48 INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



This shows that an accurate report of visibility may often serve 

 to supplement the description of the character of the weather as 

 reported in the "present weather" colnmn. 



Islands or mountains, whose distance from the observer is known, 

 should be used whenever possible as objects on which the estimate 

 of visibility may be based. Otherwise, use may be made of another 

 ship or the horizon. The latter, when viewed from the level of 

 the bridge (30 to 40 feet above sea level), appears at a distance of 

 6 to 7 nautical miles. 



Table 10 on page 119 gives the distance of visibility of objects at 

 sea. 



When isolated showers are within sight, but are not occurring at 

 the ship, the observation of visibility should be made in a direc- 

 tion where the horizon is not obscured by falling precipitation. 



TEMPERATURES 



Temperatvre of the air (TT). — Air temperatures prevailing at 

 sea are far more representative of the general thermal conditions in 

 the lower layers of the atmosphere than the temperatures observed 

 at land stations, where local influences must be taken into account. 

 Accurate reports of air temperature from ships are a valuable aid 

 in forecasting, especially when they are used in conjunction with 

 observations of water tem])erature. Because of their representative- 

 ness, they are indispensable to all studies of large-scale fluctuations of 

 climate. For these reasons it is most important, when taking an 

 observation, to make sure thfit the true value of the air temperature 

 is obtained. 



In order to obtain the true temperature of the air, it is necessary 

 that reliable thermometers be employed and that they be properly 

 exposed and accurately read. When practicable, thermometers should 

 be compared with a standard instrument, and if an appreciable error 

 is shoAvn to exist a correction should be a])plied to the readings. Com- 

 parisons may be made at any Weather Bureau office. The Weather 

 Bureau will gladly furnish information regarding the purchase of 

 reliable thermometers. 



Proper exposure of thermometers is absolutely essential. If a 

 thermometer is simply fastened to the deck house or to a bulkhead 

 it will rarely, if ever, register the true air temperature. In this 

 position the instrument may give readings that are 20° F. or more 

 loo high in the presence of sunshine and absence of wind. It is, 

 therefore, strongly recommended that a portable thermometer be used. 

 It will then always be possible to measure the air temperature on the 

 weather side of the ship, where the thermometer will be freely ex- 

 posed to the wind and unaffected by artificial sources of heat. A de- 

 tailed treatment of the problem of thermometer exposure on board 

 ship is contained on pages 25 to 27, which observers are asked to 

 read carefully. 



Care must be taken to read the thermometer correctly. It may 

 happen that the observer glances at the instrument rather hastily 

 and fails to make an accurate mental note of the positions of the 

 numbers opposite the graduations of the scale. This sometimes leads 

 to an error of an even 5° or 10° in recording the temperature. 



