initiating rotor chop. Rotor chop is a difficult to control, sometimes hazardous, up and down oscillation of the rotor blades. The peak wind speed or peak gust is the highest instantaneous wind speed or gust speed greater than  25  knots  observed  since  the  last  METAR observation. Variable Winds Variable winds occur when the wind direction fluctuates by 60° or more. While this condition occurs most frequently when the winds are very light, wind direction  fluctuations  are  most  significant  when  the wind speeds are higher (greater than 6 knots). For observation purposes,   the  wind  direction  may  be considered  variable  anytime  the  observed  2-minute mean wind speed is 6 knots or less. WIND EVENTS Certain  wind  phenomena  are  included  in  an observation even though the events did not occur during the 2- or 10-minute period during which the winds were being observed. These phenomena or events may be included in the observation if they occurred within the past  hour  and  were  not  reported  in  a  previous observation. The events include squalls and wind shifts. Squalls A squall is a sudden large increase in wind speed (usually accompanied by a change in wind direction) that lasts several minutes and then suddenly dies. For observation purposes, the wind speed must increase by 16 knots or more and the sustained wind speed after the increase must be 22 knots or more for at least 1 minute. Squalls are usually caused by large convective cells, like  those  that  produce  strong  rain  showers  and thunderstorms. Squalls may also be produced by dry frontal passages; the presence of precipitation is not a requirement. At sea, strong rain showers at a distance away from the ship are called "squalls" because squall winds  are  usually  present.  When  lines  of  thunderstorms form on or move out ahead of a cold front, the line may be called a "squall line" because of the squall winds associated with the thunderstorms. Wind Shifts A wind shift is any change in wind direction by 45° or more during a 15-minute time period. The change in direction may or may not be accompanied by a change in wind speed. However, wind shifts are only recorded when the mean wind speed is 10 knots or greater during the shift. A wind shift may be very sudden, occurring within a minute or so, or it may occur gradually over the 15-minute period. The most common cause of wind shifts is frontal passage, especially a cold-frontal passage. The onset of a sea breeze may cause a wind shift, as may other locally produced wind conditions. FOXTROT CORPIN Foxtrot Corpin is the term used to identify the best course and speed a ship should “come to” to bring the relative wind into the proper window for the launch and recovery of aircraft. For  departures  and  recoveries aboard different classes of ships, the most desirable relative  wind  speed,  acceptable  minimum  and maximum wind speeds, the most desirable relative wind direction, and the acceptable wind direction variations  are  specified  in  the  NATOPS  Flight  Manuals for each type of aircraft. Foxtrot  Corpin  is  routinely  provided  by Aerographer’s  Mates  aboard  aircraft  carriers  and amphibious assault ships and is computed by using the CP-264/U true wind computer. Detailed instructions for the procedure are printed on the reverse side of the CP-264/U. The procedure uses the true wind and the desired wind to find required ship’s course and speed. Q61. Q62. Q63. Q64. Q65. Q66. Q67. REVIEW  QUESTIONS Explain the difference between True North and Magnetic  North. Winds blowing directly off the starboard beam are coming from what relative direction? How can relative winds be manually converted to true winds? How is the mean wind speed determined? How would the observed wind speed on a ship be affected by winds blowing from dead astern? Define the term "gust." Define the term "squall." SEA AND SWELL WAVES LEARNING  OBJECTIVES:  Explain  the importance  of  sea  conditions  to  naval operations. Define duration limited seas and fetch limited seas. Define wave height, wave length, and wave period, and wave direction. Define and distinguish the difference between sea waves and swell waves. Define Romeo Corpin. l-45