E. Loomis — ( 'ontrihitthmx to Meteorology. 13 



at about ihe same rate. This is one of numerous cases which 

 appear to indicate that an area of low pressure cannot advance 

 rapidly unless an area of high pressure advances behind it. 

 The Signal Service observations also show numerous cases in 

 which a low area has remained for several days nearly station- 

 ary between the Rockv Mountains and the meridian of 100°. 

 In No. 20 there were unusually heavy rains on the north side 

 ■of the low center and a violent cyclonic movement of the winds 

 about this center, but on the west side of the low center the 

 low pressure extended to a great distance, and a high pressure 

 did not appear on the west side until April 9ih, after which 

 date the low moved eastward more rapidly. 



Thus we see that nearly all of these cases of extremely slow 

 motion apparently resulted from an unusual extension of a sec- 

 ond area of low pressure on the western side. These cases of 

 very slow motion, as well as those of very rapid motion, indi- 

 cate that the direction of movement and rate of progress of a 

 storm center do not depend exclusively upon the amount of 

 rain-fall or upon the distribution of rain fall within an area of 

 low pressure, but also upon the distribution of pressure, tem- 

 perature and humidity throughout an extensive region sur- 

 rounding the low area on all sides. Moreover the influence of 

 barometric pressure is generally much more obvious than that 

 of temperature or humidity. An examination of the Interna- 

 tional Weather maps indicates how the progress of a storm 

 center may be influenced by another storm prevailing at a dis- 

 tance of several thousand miles. We find from these maps 

 that the entire northern hemisphere, north of lat. 30°, is gen- 

 erally covered by successive areas of high and low pressure 

 which arc advancing eastward with uuepta! velocities, and are 

 daily undergoing important modifications. If one storm in- 

 creases or diminishes in intensity, this change must affect the 

 adjoining areas of high pressure, and this in turn must affect 

 the areas of low pressure which prevail beyond these areas of 

 high pressure. If one storm slackens in its rate of progress, it 

 may be overtaken by another area of low pressure which is 

 advancing from the west. If one storm advances with unusual 

 rapiditv, it may overtake another storm prevailing on its eastern 

 side and may coalesce with it. While then the precipitation of 

 the vapor of the air appears to be the chief source of that 

 maintaining power which is nee be action of 



violent storms, we cannot certainly predict the direction of 

 movement and the rate of progress of an area of low pressure 

 from a simple knowledge of the amount of rain-fall and posi- 

 tion of the rain-areas within the limits of that low area. 



The high winds which are uniformly found to prevail some- 

 where in the vicinity of a great fall of rain deserve particular 



