1913] on The Winds in the Free Air 723 



siderable distance from their point of origin. Such conditions of 

 wind are those productive of thunderstorms. This may also possibly 

 account for the superstition that gun-firing produces rain ; the sound 

 of guns is only carried to great distances under the conditions I have 

 mentioned, which are precisely the conditions favourable for heavy rains. 



A north-east wind with rain lasting many hours is a common and 

 a very unpleasant type of weather ; it is not obvious where the mois- 

 ture comes from with such a wind, for the air from the dry regions of 

 the Continent could hardly become saturated in its short passage 

 over the North Sea. I believe the moisture comes from the Atlantic 

 in a south-westerly wind in the upper air. Balloons cannot be 

 followed for any length of time in such weather, but I have sometimes 

 observed that the north-easterly wind slackens consideral^ly Ijelow the 

 cloud level, and sometimes, when breaks in the clouds have enabled 

 balloons to be followed a little farther, there have been unmistakable 

 signs of reversal. A careful watch for upper clouds, seen through 

 rifts in the lower cloud sheet, will often indicate an upper southerly 

 wind. So sure do I feel of these facts that though living only 

 12 miles from the Channel I never hesitate to send up an instrument- 

 carrying balloon in rainy weather with a north-easterly wdnd, feeling 

 sure that, though the balloons may go towards the sea at first, they 

 will ultimately return and fall on dry land. My confidence is usually 

 rewarded, the balloons coming to earth in the Midlands, or eastern 

 counties. 



The last type of wind structure to be considered is the outflow 

 that seems to take place from the upper layers over a low pressure 

 system, causing west to north winds in the upper air on the east 

 and south sides of the depression. Depressions out in the Atlantic, 

 which cause south-west winds on the surface, give rise to west or 

 north-west winds in the upper air over England ; even cyclones as 

 far off as Iceland produce such winds, and as they pass along the 

 Arctic Circle, between Iceland and Norway, they show their presence 

 by an upper northerly wind over this country. As the upper wind is 

 often quite different from that on the surface, reversals are frequent 

 in this class, and are associated as usual with rains, and with thunder- 

 storms in the summer. It may be that much of the rain that falls 

 in the cyclonic depression is due to the rising of this outward flowing 

 current over the very different surface current on the east and north- 

 east sides of the depression. 



In connexion with the subject of reversals, I may mention the 

 wave and ripple clouds that form such beautiful skyscapes. It used 

 to be supposed that these were formed by winds from different direc- 

 tions flowing over one another and setting up waves ; but the obser- 

 vations of pilot balloons have shown that between two currents from 

 different directions there is either a layer of calm, or else the wind 

 changes round gradually ; tw'o very diiferent currents are not found 

 in close juxtaposition : there is no abrupt transition between them. 



