650 THE POPULAR SCIE2iCE MONTHLY. 



that is to say, tbey do not face the wind like tlie pointer of a wind- 

 vane. 



When all this is done, we can see at a glance whether or how wind, 

 rain, cloud, and blue sky are connected with the shape of the isobars. 

 In fact, a synoptic chart gives us, as it were, a bird's-eye view of the 

 weather at the particular moment for which the chart is constructed, 

 over the whole district from which reports have been received. Sup- 

 pose, now, that after an interval of twenty-four hours another chart is 

 constructed from observations taken over the same area, then we 

 generally find that the shape of the isobars and the position of the 

 areas of high and low pressure have considerably changed, and with 

 them the positions of those areas where the weather is good or bad. 

 For instance, suppose that at 8 a. m. on one morning we find pressure 

 low over Ireland and high over Denmark, with rain over Ireland, 

 cloud over England, and blue sky in Denmark ; and that by 8 a. m. on 

 the following day we find that the low-pressure area has advanced to 

 Denmark, and that a new high pressure has formed over Ireland, with 

 rain in Denmark, broken sky in England, and blue sky in Ireland ; 

 suppose, too, that the record of the weather, say in London, for those 

 twenty-four hours had been as follows — cloudy sky, followed by rain, 

 after which the sky broke — how can an inspection of the two charts 

 help us to explain the weather as observed in London during that day? 

 Our bird's-eye view would show that the rain-area w-hich lay over 

 Ireland in the morning had drifted during the day over England, in- 

 cluding London, and covered Denmark by next morning. It would also 

 tell us that the position of the rain was identified with and moved along 

 with the low pressure. This is the fundamental idea of all synoptic 

 meteorology, but one which can only be thoroughly grasped after a 

 considerable experience in tracing actual cases. 



Such, then, is a synoptic chart. Many thousands have been con- 

 structed for all parts of the world, and by comparing them the follow- 

 ing important generalizations have been arrived at : 



1. That in general the configuration of the isobars takes one of 

 seven well-defined forms. 



2. That, independent of the shape of the isobars, the wind always 

 takes a definite direction relative to the trend of these lines, and the 

 position of the nearest area of low pressure. 



3. That the velocity of the wind is always nearly proportional to 

 the closeness of the isobars. 



4. That the weather — that is to say, the kind of cloud, rain, fog, 

 etc. — at any moment depends on the shape, and not the closeness, of 

 the isobars, some shapes being associated with good and others with 

 bad weather. 



.5. That the regions thus mapped out by the isobars were constantly 

 shifting their position, so that changes of w'eather were caused by the 

 drifting past of these areas of good or bad weather, just as on a small 



