130 GENERAL SCIENCE 



flow of air into a \owfrom the south to be deflected toward the 

 right (east) . The flow into a low from the north is likewise 

 deflected towards the right (now the west). As the inflow 

 from the other sides of the low are affected in a like manner, 

 the winds entering the area of low pressure from all sides 

 suffer deflection to the right. They move into the centre of 

 the low with a spiral motion whose direction in the northern 

 hemisphere is opposite that of the hands of a clock, or 

 counter-clockwise . 



Whether the velocity of the parts of this great whirling 

 mass of air about the central rising column is great or small 

 depends upon the relative densities and consequent pressures 

 of adjacent portions of the atmosphere. These variations in 

 pressure will of course be shown in the readings of barome- 

 ters in different localities. 



This mass of disturbed atmosphere having a progressive 

 motion in an easterly direction, and a whirling motion about 

 its rising air current, may be many hundreds of miles in 

 diameter while possibly but a few hundreds of feet in vertical 

 thickness. It is the cyclone, or " storm area/' or "low" 

 of the meteorologist. It is not destructive like the tornado, 

 and at times it moves across the country entirely unnoted 

 save by the fall and rise again of the barometer. As a rule 

 the weather changes incident to its approach and passage 

 are characteristic, and may be foretold with considerable 

 degree of accuracy for a day or two ahead of the low. The 

 average eastern progress of the cyclonic movement is about 

 thirty miles per hour. 



SUMMARY 



As moisture-laden air feeds into the rising air current at the centre 

 of a "low," the lessened density and pressure there allows its expansion. 

 The lowered temperature that results from this expansion may at times 

 result in cloud formation only, while at other times there will be more 

 or less of precipitation. 



