ship as possible, since the passage of the ship through the water tends to mix surface water with water from the keel level of the ship. The "bucket" should also be cast ahead of where the observer is standing so that the bucket  fills  as  it  drifts  by  the  observer.  As  the movement of the ship carries the bucket astern of the observer, the bucket should be retrieved. A standard thermometer is then inserted into the water sample, and the water is slowly stirred with the thermometer until the temperature reading stabilizes. The temperature is read to the nearest 1/10 degree Fahrenheit. Bathythermograph  Temperature WIND The next best method is to obtain a sea surface temperature from an  expendable bathythermograph sounding. Procedures   for   conducting   a bathythermograph sounding are covered in a later module. Bucket temperatures should be conducted occasionally   to   verify   that   the   recorded bathythermograph surface temperature is accurate. Sound velocimeterreadings may also be used in lieu of a bathythermograph  reading. Seawater Injection Temperature The  least  accurate  method  is  the  seawater  injection temperature  reading.  Seawater  injection  temperatures are read in the engineering spaces and are usually readily available by shipboard phone from the "main engine  room  control"  watch/operator. Seawater is constantly taken onboard for cooling the engines and for conversion to freshwater. The seawater injection ports are located well below the water line, sometimes as deep as 60 feet on aircraft carriers. Therefore, temperature  readings  at  that  point  do  not  accurately reflect a sea surface temperature, but rather a near surface  temperature  reading. In tropical waters, the difference between the surface  temperature  and  the  near-surface  temperature  is usually slight. But in certain regions of the mid- and high-latitudes, a strong surface thermocline may exist, which will cause a rapid decrease is temperature from the surface to the injection level. This may cause the difference between the actual surface temperature and the injection temperature to be very large. If injection temperatures are used, they should be routinely checked against  bucket  temperatures  and  bathythermograph temperatures, and adjusted if necessary. Q56.  Define wet-bulb temperature. REVIEW  QUESTIONS Q57. Q58. Q59. Q60. What is the normal result of air being cooled to below the dew-point temperature? Given an air temperature of 82.5°F and a dew- point temperature of 70.0°F, calculate the dew- point  depression. What  is  meant  by  the  term  "frost-point temperature"? List three methods for manually obtaining the sea  surface  temperature. LEARNING  OBJECTIVES:  Describe wind speed and wind direction. Define and identify how to determine true-wind direction, relative- wind  direction,  and  magnetic-wind  direction. Explain  wind  character  and  wind  event.  Define Foxtrot Corpin. The atmosphere is essentially an ocean of air surrounding earth. Temperature is unevenly distributed over the earth’s surface, varying with latitude and with the seasons. Therefore, all of earth’s atmosphere is continuously  in  a  state  of  fluid  motion.  Wind  is  the observed effect of horizontal transport of air masses over earth’s surface.   Surface winds are the movements of air within 50 feet of the ground.    The term winds usually  refers  to  both  wind  speed  and  direction. Different reporting codes require that observations be made over certain periods of time. Some conventions require  a  2-minute  observation  period,  while  others require a lo-minute. These time periods are specified in NAVMETOCCOMINST  3141.2  and  NAVMET- OCCOMINST  3144.1. Winds  are  observed  by  using  the  equipment discussed   in   chapter   2.   Automatic   observation equipment will immediately report and record winds; other wind-measuring equipment can show a detailed graph of speed and direction over time. Winds are described  by  wind  direction,  wind  speed,  and  wind character. WIND  DIRECTION Wind direction is the average direction from which the wind is blowing during a specified period. Airflow from the north toward the south is referred to as a "North wind." Wind   direction   always   shows   minor fluctuations.  These  minor  fluctuations  are  normally "averaged  out"  when  determining  a  wind  direction. 1-42