CLIMATIC CONDITIONS OF THE UNITED STATES. 207 



ture is not at all the same, as far as vegetation is concerned, as a short 

 season with the same mean. 



In studies regarding the relations between temperature intensity 

 and plant development it is necessary to measure and compare the 

 relative effectiveness of the temperature conditions at one station with 

 that of the conditions at another; it is not the temperature of the atmos- 

 phere or of the plant that primarily interests us, but it is the possible 

 effect which the various degrees of temperature may have in controlling 

 plant activity. To make the following subsection clear we shall digress 

 at this point to explain the various concepts and the teiminology which 

 will here be employed, thus presenting a tentative discussion of the 

 various sorts of temperature indices that may be used in vegetational 

 or other dynamically applied climatology. 



The word ''temperature" is used with a variety of different mean- 

 ings, some of which are very vague. Temperature and heat-content 

 are frequently confused, but the heat-content of a body is only one of 

 the conditions that determine its temperature; the heat capacity or 

 specific heat of the matter in question and the amount (mass) of matter 

 considered being also influential in determining the temperature. The 

 definition of temperature involves difficulties unless based on the 

 kinetic theory of matter, in which case the temperature of a body is 

 considered as a measure of the mean kinetic energy of its particles. 

 Temperature is often said to be the relative measure of the sensible 

 heat of the body in question, of its hotness or coldness, this conception 

 being based on the heat-sense of human beings. The latter definition 

 must assume that the matter considered does not change its state (as 

 solid, liquid, or gas) when the human sense-organ is applied to it as a 

 measuring instrument, but the heat-conductivity of the material is 

 important in this connection. Thus, a block of steel and a similar one 

 of wood seem to have different temperatures by the criterion of sense, 

 although they may be of quite the same temperature as determined by 

 a thermometer. The commonest way of measuring temperature is to 

 state it in terms of the relative volume assumed by a given mass of 

 some standard substance (such as air, mercury, alcohol, etc.) when that 

 mass of substance is in heat equilibrium with the body whose tempera- 

 ture is to be determined, this equilibrium being attained when heat 

 does not migrate in either direction between the standard mass and 

 the body under consideration. Thus air-temperature is measured in 

 terms of the relative volume assumed by the liquid in a thermometer 

 when this liquid neither gives heat to the air nor receives heat frdm 

 the air. In short, the thermometer liquid is allowed to come to the 

 same temperature as the air and then this temperature is stated in 

 terms of the volume occupied by this liquid under these conditions. 

 Since the mass of the thermometer liquid is constant for any instru- 

 ment, the different volumes assumed at various temperatures may be 



