552 THE HELM 'WIND OF CROSSFELL. 



On the Helm Wind of Crossfell. By the Rev. J. Watson. Eighth Report of 

 the British Association. 



" This wind is applied to a very violent wind, blowing frequently from some 

 eastern point of the compass, at the west side of the Crossfell range, and 

 confined, both in length and breadth, to the space between the Helm and 

 Helm Bar. The western front of the Helm is a long, large roll of clouds, 

 clearly defined, and quite separated from any other cloud on that side. 



The Bar has its eastern front as clearly defined, and at the same height. 

 On the west side of the Bar there is either no wind, or it blows in a contrary 

 direction, that is, from the west. 



Neither the Helm nor the Bar are separate or detached clouds, but rather 

 the bold, clearly defined fronts of clouds, extending eastward behind the 

 Helm, and westward from the Bar. 



The open space between these clouds may be called a very flat ellipse, eisrht 

 or ten, or even twenty to thirty miles long, and from one half to four or five 

 miles wide. They appear always united at the ends. 



The open space between the Helm and Bar is clear of clouds, with the 

 exception of small pieces breaking off now and then from the Helm, and 

 driving rapidly over to the Bar; through this open space is often seen a 

 higher stratum of clouds, quite at rest. 



It is seldom accompanied by rain in the open space, and never continues 

 long after it begins to rain heavily." 



If it should be ascertained that it rains on the top of Crossfell at the time of 

 the Helm wind, it would be easy to explain all the phenomena mentioned 

 above . 



The weight of the rain, and its cooling effect on the air under the Helm 

 cloud, would depress the air under the cloud, and press it out on all sides be- 

 low, with a velocity proportional to the height from which the drops fell. 

 This outward motion of the air below would not reach to any considerable 

 height, and the air above would be running towards the Helm cloud, and up- 

 wards in that cloud to supply new vapor to be condensed into cloud by the 

 cold of diminished pressure. The air which rushes out from the falling rain 

 below, meeting that which was coming from the west, would soon be stopped 

 in its motion outwards, and in rising up would form the Helm Bar ; and as 

 it rushed out from the very bottom of the Helm cloud itself, it would carry 

 with it portions of that cloud towards the Bar. 



Up in the region of the Bar the air at the same time would be in motion 

 towards the Helm, and would carry portions of cloud over in that direction 

 with it. 



If this is the true explanation, several other facts besides those mentioned 

 below will be found to exist at the time. Kain under the Helm cloud a 

 colder air between the Helm cloud and the Helm Bar than to the west of the 

 bar a smaller difference between the dew point and the temperature of the 

 air, between the two clouds, than on the west of the Ear ; the top of the Bar 

 vastly lower than the top of the Helm cloud ; though their bases may be on 

 the same horizontal level; the cloud that passes from the Helm to the Bar 

 underneath the one which passes from the Bar to the Helm ; the edges of the 

 Bar seldom if ever so well defined as the edges of the Helm, and never so 

 dense nor so free from motion, the lower part of the Helrn and the upper 

 part of the Bar which eross over towards each other, and frequently a jutting 

 out of the upper part of the Helm cloud, so as to overhang the Bar. If these 

 phenomena do not accompany the Helm wind then the above is not the true 

 rationale, if they do, it is. 



It may still be asked why the Bar cloud, when once formed by the upward 

 motion of the air, dissolves in a great measure on being pressed or driven 

 towards the Helm cloud by the motion of the air in that region. The answer 

 is, that the air curves downwards in going from the Bar to the Helm as it 

 does in passing across a hollow between two mountains, and in coming under 

 greater pressure the cloud is dissolved. 



