Chapter 4 — TEMPERATURE, HUMIDITY, AND PRECIPITATION 



STANDARD 



The standard air thermometer which you 

 are more than likely familiar with is the one 

 placed inside or outside of the house to see 

 how cold or warm the temperature was during 

 the day or how cool the air conditioner is 

 keeping the house. There are many common 

 uses for this thermometer which is usually 

 filled with either mercury or alcohol depend- 

 ing on its intended use. These two fluids are 

 used because they have a much greater co- 

 efficient of expansion for each degree of change 

 in temperature than glass has. (See figure 4-1.) 



The range of the standard air thermometer 

 used by the Naval Weather Service 

 is from -20° F to +120° F. 



Handle these thermometers carefully; they 

 break easily. It is important that the thermom- 

 eter stem and bulb be kept clean and free of 

 dirt, dust, and salt spray since the presence of 

 dirt or moisture on the bulb causes an errone- 

 ous indication of the free air temperature. 

 Clean the stem and bulb by wiping with a soft 

 cloth. This should be done 10 to 15 minutes 

 prior to taking a reading so the temperature 

 will have time to stabilize before the observa- 

 tion. Remove and clean the metal back as nec- 

 essary. Upon reassembly apply a drop of light 

 oil to the brass mounting screws. Renew the 

 etched graduations when faded. 



To reunite a separated mercury column, at- 

 tach a psychrometer sling to the thermometer 

 metal back and whirl it, or tap the bulb lightly 

 against the heel of the fleshy part of the hand 

 so as to jar the mercury column back together. 

 If this fails, gently heat the bulb by placing it 

 near a light bulb until the column unites. Never 

 heat the bulb over an open flame. Leave a small 

 space at the top of the tube while heating; 

 otherwise the thermometer will break. If these 



methods fail to unite the mercury column, then 

 replace the thermometer. 



Some isolated stations may still utilize the 

 maximum and minimum thermometers. These 

 are shown in figure 4-2. Procedures for their 

 use and operation are found in the Federal 

 Meteorological Handbook No. 1. 



SHELTERS 



With the increased use of automatic weather 

 stations, such as the AN/GMQ-29( ) that have 

 self-contained shelters for their sensing ele- 

 ments, the wooden-type shelters are becom'ng 

 a thing of the past. However, some stations still 

 using the AN/GMQ-14( ) and isolated overseas 

 stations may have the wooden-type shelters 

 still installed. (See figures 4-3 and 4-4.) 



These instrument shelters are used to house 

 several meteorological instruments including the 

 psychrometer and the maximum and minimum 

 thermometers. 



PSYCHROMETERS 



The several types of psychrometers have 

 as their basic construction two thermometers 

 secured as a unit to a metal back or support- 

 ing device. They may be hand-held (sling-type 

 and electric) or rotor-mounted instruments. 



The primary objective is to obtain the tem- 

 perature readings of the dry-bulb and the wet- 

 bulb thermometers, and then calculate the 

 difference between the two readings. The 

 difference is called the depression of the wet 

 bulb, which is used to find relative humidity, 

 dew point, and vapor pressure. Observations 

 are interpreted by consulting appropriate psy- 

 chrometric tables or computers. 



209.85 



Figure 4-1. — Standard air thermometer. 

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