410 METEOEOLOGICAL CORRESPONDENCE. 



From Henry M. BarMister. 



Smithsonian Institution, 



January, 1867. 

 In returning to New York from San Francisco in the present month, I ob- 

 served a curious phencmeiion in the formation of clouds over the Gulf Stream 

 We found ourselves in the stream on the morning of the 19th, the temperature 

 of the water being 70° Fahrenheit, that of the air at or below freezing point. 

 The whole surface of the sea was covered with vapor, which was most dense at 

 the level of the water, but did not much interrupt the view at the elevation of 

 the vessel's deck. The vapor ascended in forms exactly like that of a water- 

 spout, for which we at first mistook them. The long ropy column, swaying to 

 and fro, was very distinct in many instances, several being seen at one time. 

 When this was not developed we could still see a point stretching downward 

 from the cloud above, and ragged masses of mist rising to it from below. We 

 passed several of these vapor-spouts quite near enough to make sure of their 

 real nature. They did not appear to be of any density, nor were there any 

 signs of disturbance in the water or atmosphere about them. They were seen 

 on all sides, several at a time. When we had crossed the Gulf Stream, and got 

 into water of at most not more than 40'^, we left the fog and had a clear sky 

 over head, and on looking back the boundary of the stream was distinctly marked 

 by the line of clouds hanging over it. 



Rkmarks. — The phenomenon of a cloud suspended over the Gulf Stream, mark- 

 ing its boundary, must have been frequently observed during winter, when there 

 exists a great difference between the temperature of the water and that of the 

 air, as in the case described by Mr. Bannister, while the interesting appearance 

 of ascending columns, such as he has mentioned, must be of less frequent occur- 

 rence. It can probably happen only when the excess of the temperature of the 

 water above that of the air is near a maximum, and this again can only occur 

 after the air above the stream has been cooled by a northwesterly wind lollowed 

 by, at least, a partial calm. In this condition the stratum of air immediately 

 above the water would be heated, its specific gravity diminished, and the super- 

 incumbent mass brought into a state of unstable or totteiing equilibrium. In 

 the case of a small portion of air thus heated, the whole would rise in a single 

 column on account of the pressure of the surrounding atmosphere ; but in that 

 of a large extent of heated suiface, in which the temperature of the air above 

 shades off gradually at the borders, the equilibrium will be restored by small 

 ascending columns. 



A similar phenomenon is sometimes observed on extended sandy plains in the 

 form of ascending dust columns. They are interesting as serving to throw light 

 upon the causes of the water-spout and the land tornado, which have generally 

 been referred to electrical action, but which can be fully explained without the 

 introduction of this principle other than as an effect and a modifying agent. In 

 the case of the water-spout and the tornado, the unstable equilibrium may be 

 produced by the gradual heating of the lower stratum, next to the water or 

 ground, when saturated with vapor, by the rays of the sun, or perhaps in some 

 cases by the passage of a heavier stratum over a lighter one next the earth. 

 The intensity of the upward motion will generally be in proportion to the greater 

 thickness of the heated stratum and the comparative density of the upper. In 

 my meteorological essays published in the Patent Office Keports, I have en- 

 deavored to show that a thunder-storm, in accordance with the theory of Mr. 

 Eispy of the upward motion of the air in case of storms, is an upward moving 

 column, in which the ascent is produced by the causes we have explained, and 

 that the electrical effects exhibited are due to the disturbance of the equilibrium 

 of the natural electricity of the ascending column of conducting vapor by the 

 negative induction of the earth below and the positive ibfluence of space above, 



