44 



Farmers' Bulletin 1096. 



the evaporation that is taking place absorbs heat from the ther- 

 mometer and cools it to below the temperature of the air. The 

 amount of cooling depends on the amount of moisture in the air 

 and on the rate at which the air is moving past the thermometer. 



It is plain that if .a thermometer is to register the temperature of 

 the air, it must be sheltered from the sky and from direct sunlight, 



and also must be ex- 

 posed in such a way 

 that moisture from 

 any source is not 

 likely to gather on it. 

 Free circulation of 

 the air is also an im- 

 portant requirement 

 for a satisfactory 

 thermometer e x p o- 

 sure. If a shelter 

 offers much obstruc- 

 tion to air circulation 

 the " air inside the 

 shelter may cool at a 

 slower or faster rate 

 than the outside air 

 and the thermometer 

 in the shelter will 

 then fail to indicate 

 the true temperature 

 of the outside air. It 

 is essential, therefore, 

 that a thermometer 

 shelter allow as free 

 a circulation of the 

 air as possible with- 

 out sacrificing the ele- 

 ments of protection 



f 

 trom SUmiglli 



}iq U i(J O r frozen mois- 

 ' rrn i i 



ture. The standard 

 Weather Bureau shelter has a double roof to prevent undue 

 warming of the inside air by the sun's rays, and the bottom is 

 as open as possible. The sides are louvred, the openings being as 

 wide as possible without allowing the direct sunlight to reach the 

 interior. (See fig. 18.) 



All thermometers used in determining temperatures in orchards 

 should be exposed with the foregoing principles in mind. A simple 



FIG. 18. Standard type of instrument shelter used by the 

 Weather Bureau. The shelter is always placed so that 

 the door opens toward the north so that the sun can not 

 shine directly on the instruments when the door is open. 



