BÀI GIẢNG KHÍ TƯỢNG LÝ THUYẾT CHƯƠNG 25

Chapter Twenty Five CHAPTER TWENTY FIVE Brief Notes on Observations and Instruments The importance of being weather conscious Observing the weather is an essential part of the duties of a deck officer; in fact he is no seaman if he is not weather-conscious for when on watch a part of his task of keeping a good lookout is to note and record changes in wind, sea, cloud, visibility, etc In a sailing ship and any small craft, safety and progress depend almost entirely upon the wind and weather: the same is true but to a lesser degree aboard a power driven ship It is no exaggeration to say that observing and recording the weather, and the intelligent use of simple meteorological instruments is a part of seamanship The making of both instrumental and non-instrumental observations and recording them at least every four hours in the ship's log book (and at the times of any significant changes) are a normal part of an officer's watch-keeping duties NON-INSTRUMENTAL OBSERVATIONS include: Direction and force of wind Direction and description of swell (no swell should also be recorded) Visibility Weather in general terms; e.g fine, moderate rain, hail, etc State of sky; e.g., blue sky, cloudy, overcast, etc If the ship is taking spray or shipping 'green seas" Behaviour of ship; e.g., rolling heavily, pitching, pounding, engines racing In merchant ships observations of wind direction and force, state of sea, swell and visibility are all estimated INSTRUMENTAL OBSERVATIONS for ship's log book include: Barometric pressure reduced to sea level The correction for diurnal variation is only applied to readings of barometric pressure for the Master's operational use and so this correction should not be applied to the reading recorded in the ship's log book 219 Chapter Twenty Five Air temperature Sea surface temperature The method of obtaining the sea surface temperature should also be recorded OBSERVATIONS BY SELECTED SHIPS If the vessel is a selected ship under the WMO voluntary observing scheme more detailed observations are made and then coded for transmission The message is coded using an international code, details of which appear in the Ship's Code and Decode Book MET O 509 Selected ships' reports include barometric pressure barometric tendency, air temperature, dew-point (determined by table from hygrometer readings) and temperature of the sea surface for all of which special instruments are supplied on loan to the ship by the Port Meteorological Officer In a few selected ships distant reading instruments are supplied and tined for all temperature readings the recorders being sited on the bridge SUMMARY OF POINTS TO REMEMBER WHEN READING A BAROMETER (A) Aneroid barometer Tap gently read, record and check Correct only for Index Error and height (B) Precision aneroid barometer Correct only for height The correction table for height above sea level is entered with arguments height and temperature of the outside air NOT the temperature inside the chart room AN APPROXIMATE HEIGHT CORRECTION FOR ANEROID BAROMETER Hectopascals: Height in feet above sea level increased by 10 per cent and divided by 30 gives correction in hectopascals to be added BAROGRAPH A barograph is supplied by the Meteorological Office to each Selected Ship It produces a continuous graphical record of barometric pressure on a chart called a barogram Thus it shows all fluctuations in pressure which occur between the fixed times at which the barometer is read It consists of a series of flat circular sealed metal boxes which are partially exhausted of air and are very sensitive to small changes in atmospheric pressure which either increase or decrease the volume of the boxes These movements are transmitted by an arrangement of levers to vertical movement of a pen arm The pen draws an ink trace on the barogram which is fitted round 220 Chapter Twenty Five a revolving drum The drum revolves about a vertical axis controlled by a clock and one revolution takes one week, during which time a continuous record of all pressure variations are shown against GMT and date The barogram is renewed every week when the clock is rewound The instrument is contained in a glass case with a hinged lid for winding the clock, renewing the graph, refilling the inkwell, etc The barometric tendencies read from the barogram are recorded in the meteorological logbook and included in the coded weather reports for synoptic hours, The barograph should be kept at Greenwich date and time throughout the voyage HYGROMETER The hygrometer is an instrument for measuring the humidity of the air The type most commonly used aboard ship (Mason’s Hygrometer) consists of two ordinary thermometers mounted vertically inside a wooden box (Stevenson's Screen) which is louvred on all four sides to protect the inside from rain and sun also to allow air to now freely through the box The thermometers are exactly the same but one is “dry" and is used for finding the air temperature, the other (the "wet" bulb) has the bulb wrapped in a single thickness of muslin, secured round the neck of the bulb by a few strands of cotton wick, the lower ends of which are immersed in a small reservoir of distilled water placed in the screen; thus the wet bulb is kept moist by capillary action Principle The lower the humidity of the air (and hence the drier the air) the faster the evaporation from the muslin on the wet bulb, hence the lower the wet bulb temperature compared with that of the dry bulb, Conversely when humidity is high the rate of evaporation is slow and the difference in readings between the two thermometers, called "the depression of the wet bulb" is very little The accuracy of the readings depends on the flow of air past the thermometers so the screen should be exposed on the weather side of the bridge, well clear of warm air currents from deck, funnel, ventilators, skylights etc The data derived from the readings of the dry and wet bulb thermometers are the relative humidity and dew-point which are obtained from tables supplied with the instrument 221 Chapter Twenty Five SEA THERMOMETERS The sea surface temperature is an important observation for the meteorologist, as it is used for climatological work as well as for shorter term forecasting A knowledge of the sea temperature is also useful to the shipmaster It must be considered in controlling cargo ventilation and can assist in forecasting the onset of advection fog WIND FORCE AND DIRECTION Although some vessels are fitted with anemometers for measuring the wind speed these are difficult to site on a vessel as they should be kept clear of all obstructions The motion of the vessel also affects their readings The general method used is to observe the appearance of the sea surface This is then compared with a detailed description of the probable state of the sea caused by winds of various forces as given in the Beaufort Scale This sea criterion was prepared by WMO When using this method it is essential to take into account other factors which might influence the sea state These include such things as fetch tides and currents depth and precipitation DIRECTION, PERIOD AND HEIGHT OF SEA AND SWELL WAVES These observations are difficult to make but are important to the meteorologist Like wind force and direction they are particularly difficult to make on dark nights and depend largely on experience Despite the increased availability of wave height observations from satellites, observations from merchant vessels are still valuable CLOUD OBSERVATIONS The only way to become proficient in cloud identification is through careful study of the cloud descriptions and photographs, combined with much practice for which every deck officer has plenty of opportunity when on watch Detailed observations are needed from Selected Ships (See Chapter 4) VISIBILITY In Selected Ships visibility is recorded in kilometres/nautical miles on a scale of to When there is no target on the radar screen, this observation is largely a matter of judgement, having in mind the yardstick that the observer 222 Chapter Twenty Five knows the distance from his ship to the horizon which can be mentally subdivided Although the horizon method is commonly used for distances of more than miles it is not very reliable because of the possibility of abnormal refraction In a long ship objects on board of known distances can be used when visibility is low At night deteriorating visibility can be detected by a loom around the ship's navigation lights OCEAN CURRENT OBSERVATIONS These are asked for on a voluntary basis aboard Selected Ships of certain nations The method is to calculate the difference between an estimated position with no current (ie, DR position after making due allowance for leeway) and a reliable fix The result is the set and drift of the current since the previous reliable fix 223