BÀI GIẢNG KHÍ TƯỢNG LÝ THUYẾT CHƯƠNG 24

Chapter Twenty Four

CHAPTER TWENTY FOUR
Meteorological factors of planning
an Ocean Passage
GENERAL PRINCIPLES

1. 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.
2. The regions, seasons. frequency and tracks of tropical revolving storms
should be carefully noted.
3. Avoid areas where there are ice hazards or foul weather or a high
percentage frequency of fog.
4. Avoid adverse currents and obtain as much advantage as possible from
currents which are favourable.
5.Choose favourable prevailing or seasonal winds.
6. 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.

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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:
1. 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.
2. 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.
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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.

3. Ice limits
a) Minimum, average and maximum limits of pack ice.
b) Mean maximum limit of icebergs.
4. 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.
5. Fog
Isopleths show the percentage frequency of visibility of less than half a mile.
6. Low Visibility
Isopleths show the percentage frequency of visibility which is less than 5
miles.
7. Mean Air Pressure

Mean pressure for the month is shown by isobars spaced at 2 millibar
intervals.
8. Mean Air Temperature (oF)
Shown by isotherms printed at 10o intervals.
9. Mean Sea Surface Temperatures (oF) Shown by
isotherms spaced at 100 intervals.
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Chapter Twenty Four

10. Mean Dew Point Temperatures (oF)
Shown by isotherms printed at 40 intervals.
QUESTIONS
1. List the meteorological factors you would take into consideration when
planning a long ocean voyage, and name the sources you would consult for
information.

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