516 IOWA DEPARTMENT OF AGRICULTURE 



determine how much the wet bulb is cooled by evaporation. Then from 

 this data the relative humidity is obtained from prepared tables. 



To make clear the relation of the relative humidity to temperature it 

 may be well to choose a few examples. For the first case, there may be 

 assumed a cluster temperature of 60° F. (barometer, 30 in.) in an atmos- 

 phere which is fully saturated. In this event the slightest cooling will 

 cause condensation and the wet bulb in such an atmosphere (if it could 

 be circulated rapidly) would show no cooling. No evaporation can occur 

 as the atmosphere cannot take up any more moisture. If, however, the 

 wet bulb can be cooled at this temperature the relative humidity is less 

 as the readings of the wet bulb thermometer are lowered. The tempera- 

 ture to which an atmosphere must be cooled to produce condensation is 

 known as the "dew-point." This is also lowered as the humidity decreases 

 These points are illustrated in the accompanying table: 



Assumed cluster 

 temperatures Dew Relative 



Dry bulb Wet bulb point humidity 



60° P. 60° F. 60° F. lO'O per cent. 



60° F. 58° F. 57° F. 89 per cent. 



60° F. 56° F. 53° F. 78 per cent. 



60° F. 54° F. 49° F. 68 per cent. 



60° F. 52° F. 45° F. 58 per cent. 



60° F. 50° F. 40° F. 48 per cent. 



60° F. 48° F. 35° F. 39 per cent. 



With such an assumed temperature of the cluster (60° F.) only the 

 highest relative humidities would show condensation in an atmosphere in 

 which such a cluster temperature would be found, for such a cluster tem- 

 perature could occur only when the external temperature is above 57° F. 



If different temperatures are assumed for the cluster (all of which have 

 been observed under different conditions by various investigators) the 

 relative humidity of the warmer atmosphere which will show no conden- 

 sation when cooled to cellar temperature is given in the following table, 

 (barometer, 30 in.): 



Relative 

 humidity 



58 per cent. 

 48 per cent. 

 35 per cent. 

 18 per cent. 



In this second table the numbers are chosen so that the dew-point is 

 practically 45° F. in all cases, assumed as an average cellar temperature. 

 It appears that a given amount of water given off by bees at 96° F. creates 

 a much lower relative humidity (18 per cent) than the same quantity of 

 water at 60° F. (58 per cent) because the warmer atmosphere is capable 



