibility occurs between February and April.
What are the stages of ice formation?
Stage 1 - New ice
Stage 2 - Nilas
Stage 3 - Young ice
Stage 4 - First year ice
Stage 5 - Old ice
What information is included in a weather routing chart?
- Routes and distances between major ports.
- Ocean currents.
- Ice Limits.
- Load lines.
- Wind roses.
What is the opinion of shore based routing?
- Shore has greater and more up to date weather information.
- There is more time available to make the relevant calculations.
- Personnel are employed specifically for that purpose.
What is the opinion of on-board ship routing?
- The Master is more involved and knowledgeable of the vessel than some ashore.
- At the end of the day the Master is responsible for the safety and safe navigation of the vessel.
What are the advantages of weather routing?
- Fuel Saving.
- Reduction of damage to vessel/cargo.
- Reduction of voyage time.
- Comfort of crew/passengers.
What is a least time track?
- The first type of weather routing to be developed.
What is a fuel saving track?
This route is based on the weather in general including ocean current and is often not the shortest route in cases where winds/weather and current is against the ship.
What is a towing track?
This is when routing may be limited on time of departure, but usually the routing fairly conventional with advice on where and when to take shelter.
What is a least time and least damage track?
- Most common type of weather routing used.
- This is preferred by most shore-based routing companies.
- Designed to complete the passage in little time as possible whilst ensuring that the hull and cargo are not damaged.
What is wind driven currents?
Currents which are produced when wind blows over a water surface and momentum is transferred from the wind to the water at the air-sea boundary.
What is the angle between the surface current and the wind producing it?
- 45 degrees
What angle does net water transportation occur at?
- 90 degrees
What is the reason for the occurrence of ocean currents
Due to the Ekman effect the prevailing winds will cause for the sea level to rise at the poles and 30 degrees N/S causing distortion and creating currents.
What is gyre?
A Gyre is caused when currents cross the oceans from east to west from the lower latitudes and west to east in higher latitudes. this dominant action generates a general circulation in the upper layers of the ocean in an anticyclonic format.
What are equatorial currents?
At the equator the North and South Equatorial currents are driven by the Trade winds and they push water across to the western side of the ocean. This raises the sea level on the western side and produces a slope in the sea surface from west to east (some 4 cm in 100 km) in the area between the Trade winds.
What are the characteristics of the Benguela current?
- Prevailing South East trade winds.
- Flow is deflected 30 degrees to the left (285T).
- Mean transportation of the Ekman layer is SW (90 degrees left).
- Current flow 315T.
What are the characteristics of the Peru current?
- Surface water will flow away from the coast.
- Ekman layer will flow to the west away from the coast.
- Gradient current will flow parallel to the original wind direction.
What are the characteristics of the California current?
- Sluggish South Easterly current.
- The current is seasonal, occurring in spring and early summer with N/NW prevailing winds.
- In winter the current flows northwards caused by El Nino.
What are the characteristics of the Canary current?
- At approx 20 degrees and area of upwelling occurs of the west coast of Africa which is produced by prevailing NE winds.
- Mean transport is NW
- Direct surface and gradient current flow SE
What are factors affecting the circulation of the Atmosphere?
- The angle of incidence of the isolation reaching the Earths surface at different latitudes.
- The thickness of the atmosphere through which the insolation must pass before reaching the earths surface.
- The inclination of the Earth’s axis of rotation which causes the seasons and associated differences in the length of day throughout the year in any location.
- The rotation of the Earth which creates wind belts moving around the Earth as a result of the effect of Coriolis.
- Declination of the sun.
What is the associated weather in the ITCZ?
- Very moist air which rises due to the heat of the sun, maintaining a low pressure over the area.
- Periods of variable light or clam winds changing in the presence of thunderstorms or squalls.
- Conditions will be worst when the trade winds are at their strongest in spring in each hemisphere.
- Wind can disappear all together.
What is the associated weather in the trade wind belts?
- Winds tend to blow consistently NE in the northern hemisphere and south east in the southern hemisphere.
- Average force 3-4.
- Maximum windspeed is reached at the end of winter and early spring due to an increase in strength of the SE trade winds.
- Winds weaken as they approach the ITCZ.
- Visibility may decrease on the eastern side of the oceans due to fog and mist over the cold currents.
- Cloud cover will increase towards the ITCZ.
