LO2/4 take 2 Flashcards
What is a monsoon?
A seasonal prevailing wind in the region of South and SE Asia, blowing from the south-west between May and September and bringing rain (the wet monsoon), or from the north-east between October and April (the dry monsoon).
What are the main factors that cause monsoons?
- Differential heating.
- Migration of the ITCZ.
- The modification of the sub tropical jet stream
What are the causes of differential heating?
- The land has a lower specific heat capacity than water meaning, that there is more energy required to raise the temperature of water by a degree than it is the land.
- Land is a poor conductor of heat meaning the land will heat quicker than water but equally lose it quicker due to conduction to the air, lower layer of rock and soil below the surface.
- The depth of penetration of solar radiation. Insolation can only affect the top few centimetres of soil whereas the transparency of water allows the insolation to extend to greater depths.
- Turbulence. Turbulence only affects water and will cause water which has been warmed at the surface to be mixed downwards through a layer which may be some 100 m or so deep.
What are the changes in pressure over Asia’s landmass?
- Under the influence of summer heating an area of low pressure develops over a land surface.
- Under the influence of winter cooling an area of high pressure develops over a land surface.
What is the cause of seasonal migration of the ITCZ?
- Migration is caused by the declination of the sun.
- Greatest migration tends to be over large land masses in the northern hemisphere where land area of continents is greater and the effects of surface heating are much greater than over the oceans.
- Longer term changes in the ITCZ can result in severe droughts or flooding in nearby regions.
What is the ITCZ?
- The ITCZ, or equatorial trough as it is often termed, denotes the belt of convergence of the northeast and southeast trade winds.
- When the trade winds cross the equator, Coriolis effect
deflects these winds and can also increase or decrease their relative strength as well as change their direction.
What is the subtropical Jetstream?
A narrow band of fast-moving air flowing from West to East in the northern hemisphere and flows between 25-35 degrees North.
How does the subtropical Jetstream influence Monsoons?
- As the summer season progresses, the STJS which flows from west to east over northern India weakens and moves northwards to re-locate far north of the Himalayas.
- A new jet forms over the southern half of the Indian peninsula which flows from east to west, known as the Tropical or Equatorial Easterly jet (EEJ). This modification to the jet stream has a significant effect on the formation of the SW, or summer, monsoon.
What are the weather conditions in the Arabian sea when under the influence of a North East Monsoon?
- Prevailing winds are North easterly Force 4.
- Weather is generally fine and clear with cloudiness and rainfall increasing towards the south and east.
- Visibility generally good but may be reduced by dust and haze in the north and east.
What are the weather conditions in the South China sea when under the influence of a North East Monsoon?
- Prevailing winds are N to NE and reach Force 5 to Force 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 occur between February and April.
What are the weather conditions in the Bay of Bengal when under the influence of a North East Monsoon?
- Prevailing winds are North Easterly Force 4.
- Weather is generally fine and cloud amounts are small.
- In the extreme SW of the Bay, its cloudy.
- Visibility generally good, but in the northern part of the Bay it may be reduced by smoke.
What are the weather conditions in the Bay of Bengal when under the influence of a South West Monsoon?
- Winds are SW moderate to fresh, Force 4-5.
- Weather is generally cloudy and unsettled with less cloud and rainfall near the eastern coasts of India and Sri Lanka.
What are the weather conditions in the Arabian Sea when under the influence of a South West Monsoon?
- Winds are SW.
- Over most of the Arabian Sea winds are Force 4 to 5, but in the western part the average is Force 6 and may reach Force 8.
- Weather is cloudy and unsettled with heavy rainfall on the western coasts of India and Burma.
What are the weather conditions in the South China Sea when under the influence of a South West Monsoon?
- 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 are the conditions that lead to El Nino conditions?
- The average air pressure is higher in Darwin than Tahiti.
- Trade winds decrease in strength and may even reverse direction resulting in less upwelling off South America.
- When the water is 0.5 degrees warmer than normal in the East Pacific.
