WIND. 15 



pretension to accuracy, can be made. Notwithstanding this, 

 however, a solitary observer, by making use of all the indications 

 at his disposal, may often make a very good guess at what the 

 weather is going to be. 



The first step towards this is to carefully record the move- 

 ments of the barometer and thermometer, which, if possible, 

 should be self-recording instruments. 



The indications given by these, coupled with the observation 

 of clouds, and a consideration of the usual local sequence of 

 weather under varying conditions, will often enable fairly correct 

 forecasts to be made. Such forecasts are, however, obviously 

 not so reliable as those emanating from a study of charts, like 

 those just described. 



Since as has already been pointed out (see Fig. 3, p. 5) 

 the central, and at the same time the most dangerous, part of 

 a storm is a low-pressure area, it follows that as such area 

 approaches the barometer will fall; and since the velocity of 

 wind is proportionate to the barometric gradient down which it 

 blows, if successive readings of the barometer, on being plotted, 

 show a convex curve or increasing rate of fall, the violence of 

 the gale may be expected to increase. If, on the contrary, a 

 concave curve or decrease in the rate of fall is indicated, the 

 wind may be expected to moderate. 



A quickly falling barometer inasmuch as it indicates a 

 steep gradient, which is synonymous with high wind velocity 

 almost invariably forebodes bad weather. 



There is a law in meteorology, known as " Buys Ballot's 

 law," which is as follows : " Stand with your back to the wind, 

 and the lowest pressure will always be on your left hand in the 

 northern hemisphere, and on your right in the southern hemi- 

 sphere." Thus the position of a storm centre may be approxi- 

 mately determined. 



The manner in which the wind either veers or backs as a 

 storm advances depends upon the position which the observer 

 occupies in the cyclonic area. Thus, wind in the northern 

 hemisphere appears to back if the observer is stationed to the 

 left of the centre line of the storm's path, and to veer if to the 

 right of such line. This will be evident on reference to Fig. 4, p. 6, 

 it being assumed that the storm is travelling along the line AB 

 in the direction of B. In the southern hemisphere the converse 

 of this obtains. 



