48 Rev arks on the Formation of Fogs. 



Therefore the decrement of atmospheric temperature will be 

 greater than that of the dew-point temperature, by (c-J-qo'X?) ~~ f') 

 -[-/?. If it be denied that radiation has any influence in this case, 

 we still have the decrement of atmospheric temperature greater 

 than that of the dew-point temperature, by (c-)-9£/)(<p - 9'). Now 

 this concurrent diminution, by unequal decrements, that of the 

 atmospheric temperature being greatest, tends to reduce this tem- 

 perature to equality with that of the dew-point ; and after the 

 two temperatures have in this way become equal, any farther 

 diminution in the same ratio must result in the production of vis- 

 ible vapor in the form of fog. This explanation, if correct, will 

 apply equally to fogs which form over cold waters, and many 

 which form over land. Over land they occur most frequently 

 towards morning, and are preceded, and perhaps at first accom- 

 panied, by the deposition of dew. They are most common in 

 autumn and spring. When they form over large fields, or over 

 our extensive prairies, they first begin to appear as a thin haze 

 near the ground, and become deeper by additions above ; but in 

 dense forest regions, the condensation first commences about the 

 tops of the trees, which taken together constitute in fact, in the 

 present instance, the radiating and refrigerating surface. 



When heavy and extensive fogs are dissipated in the morning 

 by the heat of the sun, the visible is converted into invisible 

 vapor, and becomes a part of the atmospheric air; but this change 

 is not usually permanent, for the mixed atmosphere is heated, and 

 rising in consequence of diminished specific gravity, it soon ar- 

 rives at a region where the temperature is as low as its dew-point, 

 when it again takes the form of visible vapor. Clouds thus pro- 

 duced, very often succeed to heavy fogs in this country during 

 spring and autumn ; they are generally formed at very short dis- 

 tances above the earth. Often, indeed, the vapor does not lose 

 the visible form at all, but rises slowly until it arrives at a small 

 elevation, when it commences to move as an ordinary cloud. 

 This horizontal motion commences, generally, when the vapor 

 has risen to the height of from one hundred to three hundred 

 feet, but this region of clouds attains a greater elevation as the 

 day advances, until it reaches the ordinary height. This kind 

 of cloud is generally carried over this part of the country, towards 

 the north or northwest, and sometimes continues to pass over 

 during the earlier part of the day, until 10 or 11 o'clock, a. m. 



