STORMS AND WEATHER FORECASTS 69 



surface of the mercury in the cisterns of the instruments, thereby 

 sustaining the columns of liquid metal at a greater height in 

 the vacuum tubes. Over another considerable area the l)arom- 

 eters are falling, as increasing temperature rarefies and expands 

 the volume of the air, causing it to press upon the instruments 

 with less force. This chart is extremely useful to the forecaster, 

 since, in connection with the general weather chart, it indicates 

 whether or not the storm centers are increasing or decreasing in 

 intensity, and, what is of more importance, it gives in a great 

 measure the first warning of the formation of storms. 



A third clerk constructs two charts, one showing the humidity 

 of the air and the other the cloud areas, with the kind, amount, 

 and direction of the clouds at each station. It is often interest- 

 ing to ol)serve at a station on the cloud chart high cirrus clouds 

 composed of minute ice spicule moving from one direction, lower 

 cumulo-stnitus composed of condensed water vapor moving from 

 another direction, and the wind at the surface of the earth blow- 

 ing from a third point of the compass. Such erratic movements 

 of the air strata are only observed immediately l)efore or during 

 rain or wind storms. 



A fourth clerk constructs a chart called the general weather 

 chart, showing for each station the air temperature and ])ressure, 

 the velocit}'^ and direction of the wind, the rain or snow fall since 

 the last report, and the amount of cloudiness. The readings of 

 the barometer on this chart are reduced to sea-level, so that the 

 variations in pressure due to local altitudes may not mask and 

 obscure those due to storm formation. Then lines, called isobars, 

 are drawn through ])laces having the same pressure. By draw- 

 ing isobars for each difference in pressure of one-tenth of an inch 

 the high- and the low-pressure areas are soon inclosed in their 

 proper circles. The word "high " is written at the center of the 

 region of greatest air pressure and the word "low " at the center 

 of the area of least pressure. Under the influence of gravity the 

 air presses downward and outward in all directions, thus caus- 

 ing it to How from a region of great pressure toward one of less. 

 The velocity with whicli the wind moves from the high toward 

 the low will depend largely on the difference in air pressure. 

 To better illustrate: If the barometer read 29.5 at Chicago and 

 30.5 at Bismarck, North Dakota, tlie difference of one inch in 

 pressure would cause the air to move from Bismarck toward 

 Chicago so rapidly that after allowing for the resistance of the 

 ground there would remain a wind at the surface of the earth of 



