Admiralty Routing charts are used in passage planning for ocean voyages, Routeing Charts include ocean routes and distances between major ports, ocean currents, ice limits, load lines and wind roses. They also contain expected meteorological and oceanographic conditions for each month of the year.
The data that is available on a Routing Chart are as follows :
Wind Rose on Routing Charts
Wind Rose is graphical depiction of wind data as regard to direction and force of wind likely to be encountered in an area, in a particular period of month.
With the help of the wind rose, it can be known at a glance the likelihood of encountering wind from a particular direction at a given force.
Wind roses are shown in red on routing charts, in the form a circle having arrows pointing radially towards center. The arrows fly with the wind, and this fact provides the direction. The length of the arrow indicates the percentage frequency of occurrence on the scale provided on the chart.
The thickness of the arrow indicates the wind force.
Ocean Currents - predominant direction and speed in knots
An Ocean current is the general movement of body of sea water on a permanent, semi-permanent or seasonal basis. Currents flow at all depths in the oceans, but usually the stronger currents occur in an upper layer.
The blue arrow show the predominant direction of sea surface currents during the period the Chart is prepared for. The Arrows show the constancy of the current. The average rate of the current is mention on the arrow on the chart in figures.
The general surface current circulation of the world is shown on the World Climatic Charts in NP136 - Ocean Passages of the World and also described in Admiralty Sailing Directions.
Recommended Ocean Routes and distances
These recommended ocean routes marked on the Routing charts are between the Major Ports considering the statistics on weather, currents and climate.
Distances for the routes are between the indexed arrival and departure positions, to the nearest 10 miles for passages of more than 1000 miles and to the nearest 5 miles for distance less than 1000 miles. The arrival and departure positions are usually pilot grounds or anchorages, as mentioned in the Admiralty Sailing Directions.
Ice information - Maximum Ice Limits
Ice Limits is the climatological position of the extreme minimum or maximum of sea ice extent. Admiralty Routing Charts provides information regarding minimum limit of pack ice, average limit of pack ice, Maximum limit of pack ice and Mean maximum iceberg limit.
Mean Sea Temperatures / Dew point Temperature
Mean Sea and Dew point temperature are shown on a small Box on the Right side of the Routing chart. This is a pictorial representation using lines. Below image will help you understand.
Mean Air Temperature / Mean Air Pressure
Mean Air temperature is also represented on a small chart on Routing box on Right Side.
Load Line limits
Load Line limits are marked on Routing charts as per the Load line charts with respect to the Area covered in Routing Chart. Below is the pictorial representation
Percentage Frequency of Wind of Beaufort force of 7 and Higher / Storm Tracks
Low Visibility / FOG
Low Visibility mentioned on routing chart is the percentage frequency of visibility less than 5 miles and fog mentioned as percentage frequency of visibility less than 0.6 miles approx.