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CHAPTER VII. TEMPERATURE 



7000. GENERAL 



7001. Temperature readings are made at sea to determine the free- air 

 temperature, the dew-point temperature, and the temperature of the sea 

 surface. 



7100. AIR- TEMPERATURE MEASUREMENTS 



7110. DEFINITIONS. The following terms are used in connection with 

 air-temperature measurements. 



7111. Free-air temperature. The temperature (or dry bulb measured by 

 an ordinary thermometer exposed to the free air on the windward side of 

 the ship under conditions that eliminate as completely as possible the effects 

 of extraneous sources of heat. 



7112. Wet-bulb temperature. The lowest temperature to be secured by 

 evaporating water from a muslin- covered bulb of a thermometer at a 

 specified rate of ventilation. The wet- bulb temperature differs from the 

 dry-bulb temperature in an amount dependent on the temperature and relative 

 humidity of the air. 



7113. The dew point. The temperature at which condensation would occur 

 if the air were cooled. The dew point can be computed when the difference 

 between the dry-bulb and the wet-bulb temperature is known. 



7120. READING THE THERMOMETER. Temperatures indicated by any 

 mercurial or spirit-filled thermometer may be determined as follows: 



7121. Stand as far from the thermometer as is consistent with accurate 

 reading, to prevent body heat from affecting the instrument. 



7122. Insure that the line of sight from the eye to the top of the liquid 

 column makes an angle of 90° with the thermometer tube. This will avoid 

 an error of parallax. 



7123. Read the thermometer to nearest 0.1°. A degree interval begins 

 at the middle of the de^^ree marking etched on the tube. 



7130. PSYCHROMETERS. Air-temperature measurements may be made 

 with a psychrometer. Psychrometers consist of one dry and one wet-bulb 

 thermometer mounted on a single frame. The psychrometer should be so 

 ventilated that the minimum speed of air passing over the thermometer 



