b. Microbarograph . This instrument, manufactured by the Belfort Instru- 

 ment Company, Baltimore, Maryland, is located inside the building and is used 

 to measure the atmospheric pressure and pressure tendency (Fig. 21). The 

 device is an aneroid sensor which responds to pressure changes on the order of 

 0.005 inches of mercury. The pressure tendency can be determined from the 

 chart record; i.e., the total change in pressure over a 3-hour period and the 

 direction of change (increasing, decreasing, or some combination). 



Figure 21. Microbarograph. 



c. Barometer . A NWS barometer is also used for monitoring atmospheric 

 pressure (Fig. 22). This device is an aneroid barometer with a 2-inch-diameter 

 bimetallic wafer which expands or contracts with changes in pressure. The 

 pressure (in inches) is read directly off the gage. Additional details on the 

 barometer can be provided by the Basic Observations Branch, NWS. 



d. Kygro thermograph . The hygro thermograph (Fig. 23) is located in an 

 outdoor, ventilated instrument shelter (Fig. 24). This instrument, also manu- 

 factured by the Belfort Instrument Company, is used to measure and record both 

 air temperature and relative humidity. The temperature sensor consists of a 

 bimetal assembly made from two dissimilar metals which react to temperature 

 changes at different rates. The recording styles (or pens) are driven by the 

 expansion or contraction of the assembly. The relative humidity is sensed by 



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