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CHAPTER Observation of Wind CHAPTER Observation of Wind Wind is an important meteorological element, closely related with the pattern of pressure Wind speed is proportional to pressure gradient and wind direction deviates generally about 15 degrees from the direction of isobars over the ocean in mid-latitude Observations of wind over the ocean are reported on real time for analysis, forecasting and warning of weather They are also utilized as statistical data for navigation of ships and airplanes 4.1 Definition and unit Wind is a vector with direction (wind direction) and magnitude (wind speed) Wind is defined as an averaged value over 10 minutes immediately preceding the observation time If you have discontinuous changes of wind direction during the ten minutes, adopt the wind direction averaged over a period between the last discontinuous change and the observation time Wind direction is the direction from which wind is blowing In marine meteorological observation, the wind direction is measured as one of the 36 direction codes, with the east as 9, the south as 18, the west as 27 and the north as 36 The scale of the direction code increases clockwise with an increment of for each angle increment of 10 degrees If the wind speed is less than 0.2 m/s (O.4knot), the wind direction should be recorded as "calm" and coded as 00 for the logbook If wind is indeterminate, put 99 as the direction code As for wind speed, "knot" is used for its unit Note the following relationships: knot = nautical mile/hour = 1852 m/hour = 0.5144 m/ s; m/ s= 1.9438 knot CHAPTER Observation of Wind CHAPTER Observation of Wind Wind is an important meteorological element, closely related with the pattern of pressure Wind speed is proportional to pressure gradient and wind direction deviates generally about 15 degrees from the direction of isobars over the ocean in mid-latitude Observations of wind over the ocean are reported on real time for analysis, forecasting and warning of weather They are also utilized as statistical data for navigation of ships and airplanes 4.1 Definition and unit Wind is a vector with direction (wind direction) and magnitude (wind speed) Wind is defined as an averaged value over 10 minutes immediately preceding the observation time If you have discontinuous changes of wind direction during the ten minutes, adopt the wind direction averaged over a period between the last discontinuous change and the observation time Wind direction is the direction from which wind is blowing In marine meteorological observation, the wind direction is measured as one of the 36 direction codes, with the east as 9, the south as 18, the west as 27 and the north as 36 The scale of the direction code increases clockwise with an increment of for each angle increment of 10 degrees If the wind speed is less than 0.2 m/ s (O.4knot), the wind direction should be recorded as "calm" and coded as 00 for the logbook If wind is indeterminate, put 99 as the direction code As for wind speed, "knot" is used for its unit Note the following relationships: knot = nautical mile/hour = 1852 m/hour = 0.5144 m/ s; m/s= 1.9438 knot CHAPTER Observation of Wind 4.2 Measuring instruments and installing conditions To measure wind, wind vane/ anemometer is generally used It should be installed at a place where wind is least affected by your ship structure and where you can repair the instrument easily in case of malfunction 4.3 Measurement of wind direction and wind speed If you use an anemograph, examine a part of the data recorded for 10 minutes immediately preceding the observation time You can find the wind direction as the averaged direction at the centre of the most densely dotted part in the record You can also get the wind speed at the centre of the fluctuating values of wind speed If you are not using an anemograph, examine the average of the values of wind direction shown by the indicator for about minute Regarding wind speed, discard the maximum and the minimum values and adopt the value around which fluctuations are almost constant 4.4 Calculation of true wind direction and speed Unless your instrument is capable of measuring the wind with compensation for ship's motion (true wind), the observed wind obtained on board ship during navigation indicates an apparent value (apparent wind) The true wind is the vector sum of the apparent wind and ship's motion It is convenient to make use of a circle graph Plot angle scales on the circumference with each increment of 10 degrees Then plot speed scales on a diameter in the unit of knot with increment of 10 knots Let's take the following example (Fig.4.5) to see how to get the true wind: 18 CHAPTER Observation of Wind true wind direction (25 degrees) from which the wind blows You can make sure your calculation of true wind direction by observing wind waves, because they are traveling from almost the same direction as the wind direction True wind can be also calculated using a wind velocity scale (see Fig.4.6), which is composed of two turning disks and one rectangular scale and they are fastened at the center of the disks The lower disk is called a compass disk, and the upper disk is called a wind direction disk, and the rectangular scale is called a wind speed ruler In order to calculate true wind, 1) Turn and adjust "a" on the wind direction disk to a direction of ship's bow on the compass disk 2) Turn the wind speed ruler to a direction of apparent wind on the wind direction disk 3) Mark a point" A" on the wind speed ruler with a distance of the wind speed from the center, and next, mark a point "C" going down from" A" with a distance of the ship's speed in parallel with vertical lines of the wind direction disk 4) Turn the wind speed ruler to "C" Then you find the true wind speed by reading the scale of the ruler at "C" 5) At the cross point of the wind speed ruler and the wind direction disk, the wind direction disk shows the true wind direction from the bow, and the compass disk shows the true wind direction from true north CHAPTER Observation of Wind vane/ anemometer, you need to measure wind direction as shown below and wind speed through wind force defined below 4.5.1 Wind direction True wind direction may be determined from streaks of foam on the sea surface and/ or sea (wind) wave direction by using a gyro compass You need to take the following into consideration for the non-instrumental observation: 1)Observe wind waves located beyond a certain distance from your ship so as to free from her influence; 2) When the wind direction changes abruptly, watch the direction of streaks of foam or the orientation of the crests of sea waves, other than overall wave shapes since character of wave remains affected by preceding wind for some time; 3) When wind is too weak to cause perceptible waves, estimate wind direction for yourself on deck considering the direction of ship's movement; 4) Do not rely on wind waves near shore or sea ice which not give you proper wind direction; 5) In case of night time or bad visibility (such as heavy rainfall, snow, fog etc.), estimate wind direction by the statement of smoke, tape and so on; 6) Though upstream side is preferable in general, observe wind at lee side exceptionally only when you have to avoid dazzle caused by the sunbeam or its reflection from the sea surface 4.5.2 Wind force In order to estimate wind force from the sea surface situation, follow the Beaufort Scale of Wind Force (see TabA.1) and also refer to photographs The velocity equivalent in the Tab.4.1 is at a standard height of 10 meters above the sea surface When you use the table, you should take caution as stated in the following: 1)Even while a wind is getting stronger, there are certain time lags before actual changes of sea surface situation (wave length, wave height, etc.) take place; 2) The scale of a wave depends on the fetch (the distance along which a wind is blowing straight) of the wind in question This is why a wave with a land on its upstream side has different scale from an offshore wave; 3) When a tidal or ocean current is predominant, the sea surface situation is not necessarily correspondent to the wind force Especially, unusual waves are remarkable near an ocean front; 4) The sea depth affects the way how a wave is formed; 5) When a swell is prevailing, it becomes more difficult to discern wind waves; 6) At night, the estimation of wind force is not easy Since you can normally recognize 21 CHAPTER Observation of Wind white caps less easily, you need to slightly overestimate wind force from wind waves; 7) Rainfall tends to make the sea surface smoother Severe rainfall is particularly effective Therefore you need to overestimate the wind force to some extent in such a case; 8) The height of a wind wave depends on the difference between air temperature and sea surface temperature So you had better adopt a larger value than the wind force estimated from wind waves when the air temperature is lower than the sea surface temperature 22 23 CHAPTER Observation of Wind 24 29