CIRCULATION OF THE ATMOSPHERE 453 



circulating on practically the same vortical laws, but unfortunately it 

 shows indications of not being able to follow the law strictly, especially 

 in the inner portions of it. The outer part of a strong ocean cyclone, 

 where the barometer drops to 28 inches of pressure at the center, is 

 very much like an enormous hurricane in its formation, but near the 

 center the angles and the velocities begin to break away from the pure 

 vortex law. This is probably due to the great extent of the wind areas, 

 and consequently the congestion, and to the fact that the ocean cyclone 

 is not deep enough, although it may be 3 or 4 miles high, to carry out 

 fully the requirements of so large a vortex of a pure type. It is known 

 that hurricanes are vortices which are 6 or 7 miles deep. The large 

 ocean cyclone is probably not more than 4 miles deep, and the great 

 land cyclone is rarely more than 2 or 3 miles deep. 



The land cyclones in the United States conform to the pure vortex 

 law less perfectly than does the ocean cyclone. The pressure in the 

 land cyclone usually stops at about 29 inches near the center. Its 

 depth is usually about 2 or 3 miles. It may cover a diameter of 

 2,000 miles. These dimensions are evidently unfavorable for the devel- 

 opment of a pure vortex. Furthermore, the distribution of the tem- 

 perature in the land cyclone is entirely different from that in the pure 

 hurricane, and this too prevents the land cyclone from developing accord- 

 ing to the perfect law. Furthermore, the cyclones of the temperate 

 zone develop in the lower levels of the great eastward drift. In these 

 lower levels the eastward velocity of the drift is not very high; some- 

 thing like ten meters per second. At the height of two or three miles 

 the eastward drift is something like twenty to forty meters per second. 

 It becomes evident, then, that a vortex which develops in the lower 

 levels, from any set of causes, must lift its head into a rapidly flowing 

 stream of air, and this necessarily will tend to break down the intruding 

 head by stripping off portions of it and detaching the upper portions of 

 the vortex from the lower portions. Now a vortex can not develop 

 except as a complete individual. If it is intruded upon by cutting off 

 the lower section, as in the hurricane over the ocean, or by the upper 

 sections thrusting themselves into the stream of the rapidly flowing 

 eastward drift, it is evident that this is a sufficient cause for the par- 

 tial destruction of the vortex system. In the theoretical vortex, above 

 the middle section, the wind has an outward component increasing with 

 the height, as already explained. Below this section it has in every 

 cyclone an inward component. Now as a result of the cloud observa- 

 tions which were undertaken by the U. S. Weather Bureau during the 

 international cloud year 1896-7, in which between 6,000 and 7,000 

 observations were made by means of theodolites upon the direction of 

 motion in the different cloud levels, it was found that there was an 

 inward component over the cyclones ia all levels from the ground up 



