HUMPHREYS: METEOROLOGICAL PARACOXES 165 



would, therefore, never be fully attained, but forever approached 

 asymptotically. Clearly, then, if the temperature of the shell 

 were T and that of the enclosed object T -\- t, the latter would 

 continue to grow colder through any finite time unless, and un- 

 til some time after, the temperature of the shell were raised 

 above the then temperature of the enclosed object. 



The reasoning in this special case applies also to the normal 

 daily temperature of the atmosphere (substantially that of the 

 surface of the earth), provided, as will be assumed for the mo- 

 ment, that there is neither circulation nor any thermal effects due 

 to water transformations — freezing, thawing, etc. It applies 

 because the normal daily loss of heat through radiation to space 

 by any given region is as though it were a full radiator at a cer- 

 tain temperature, and its normal daily gain of heat from the 

 outside as though it were completely canopied by another full 

 radiator also at a certain (generally different) temperature. 



During the autumn, therefore, while there is still stored in 

 the earth much of its summer gain of heat, and while the daily 

 supply of energy from the sun is growing less and less per unit 

 area, the average 24-hour temperature of the surface, and of the 

 surface air, must be appreciably higher than that of equilibrium 

 with the simultaneous incoming radiation^ — -higher because of 

 the additional supply of heat by conduction from its reservoir 

 beneath the surface — and as the summer storage of heat in the 

 earth is very large and also near the surface (but little penetrating 

 beyond a depth of 5 or 6 meters) it is obvious, frdm the pre- 

 liminary explanation above, that the minimum temperature can- 

 not occur until some time after winter solstice, or when the days 

 have again grown longer, and that the delay must depend on 

 latitude, nature of surface, and a number of other factors. 



The date of this minimum temperature is still further delayed, 

 in many places, by the trend of warm ocean currents and the 

 warmer surface drifts toward the higher latitudes, and by on- 

 shore winds. It is also affected, though probably but slightly, 

 by the thermal effects of freezing, thawing, evaporation, and con- 

 densation. 



