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- Table III. 2 BEAFORT SCALE OF WIND
The rules to be followed at a sea station are the same as for a land station. 3.2.2.3.2 Wind direction and speed The methods and instruments for the wind observations at sea station are described in the Guide to Meteorological Instruments and Methods of Observation (WMO-No. 8), Part II, Chapter 4, section 4.2.5. The observations of wind speed and direction may be made either by visual estimates or by means of anemometers or anemographs. Visual estimates will normally are be based upon the appearance of the surface of the sea. Observers should be aware that the wave height in itself is not always a reliable criterion since it depends also on the fetch and duration of the wind and on the presence of the swell. The criteria to be used for visual estimation of the wind speed, using the Beaufort scale, are given in Table III.2. Wind direction is determined by observing the orientation of the crests of the wind-driven sea waves. Wind measurements made by means of anemometers and anemographs are done in the same way as for land stations, but it may be difficult to avoid local effects, for instance those produced by a ship's superstructure. For this reason, sited the instrument should be sited on board of the a mobile ship should be as far forward and as high as possible. For a moving ship, it is necessary to distinguish between the wind relative to the ship and the true wind. For meteorological purposes, the true wind should always be reported. The true wind may be obtained from the apparent wind by using the parallelogram of velocities as shown in the figure below (Figure III.8). 
Figure III.8 - Parallelogram of velocities Source: (Marine Observer's Handbook, Meteorological Office, United Kingdom). The apparent wind speed measured on board a moving ship is to be corrected for the course and the speed of the ship, in order to obtain the speed of the true wind, which is to be reported. The correction can be made on the basis of the parallelogram of velocities or by means of special tables (see the Manual on Codes (WMO-No. 306), Volume I.1, Part A, Regulation 12.2.2.3.3). For fixed platform stations and anchored platform stations, special rules must be applied for determining the wind due to the fact that their heights may be more than 100 m above sea level while surface wind is defined as the horizontal component of the wind vector measured 10 m above the earthground or the sea surface. If the wind sensor is exposed at a higher elevation, the readings must be corrected. The following scale should be applied for reporting mean wind speed (over 10 minutes): Elevation (height in m) 20 30 40 50 60 70 80 90 100 Coefficient (r) 1.10 1.15 1.20 1.23 1.26 1.29 1.31 1.33 1.35 Example: Sensor at 75 m, indicated speed 50 kt Coefficient 1.30 (interpolated) Wind reduced to 10 m = 50/1.30 = 38.46 or 38 kt.
* This table is only intended as a guide to show roughly what may be expected in the open sea, remote from land. It should never be used in the reverse way; i.e. for logging or reporting the state of the sea. In enclosed waters, or when near land, with an off-shore waves, wave heights will be smaller and the waves steeper. Figures in brackets indicate the probable maximum height of waves. 3.2.2.3.4 Visibility
The use of a sling or aspirated psychrometer exposed in the fresh airstream on the windward side (of the bridge) is normally recommended. Automatic observing stations require a fixed exposure and should be exposed as far away from any artificial source of heat as practicable. This calls for two well-ventilated screens, as indicated in Figure III.1. The requirements for thermometers for psychrometry at sea are as laid down in the Guide to Meteorological Instruments and Methods of Observation (WMO-No. 8). The thermometers should be exposed on the windward side and adequately shielded from radiation, precipitation and spray. For psychrometers the recommendation in the same Guide (section 5.2.4) should be followed. For fixed platform stations and anchored platform stations whose heights may be more than 100 m above sea level, it is not necessary to take into account the variation of temperature and humidity with height when reporting these variables.
Pressure may be measured either by a precision aneroid or by a mercury barometer. The characteristic of the tendency is obtained from a barograph. The requirements and instruments for atmospheric pressure observations at sea station are described in the Guide to Meteorological Instruments and Methods of Observation (WMO-No. 8), Part II Chapter 4, section 4.2.6. 3.2.2.3.7 Ship's course and speed For mobile ships the course shall be given as the direction from the position three hours previous to the present position, regardless of the real track between the two positions. The position, course and speed of ships are taken from its navigation system or are computed independently using a satellite navigator (e.g. global positioning system).