- Rainfall becomes more common in the central Pacific
while the Western Pacific becomes relatively dry.
What are the conditions that lead to El Nina conditions?
- The average air pressure is higher in Tahiti than Darwin.
- When the water is cooler than average in the East Pacific.
- Trade winds blowing westward across the tropical Pacific are stronger than normal.
- An increase in upwelling off South America meaning lower than normal sea temperatures.
- Prevailing rainfall pattern shifts further west.
- These winds pile up warm surface water in the Western Pacific.
How does the El Nino southern Oscillation affects the Jetstream and the consequential effects on weather in locations around the Earth?
As the position of the warm surface water along the equator shifts east and west so does the position of greatest evaporation of water into the atmosphere. This greatly effects the position as it will create the trough to move further east in the Pacific and increases the likelihood of associated storms.
How does the El Nino southern Oscillation affects the weather in Southern California?
- Brings huge amounts of tropical moisture.
- Very strong El Ninos will cause the trough of the Jetstream to shift further south with the average storm track position moving into Southern California resulting in higher than normal rainfall, flash flood occurrences. With the storm track shifted south, the Pacific Northwest becomes drier and drier as the tropical moisture is shunted south of the region.
What are the implications of the El Nino Southern Oscillation on the commercial fishing activities in certain parts of the world?
- It marks a time of poor fishing conditions as the nutrient rich waters off the northwest coast of South America remained very deep.
- Usually immediately after Christmas .
What are three causes of ice accretion occurring on a ship?
- Spray or seawater breaking over a vessel when the air temperature is below the freezing point of seawater (-2).
- Freezing drizzle, rain or wet snow.
- Fog, including fog formed by evaporation from a relatively warm sea surface, combined with freezing conditions.
What are the factors that influence the rate and amount of icing that might be experienced?
- Strong winds and air temperatures of -2 and below.
- Freezing rain or snowfall increases the hazard.
- The rate of accumulation also depends on the vessels speed and course relative to wind and waves.
- Design of vessel.
What must the Master do under SOLAS chapter 5 in the event of dangerous icing conditions?
-The master of every ship which meets with dangerous ice, a dangerous derelict, or any other direct danger to navigation, or a tropical storm, or encounters sub-freezing air temperatures associated with gale force winds causing severe ice accretion on superstructures, or winds of force 10 or above on the Beaufort scale for which no storm warning has been received, is bound to communicate the information by all means at his disposal to ships
in the vicinity, and also to the competent authorities. The form in which the information is sent is not obligatory. It may be transmitted either in plain language (preferably English) or by means of the International Code of Signals.
- Contracting Governments to promulgate danger information.
- Messages free of charge to ships.
How does the direct effect of wind cause the net transport of surface water that can contribute to create an ocean current?
-The most constant drift currents are those caused by permanent and semi-permanent winds, the next are those resulting from predominate winds such as the westerlies of temperate latitudes.
Why are the currents on the western ocean margins tend to be stronger than those eastern margins?
- Gyres are are not symmetrical and displaced towards the west.
- Coriolis is stronger in higher latitudes, eastward flowing waters are strongly deflected back toward the equator causing an infestation of the currents on the western margins of oceans and increased current rates.
- The reason for this being that the rotation of the Earth exerts a torque on the ocean as a whole and shifts the centre of rotation westwards; this causes a western intensification of currents, whereby currents on the western side flow more quickly while currents on the eastern side are sluggish and more diffuse.
What is the essential information required to incorporate weather routing in a ships passage plan?
- Weather.
- Climate.
- Winds.
- Currents.
- Swell.
- Seasonal factors and ice hazards.
- Calculation of arrival times.
What are the advantages of weather routeing?
- Fuel saving.
- Reduction of damage to vessel and cargo.
- Reduction of voyage time.
- Comfort of personnel and passengers.
When are the benefits of weather routeing seen most?
- Voyages of 1,500 and over.
- Where the weather is a major factor in deciding the route.
- In unrestricted navigable waters.
- Where extremes of weather occur in changing patterns.
