64 MANUAL OF MARINE METEOROLOGICAL OBSERVATIONS 



7134, Psychrometric Readings. Readings from the dry and wet-bulb 

 thermometers should be obtained in accordance with the following instructions. 



7134.1. Saturate the muslin of the wet bulb with clean water. Use care 

 to avoid wetting the dry-bulb thermometer, which must be thoroughly dry be- 

 fore the readings. Ventilate the psychrometer for about 10 seconds and 

 quickly read both thermometers, the wet-bulb thermometer first. Repeat 

 the alternate ventilating and reading until two successive readings of the wet- 

 bulb thermometer are the same and indicate that the thermometer has reached 

 its lowest temperature. 



7134.2. When the wet-bulb temperature is below 0°C., moisten the wet- 

 bulb thermometer with water having a temperature between 18° and 24° C. 

 (65°-75°F.) in order to melt completely any accumulation of ice on the muslin, 

 since a very thin coating is necessary for accurate data. When the psychrom- 

 eter is ventilated at below freezing temperatures, and there is doubt as to 

 whether the muslin is frozen, the muslin should be brought to the frozen 

 state by touching it with clean ice or snow, or some other object whose 

 temperature is approximately the same as or less than the dry-bulb temper- 

 ature. After the muslin has become ice- covered, continue to ventilate the 

 psychrometer until successive readings indicate that the lowest wet- bulb 

 temperature has been reached. 



7140. COMPUTATION OF THE DEW-POINT TEMPERATURE. The 

 temperature of the dew point is computed with tables furnished with this 

 manual (see J7144). To use the tables, it is necessary to know the temiper- 

 ature of the wet bulb and the wet- bulb depression. 



7141. Depression of the Wet-Bulb Thermometer. The depression of 

 the wet-bulb thermometer is the algebraic difference between the dry and 

 wet-bulb temperatures. 



Examples: 



(1) Dry-bulb temperature 40.6 



Wet-bulb temperature 34.1 



Depression 6.5 



(2) Dry-bulb temperature 1.2 



Wet- bulb temperature -0.7 



Depression 1.9 