The measurement of sea-surface temperature is described in the Guide to Meteorological Instruments and Methods of Observation (WMO-No. 8), Part II, Chapter 4, section 4.2.11. The method used at manned sea stations for measuring sea-surface temperature shall be recorded in the relevant meteorological log book. The temperature of the sea surface may be observed by: (a) Taking a sample of sea-surface water with a specially designed sea bucket; (b) On a ship, reading the temperature of the condenser intake water; (c) Exposing an electrical thermometer to the sea water either directly or through the hull (of a ship); (d) Using an infra-red radiometer mounted to look down on the sea surface. The principal methods used for many years have been (a) and (b). Method (c), however, is more widely used now, especially for large ships where method (a) is impractical and has proved to give the most consistent results. Method (b) should be avoided because great care is needed to obtain reliable results. (For further information, see section 17.8 of the Guide to Meteorological Instruments and Methods of Observation (WMO-No. 8)). Observation of sea-surface temperature may be difficult from a platform or a rig. The best method of exposing a seawater thermometer is to attach the sensor to a buoy which is kept floating on the water during the reading period. Transmission of signals could then be either by radio or by cable. The buoy could also be equipped with a wave recorder to give sea-state observations as well. 3.2.2.3.9 Direction and movement of Ocean waves and swell The observation of waves and swell is described in the Guide to Meteorological Instruments and Methods of Observation (WMO-No. 8), Part II, Chapter 4, section 4.2.12. The following definitions apply: Sea - System of waves observed at a point which lies within the wind field producing the waves. Swell - System of waves observed at a point remote from the wind field which produced the waves, or observed when the wind field which generated the waves no longer exists. Wave length - The horizontal distance between successive crests or troughs (equal to the wave period multiplied by the wave speed). Wave height - The vertical distance between trough and crest. Wave period - The time between the passage of two successive wave crests past a fixed point (equal to the wave length divided by the wave speed). Wave speed - The distance travelled by a wave in a unit of time (equal to the wave length divided by the period).The characteristics of a simple wave are shown in Figure III. 9:below. 
C= wave speed L=wave length H= wave height Figure III.9 - Characteristics of a simple wave Observation of waves should include the measurement or estimation of the following characteristics: (a) Direction from which the waves come on the scale 01-36, as for wind direction; (b) Period in seconds; (c) Height in metres or feet. The direction from which the waves are coming is most easily found by sighting over a compass along the wave crests and then turning 90° to face the advancing waves. The period can be measured by means of a stopwatch. A small floating object is observed and the time it takes for the object to move from one crest to the next is noted. A distinctive patch of foam from a breaking wave may be used as "floating object". For estimation of waves shorter than the ship, the observer should take a position as low down as possible, preferably amidships. Height marks (at every .5 m vertically) painted on the ship's side are recommended. For waves longer than the ship, this method fails because the ship moves as a whole with the waves. The observer should move up or down in the ship until the oncoming waves appear just level with the horizon when the ship is vertical; otherwise, one must compensate for the tilt. (See (a) and (b) in Figure III.10.) Some experience is needed to obtain fairly good estimates.  Figure III.10 - Observation of wave heightWave length can be observed by streaming a buoy for a distance astern such that the crests of two successive waves simultaneously pass the buoy and the observer, taking into account the angle between the wave direction and the line joining the buoy to the ship.
