42 



Farmers' Bulletin 1096. 



Small green lemons are injured when the temperature remains at 

 29 F. for some time. Ripe lemons are damaged at 27 F. Lemon 

 blossoms are killed at about 30 F. 



METEOROLOGICAL INSTRUMENTS AND EXPOSURES 



It has been stated that substances when exposed to a clear sky, 

 steadily lose heat by radiation. Not only does the rate at which ra- 



diation takes place vary greatly on different nights, but every in- 

 dividual substance has its own rate at which it radiates heat. There- 

 fore, under conditions favorable for radiation, a thermometer of 

 dark metal would show a lower temperature than one of mercury 

 inclosed in glass. 



The only method whereby temperature measurements which are at 

 all comparable can be secured is to obtain, not the temperature of 

 the thermometer under radiation conditions, but as nearly as pos- 

 sible the temperature of the free air surrounding the thermometer. 



