HEAT. 



23 



or isobars, shows the distribution of barometric pressure, and when the 

 direction of the wind is marked on the map, it is found that it blows, in 

 general, from higher to lower pressures, but not straight down the slope. 

 Through the rotation of the earth, the wind has a tendency to go to the 

 right of the area of lowest pressure, as explained already in the case of 

 the trade-winds. One of the commonest arrangements of the isobars is 

 that of a series of oval curves, round the area of lowest pressure. This 

 will be seen from Fig. 44, which gives the map for November 23, 1874, 

 in which the lowest pressure was over the Midlands. The arrow-heads 



giving the direction of the wind, 

 show us also that it is every- 

 where blowing towards the right 

 of this low pressure area, and so 

 much so that the whole of the 

 system constitutes a whirlwind 

 or cyclone. The rotation in such 

 a cyclone is, in the northern 

 hemisphere, always counter- 

 clockwise. Through the in- 

 draught of air to the centre, the 

 depression is gradually filled up ; 

 but not so rapidly as we should 

 expect, if the air merely moved 

 to the centre, and stopped there. 

 It appears from observations 

 made on high clouds that, in 

 general, it ascends there, and 

 flows outwards again in the 

 upper strata, forming an " anti- 

 cyclone." 



A study of cyclones has shown 

 that in the various parts, there 

 are not only characteristic winds 

 but also characteristic kinds of 

 weather. Thus, to the east of the 

 centre, the wind is usually from 

 FIG. 44. The isobars and wind in a Cyclone. tne south or south-east, and, far 

 (From Abercromby's Forecasting by from the centre, the weather is 

 Weather Charts.) nne> Moving westward towards 



the centre, the weather gradually 



gets damper ; the sky becomes overcast ; and near and at the centre, 

 there is usually rain. Passing the centre, the wind is now from the 

 north or north-west ; it is colder and drier ; and some distance from the 

 centre the sky usually becomes clear. To the south of the centre, the 

 wind is usually south-west, and north of it north-east. 



The distribution of weather in a typical cyclone is represented in 

 Fig. 45. 



Cyclones usually move in an easterly direction most commonly to 

 the north-east in our latitude, the centre travelling at any rate up to 

 70 miles an hour. But with a given cyclone the rate is often nearly 

 constant for some time. 



November 23, 1874. 