Wave speed can be obtained by noting the time of the passage of a wave from the stern to the buoy, allowance being made for the ship’s speed. NOTES: 1. Care should be taken to distinguish between separate wave systems, bearing in mind the distinction between sea and swell as defined above. 2. For checking the observations, note that the wave length divided by the period should be equal to the wave speed. 2. Observations of waves made from island and coastal stations are not representative because of the shallow water, the effects of shelter by the shore, etc. 3.2.2.3.9.1 Observations of waves made from coastal stations These observations cannot be expected to be representative of conditions in the open sea because of the water depth, tides, sheltering effects, etc. If the observations are made despite these difficulties, they should be made in the same way as at sea. 3.2.2.3.9.2 Use of instruments for wave measurements In recent years suitable sea-wave recorders have been developed for measuring the height and period of waves. Different methods are applied: (a) Surface-following buoys measuring acceleration; (b) Ship-borne wave recorders measuring pressure and acceleration; (c) Wave staffs, based on measurement of electrical resistance or capacity; (d) Radar measurement instruments mounted on a platform or on land. It is strongly recommended that such recording instruments should be used at ocean weather stations, research ships and fixed platforms. NOTE: Further detailed information on wave observations is given in the Handbook on Wave Analysis and Forecasting (WMO-No. 446) and the Guide to Meteorological Instruments and Methods of Observation (WMO-No. 8). 3.2.2.3.10 Sea ice and/or icing of ship's superstructure The observation of ice is described in the Guide to Meteorological Instruments and Methods of Observation (WMO-No. 8), Part II, Chapter 4, section 4.2.13. 3.2.2.3.10.1 Sea ice Although general knowledge of the extent of sea-ice cover has been revolutionized by satellite imagery, observations from shore stations and ships are still of great importance, especially in establishing the "ground truth" of satellite observations. The observations of floating ice depend almost entirely on visual estimation.
Sea ice may be reported in plain language or by the use of codes (FM 13-IX SHIP in the Manual on Codes (WMO-No. 306)). Two basic rules for observation of sea ice from ships and shore stations are: (a) Increase the field of view of the observation from the highest point above sea surface (e.g. the top of a lighthouse, the bridge or crow's-nest of a ship); (b) Do not attempt to report sea-ice conditions beyond a radius of more than half the distance between the point of observation and the horizon. Other forms of floating ice may be encountered at sea: river ice and ice of land origin, i.e. icebergs. 3.2.2.3.10.2 Ice formation on ships and other marine structures There are two main types of icing at sea: icing from seawater and icing from freshwater. Icing from seawater may be due either to spray and to seawater thrown up by the interaction between the ship or installation and the waves, or else to spray blown from the crests of the waves. Icing from freshwater may be due to freezing rain or drizzle or freezing fog. Reports on ice accretion at sea may be made either in plain language or in code form (FM 13-IXI SHIP, the Manual on Codes (WMO-No. 306), Volume I.1, Part A). 3.2.2.3.11 Special phenomena WMO has been requested to give assistance by providing, through its Voluntary Observing Ships Scheme, observations of a special nature. Examples of these are: (a) Observations of locust Swarms in sea areas around Africa, the Middle East, Pakistan and India: (b) Observations of freak waves which constitute present a great danger to shipping: (c) Sea-surface currents, which can be determined from measurements of a ship's set and drift and which are of value to research and climate studies. Further details on the reporting of these three observations are contained in the Guide to Marine Meteorological Services (WMO-No. 471), Part II, section 1.4.5 and its aAnnexes. Water spouts should be reported as a special observation. When describing a water spout, the direction of rotation should always be given as if seen from above. Directory: pages -> prog -> www -> OSY www -> Cyclone programme www -> World meteorological organization technical document www -> Regional Association IV (North America, Central America and the Caribbean) Hurricane Operational Plan www -> World meteorological organization ra IV hurricane committee thirty-fourth session www -> World meteorological organization ra IV hurricane committee thirty-third session www -> Review of the past hurricane season www -> Ra IV hurricane committee thirty-fourth session ponte vedra beach, fl, usa www -> World meteorological organization ra IV hurricane committee thirty-second session OSY -> Implementation plan for the evolution of the surface- and space-based sub-systems of the gos OSY -> Commission for basic systems open programme area group on integrated observing systems expert team meeting Download 2.86 Mb. Share with your friends:
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