32 INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



of a bucket is, -within a very few minutes, apt to become somewliat 

 different from the true temperature of the ocean surface, and a 

 thermometer installed in the condenser intake may be affected by the 

 heat of the engine room. 



It is important also to state whether the temperature of the air 

 and also the wet-bulb temperature are measured with stationary 

 thermometers or with a psychrometer (i. e., a pair of ventilated 

 thermometers), because stationary thermometers, as pointed out in 

 Part III, give much less accurate results than the psychrometer. 

 Furthermore, the formula employed for determining the relative 

 humidity from the readings of a psychrometer is not applicable to 

 the readings of a stationary wet-bulb thermometer. 



TIME AND POSITION 



Day of month. — The date given in the column at the left of each 

 page should be the civil day, beginning at a given midnight and 

 ending at the following midnight. 



In crossing the ISOth meridian, observei's aboard westward-bound 

 vessels sometimes make the mistake of dropping a day from the 

 record of the Greenwich mean noon observations, and conversely 

 of using the same date twice when eastward bound. This is in- 

 correct. In crossing the date line, 24 hours are dropped or repeated 

 as the case may be, in local time reckoning, but not, of course, in 

 the dates of the Greenwich noon observations, which should run 

 consecutively. To avoid any confusion regarding the dates of ob- 

 servations, the observer should remember that when Greenwich noon 

 observations are taken regularly there should never be two of the 

 same date and never any date without an observation. 



"When radio observations are taken regularly at Greenwich mid- 

 night there may be two midnight observatioiis on the same local date, 

 or none at all, depending on the vessel's course, whether eastward 

 or westward when crossing the 180th meridian. The Greenw^ich 

 date of the midnight observations is always the day just beginning 

 at Greenuiich. 



Local f<hip\s time. — The small chart given on page 120 shows the 

 local time corresponding to Greenwich mean noon for each 15° of 

 longitude east and west, i. e., the local (ship's) time at which the 

 daily observation should be taken to the nearest hour. The exact 

 local time at which the observation in any longitude, east or west, 

 should be taken may easily be found from the table of longitude 

 nnd time (table 9). In east longitude, the Greenwich noon obser- 

 vation should always be taken during the afternoon hours, local 

 time ; in west longitude, during the forenoon hours. 



Day of week (I"). — One figure only should be entered in this 

 column to indicate the day. The appropriate code figures for each 

 day of the week are given in the table for Y. The observer should 

 be careful to record the day of the week at Greenwich and not that 

 at the ship, unless the Greenwich day and local day happen to be 

 the same. When an observation is taken at 0000 G. C. T. (Green- 

 Avich midnight), the day of the week should be coded as the day 

 just beginning and not the day just ended. 



