46 Remarks on the Formation of Fogs. 



earlier summer months, the temperature of the water was lower 

 than that of the air ; and the air over the land warmer than that 

 over the water. These rarely extend over land much, and are 

 most frequently confined to the air over the bed of the river. 



8. In all the diurnal and in most of the nocturnal fogs of the 

 spring and first part of summer, observed by me, on the Missis- 

 sippi River, the production of visible vapor commenced near the 

 surface of the water, and increased in depth by additions above. 



It appears therefore that the fogs of the Mississippi River may 

 be considered under two heads : — 1st. Those which originate 

 when the water is warmer than the air. 2d. Those which origi- 

 nate when the water is colder than the air. The first kind pre- 

 vails mainly during the fall and earlier part of the winter; the 

 second kind are most common during the spring and early part of 

 the summer. The first unquestionably results from the conden- 

 sation of aqueous vapor, which rises constantly from the warmer 

 water into the air, in quantities more than sufficient to saturate it 

 at its temperature. The second, it appears to me, must result 

 from the condensation of aqueous vapor already in the air, in con- 

 sequence of the extension of refrigeration, from the cold surface 

 of the water, through the warm and humid air above. 



Among fogs which form over land, likewise, a large number 

 have their origin independent of the causes assigned by the the- 

 ory of Hutton. During almost every portion of the year, the 

 high temperature of the air and earth during the day-time, in- 

 creases the quantity of watery vapor in the air, and the dew- 

 point temperature rises towards the hottest part of the day, so 

 that the complement of the dew-point, except in winter, is rarely 

 more than 10° or 15° Fahr., and most frequently not more than 

 5° or 10°. The temperature of the air gradually declines until 

 about sunset, while the temperature of the dew-point remains 

 almost fixed ; thus diminishing the complement. After sunset, 

 however, the earth cools rapidly by radiation, and the air partici- 

 pates, though not to the full extent, in the diminution of tempe- 

 rature.* It is not unusual to see the temperature of the air di- 

 minished by 15° or 20° between 3'clock p. m. and midnight, and 



* " In every calm, still night, the air nearest the earth is colder than that which 

 Is more distant from it, to the height of at least 220 feet, this being the greatest 

 height to which M. Six's experiments relate." — Wells, on JDeic,p. 95. 



