BÀI GIẢNG KHÍ TƯỢNG LÝ THUYẾT CHƯƠNG 24

Chapter Twenty Four CHAPTER TWENTY FOUR Meteorological factors of planning an Ocean Passage GENERAL PRINCIPLES Choice of route must be balanced between considerations of safety and time with emphasis on SAFETY The direct route is not always the shortest in time A longer route with more favourable winds, currents and weather will often prove faster The regions, seasons frequency and tracks of tropical revolving storms should be carefully noted Avoid areas where there are ice hazards or foul weather or a high percentage frequency of fog Avoid adverse currents and obtain as much advantage as possible from currents which are favourable 5.Choose favourable prevailing or seasonal winds There is always the possibility of a long delay and/or damage resulting from bad weather Thus it is prudent to plan for possible diversions for repairs, refuelling, etc RECOMMENDED BOOKS * Admiralty Sailing Directions (Pilots) for any coastal or "small sea" passages (e.g Caribbean, West Indies) These amplify in great detail the information given on the charts * Ocean Passages for the World: As its name implies, this is specifically designed for mariners planning an ocean passage It gives recommended routes and distances between the principal ports of the world Details of winds, currents, weather, ice hazards, etc along each route are described Much other useful information is included For coastal passages the navigator is referred to the appropriate Sailing Directions for the particular passage in question * Obtainable from Admiralty Chart agents 215 Chapter Twenty Four * The Mariner's Handbook: This is an invaluable reference and text book containing chapters on The Sea which includes information on tidal movements and currents one on General Meteorology including a reference to weather routeing and two describing the full range of ice likely to be encountered by mariners together with recommendations and duties when encountering ice INFORMATION FROM ROUTEING CHARTS * Monthly Admiralty Routeing Charts present the following information which is essential in route planning and on passage: Ocean Currents Predominant directions of flow for a stated period of the year are shown by arrows Percentage constancy and mean rate (in knots) are also indicated Winds The following information is presented in each small area of the ocean, by means of a diagram called a wind rose (See Figure 24.l(a)): Fig 24.1(a) It can be seen in this illustration that the wind blows most frequently from NE during the month for which the chart refers The total length of the arrow against the scale shows a 40% frequency a) The direction from which the wind most frequently blows during the month is shown by arrows which fly with the wind b) Percentage frequency of each wind vector (direction and force) is indicated by the length of the arrow measured against a scale which is printed on the chart (See Figure 24.1(b)) * Obtainable from Admiralty Chart agents 216 Chapter Twenty Four c)The force of the wind (Beaufort Scale) is indicated by the thickness and shading of the arrow shaft (See Figure 24.1 (c)) d) The upper figure in the circle shows the number of observations The middle figure gives the percentage frequency of variable winds and the lower figure calms Note: For the purpose of clarity Figures 24.1(a) (b) and (c) have been drawn to a larger scale than that on the roureing charts Ice limits a) Minimum, average and maximum limits of pack ice b) Mean maximum limit of icebergs Shipping routes and distances, either between principal ports or the ends of great circle tracks The follo1w'ing information is presented on small ocean charts inset over land areas on the main routeing charts Fog Isopleths show the percentage frequency of visibility of less than half a mile Low Visibility Isopleths show the percentage frequency of visibility which is less than miles Mean Air Pressure Mean pressure for the month is shown by isobars spaced at millibar intervals Mean Air Temperature (oF) Shown by isotherms printed at 10o intervals Mean Sea Surface Temperatures (oF) Shown by isotherms spaced at 100 intervals 217 Chapter Twenty Four 10 Mean Dew Point Temperatures (oF) Shown by isotherms printed at 40 intervals QUESTIONS List the meteorological factors you would take into consideration when planning a long ocean voyage, and name the sources you would consult for information 218