DISCOVERY 



269 



north of Ireland. In this particular case the isobars 

 were fairly wide apart, and light or moderate winds 

 prevailed ; except where, off the north-west of Ireland, 

 the lOiS mb. isobar just mentioned closed into a 

 1016 mb. isobar, which came down from the north to 

 meet it. Here the wind is seen to be blowing in freshly 

 from the westward, illustrating one of the points of the 

 utility of the weather map.' Barometrical gradients 

 are measured in hundredths of an inch (or about J of 



a miUibar) to fifteen nautical miles. According to the 

 "steepness" of the gradient, so will proportionately 

 be the strength of the wind. If the difference between 

 the readings of two barometers (and therefore the 

 denomination of the two resulting isobars) amounts to 

 ■07, the wind will be less strong than on a day when 

 they differ by '14, and so on in proportion. 

 The following table of gradients is useful : 



Gradients are calculated in the following manner : 

 If a barometer at one station, which we will call (a), 

 reads 29- 14 in., and at another station, called (6), 



• Although the wind was generally light inland, on the coasts 

 there were Jresh local sea breezes, especially in the north of 

 Scotland. 



29"2S in., station (6) being thirty nautical miles distant 

 from (a), then the difference in barometer readings is 

 fourteen hundredths (■14) of an inch in thirty nautical 

 miles, or seven hundredths ("07) in fifteen n.m. ; hence 

 the gradient is said to be seven.^ 



This system of gradients, in conjunction with Buj'S 

 Ballot's Law, may be said to be the "backbone " of 

 the synoptic weather chart. This law, which may 

 be called the " Law of Storms," tells us that if we stand 

 with our backs to the wind, low barometric pressure 

 will be to your left hand and high on your right. This 

 is only the case, however, in the northern hemisphere ; 

 south of the equator the opposite is true, and when we 

 have our backs to the wind, the low pressure would be 

 on our right hands and high on our left. 



For the rest, these weather maps show us direction 

 and force of wind by means of arrows flying with the 

 wind, the force being shown by the number of barbs. 

 A calm is indicated thus ; 0. The figures in the 

 circles at the end of the arrows, i.e. where the point 

 should be, show the velocity of the wind in miles per 

 hour. 



The figures just beside the arrows show the tempera- 

 ture at the time of observation. The small letters 

 show the state of the weather in the Beaufort Notation, 

 thus : — 



Since the conclusion of the war the Meteorological 

 Office has come under, and we might say become part 

 of, the Air Ministry, with Sir Napier Shaw, F.R.S., as 

 Director, and Lieut. -Colonel Gold as Superintendent of 

 the Forecast Division. It is quite in accordance with 

 the fitness of things that the Air Ministry and the 

 Meteorological Office should " join hands," so to speak, 

 as in peace, even more than in war, meteorology and 

 aviation must necessarily be of mutual benefit the one 



' Inches of mercury can be converted into millibars if we 

 take '03 in. as equal to one millibar. — D. W. H. 



'The Beaufort Notation must not be confused with the 

 International Weather Symbols, in which V = rime frost, and 

 the letter v, which used to stand for exceptional visibility, was 

 abolished at the International Meteorological Conference in 

 October last, the sign O being substituted for it. 



* Humidity less than 60%. 



