654 THE POPULAR SCIENCE MONTHLY. 



parallel to the isobars. The velocity or force of the wind will depend 

 on the closeness of the isobars. In the diagram they are much closer 

 set in rear than in front of the cyclone, and therefore the wind is 

 strongest behind the center. 



The weather in a cyclone is somewhat complicated. Some charac- 

 teristic features depend on the position of the trough, and have noth- 

 ing to do with the center. For instance, the weather and sky over 

 the whole front of the cyclone — that is, all that lies in front of the 

 trough — is characterized by a muggy, oppressive feel of the air, and a 

 dirty, gloomy sky of a stratiform type, whether it is actually raining 

 or only cloudy. On the other side, the whole of the rear is character- 

 ized by a sharp, brisk feel of the air, and a hard, firm sky of cumulus 

 type. 



But, on the contrary, other characteristic features are related to the 

 center, and have little to do with the trough. The rotation of the 

 wind, though slightly modified near the trough, is in the main related 

 to the center, and the broad features of the weather in a cyclone are — 

 a patch of rain near the center, a ring of cloud surrounding the rain, 

 and blue sky outside the whole system. The center of the rain-area 

 is rarely concentric with the isobars. It usually extends farther in 

 front than in rear, and more to the south than to the north, but is still 

 primarily related to the center. 



This will be readily seen by reference to the diagram ; there the 

 drizzle and driving rain extend some distance to the right front, while 

 almost directly behind the center patches of blue sky become visible. 

 Thus a cyclone has, as it were, a double symmetry ; that is to say, 

 one set of phenomena, such as warmth, cloud character, etc., which 

 are symmetrically ' disposed in front and rear of the trough ; and 

 another set, such as wind and rain, which are symmetrically arranged 

 round the center. There is reason to believe that what we may call 

 the circular symmetry of a cyclone is due to the rotation of the air, 

 while the properties which are related to the trough are due to the 

 forward motion of the whole system. 



We have marked on the diagram the kind of weather and cloud 

 which would be found in different parts of a cyclone. The first thing 

 which will strike us is that the descriptive epithets applied to the sky 

 contain the phraseology of the most familiar prognostics. At the 

 extreme front we see marked " pale moon," " watery sun," which 

 means that in that portion of a cyclone the moon or sun will look 

 pale or watery through a peculiar kind of sky. But all over the 

 world a pale moon and watery sun are known as prognostics of rain. 

 Why are they so ? The reason we can now explain. Since a cyclone 

 is usually moving, after the front part where the sky gives a watery 

 look to the sun has passed over the observer, the rainy portion will 

 also have to come over him before he experiences the blue sky on the 

 other side of the cyclone. 



