ADDITIONAL    WEATHER-RELATED RESPONSIBILITIES.—In addition to the assistance you may provide during heavy weather, you will have other POOW responsibilities concerning weather. Some of those responsibilities are the measuring of temperature, atmospheric pressure, and wind speed. You also must have a working knowledge concerning weather flags and pennants. Measuring Temperature.—You probably don’t need to be told that a thermometer is an instrument for measuring temperature. The ship’s thermometer is usually located on the bridge, but on some small ships it is located near the quarterdeck area. Most Navy thermometers are liquid-filled and practically all of them use the Fahrenheit (F) scale which is based on the freezing point of water being 32F and the boiling point being 212F. A thermometer must be read properly for you to obtain an accurate result. First, if you must handle it, be sure that you do not touch the lower part of the glass containing the liquid. The heat from your body can affect the height of the liquid column and give an erroneous temperature reading. Make sure that the top of the column is level with your eyes; otherwise, you will be reading a higher or lower graduation than the one actually indicated. Another weather device that you must use to forecast the approach of a storm is the barometer. Measuring Barometric Pressure.—The aneroid barometer, as shown in figure 8-2, contains a small metallic cell, called a Sylphon cell, which encloses a partial vacuum. As atmospheric pressure increases, the Sylphon cell contracts; as pressure decreases, it expands. As the Sylphon cell expands and contracts, it communicates motion to an indicating pointer on a graduated scale. The aneroid barometer can be read in inches of mercury and in millibars (mb). Both inches and millibars are measurements of the weight of the atmosphere at a given time and point. The average atmospheric pressure at sea level is 29.92 inches or 1,013.2 millibars. Figure 8-3 shows the comparative readings on the inch and millibar scales. The aneroid barometer normally can be read no closer than 0.01 inch. Aneroid barometers are the standard pressure-indicating instrument aboard ship and  the  type  of  barometer  that  the  POOW  will encounter most frequently. Barometers are normally checked (calibrated) yearly by the quartermaster division, following the preventive maintenance system (PMS) procedures. Another important forecasting tool is the wind speed and direction indicator. Measuring Wind Speed and Direction.—An installed anemometer is shown in figure 8-4. It is an instrument fixed somewhere aloft, usually at the masthead of the ship. The wind blows on a propeller attached to one end of a wind vane that pivots. The whirling propeller revolves a spindle, communicating with a synchro repeater on the pilothouse or chart house bulkhead. Figure 8-5 shows one type of synchro repeater. 8-16 MRP3F0502 Figure 8-2. Aneroid barometer. 1050 1047 1044 1041 1038 1035 1032 1029 1026 1023 1020 1017 1014 1011 1008 1005 1002 999 996 993 990 987 984 981 978 975 972 969 966 963 960 957 954 951 948 31.0 30.9 30.8 30.7 30.6 30.5 30.4 30.3 30.2 30.1 30.0 29.9 29.8 29.7 29.6 29.5 29.4 29.3 29.2 29.1 29.0 28.9 28.8 28.7 28.6 28.5 28.4 28.3 28.2 28.1 28.0 1013 MILLIBARS OR 29.92 INCHES AT SEA LEVEL MRP3F0503 Figure 8-3. Inches and millibars.