- On westbound passages because there are more chances of encountering variable weather, head winds and sea on a westbound passage that an eastbound one.
What information is included in a vector mean chart?
- The net movement of water over a long period of time.
- The average of all the individual observations at that place over the relevant period of time (e.g. a month). The speed and direction are taken into account and displayed.
What information is included in a current rose chart?
- Further detail on the variation of currents such as frequency of various speeds within each direction.
- Average percentage of current setting.
What information is included in predominant current chart?
- This chart represents the most frequent or ‘most likely’ current in a certain area.
How is a least time track constructed?
- Draw a straight line between the arrival and departure points (including waypoints) on a polar stereographic projection chart to represent the circle track.
- Using the wave height forecast chart for the next 12 hours and the ship’s performance curve, the distance along the great circle that the ship can travel in the next 12 hours is calculated. This distance is then laid off along the great circle track.
- The calculation is then repeated, for several courses at 10°or 15° intervals on either side of the great circle track. These will produce a series of positions, which the vessel could reach in 12 hours depending upon the relative direction and height of the significant waves as forecasted.
- A curve is drawn through the estimated 12-hour positions; this being termed the 12-hour locus or 12-hour time front. Positions too far from the great circle track are rejected at this stage.
- Using the wave height forecast for the period 12-24 hrs, and choosing several points on the 12-hour time front as starting points, various positions are found from each; subsequently a 24-hour time front is constructed.
- The process is repeated using updated wave data charts until a 72-hour time front is plotted (72 hours being the usual maximum extent of prediction).
- Once the final time front has been drawn, the point on the time front which is closest to the destination is found. The vessel should then make for this locus point.
What is the development of the summer/SW monsoon?
- Asian continent heats up as the suns declination moves north.
- The Southern Tropical Jetstream, which flows from east to west weakens and moves northwards to above the Himalayas.
- A new jet forms over the southern half of the Indian peninsula which flows from east to west, (Equatorial Easterly Jet).
- Sea surface temperature reach nearly 30 degree C over the ocean which is cooler than the air over the land.
- Temperature over the land mass reaches 50 degrees C.
- Air now moves from HP to LP and the low-level Somali jet off the East African coast enforces the SW monsoon.
- The south-westerly winds arrive over the west coast of India in May to early June and this occurrence is referred to as the ‘burst’.
- The monsoon winds push northwards reaching northwest India by mid-July.
- The high ground of the Himalayas acts as a barrier preventing the winds from
progressing further. - The monsoon trough marks the area of the heaviest rain.
What is the development of the Winter/SW monsoon?
- From September the Asian continent starts to cool and the air density and surface pressure increases. As the land cools more quickly than the sea there is a reversal of the temperature and pressure gradient.
- The Equatorial Easterly jet (EEJ) rapidly disappears.
- The STJS moves southwards and becomes re-established over northern India.
What is the formation of winter monsoons over West Africa?
The NE winds are produced by the pressure gradient between the sub-tropical anticyclone and the equatorial low-pressure areas. The ITCZ is just north of the equator and the NE winds blow over almost the entire region. The NE wind is known locally as the Harmattan.
What is the weather associated with West African North Easterly Monsoons?
- Average winds of force 3-4.
- Wind becomes cooler and humidity increases as it moves further away from the coast.
- Cloud cover is 3/8 with low rainfall and humidity.
- Visibility is frequently reduced by haze due to dust carried by the wind.
What is the weather associated with West African South Westerly Monsoons?
- Winds are force 3, but may increase to force 6 during thundery squalls.
- Thick cloud cover and thunderstorms are common in the south.
- Average max temperature is below 30 degrees in most parts.
- Humidity is high, especially in July.
- Torrential rain at 5-12 degrees North.
What is the formation of summer monsoons over West Africa?
- In northern summer the West African land mass heats more rapidly than the adjacent sea, producing an area of surface low pressure over the land.
- The ITCZ moves further north reaching about 15°N in July.
- The SE trade winds cross the equator, are deflected by Coriolis force and become SW winds.
