Page:The National Geographic Magazine Vol 16 1905.djvu/333
Forecasting the Weather and Storms
297
developing the ■ normal storm tracks ' for a given locality. In this figure, drawn to to the scale of the Washington weather map, the vector X represents the direction and movement in 24 hours of the storm that was centered near Amarillo, Texas, 8 a.m., May 26, 1903. The vector is the resultant of the pressure acting from the north, northeast, east, etc., in the direction indicated, and was determined from an increase in pressure from the storm center outward at 8 a.m., May
Chart XVI — Showing method followed in developing normal storm tracks and in ascertaining the correct value to represent the pressure exerted on a storm center that causes it to depart from a normal track.
26, as follows : To the north, o. 10 inch ; northeast, o 00 ; east, 0.60 ; southeast, 0.30; south, 0.30; southwest, 0.30; west, 0.30, and northwest, 0.20, each tenth of an inch being given a value of one centimeter. The vector Y represents the 24-hour movement of the storm that was centered over Amarillo, Texas, at 8 a. m., May 28, 1903. The vector Yj is the resultant of the pressure exerted on the storm center, determined, as above, from an increase in pressure from the storm center toward the several directions, as follows : To the north , o. 30 inch ; northeast, o. 20 ; east, 0.60; southeast, 0.30 ; south, o. 10 ; southw T est, o. 10 ; west, o. 10, and northwest, 0.40.
" The vector X being the resultant of the forces that propelled the storm in the direction and to the point indicated in 24 hours, it is possible to eliminate the pressure influence (if it be given an appropriate value) by resolving the vector representing the track of the storm into its two components, one of which, Xj, being the resultant of the pressure exerted on the storm, the other, X 2 , will represent the 24-hour value that should be given the general circulation of the atmosphere that carried the storm with it. Similarly, Y 2 represents the 24- hour value that should be given the general circulation that carried the storm of May 28 with it. It will be observed that X 2 and Y 2 , representing the 24- hour values of the general circulation on the two dates, are of equal length and vary not more than 3 degrees 30 minutes in direction, from which it may be assumed that the general circulation of the atmosphere in May that carries the storms of the region of New Mexico and northwest Texas with it may be represented by a mean of a number of vectors determined as above. It is manifest, therefore, that should a storm in May in the region indicated be acted upon by a distribution of pressure whose resultant is zero its 24-hour direction and rate of movement will be that of the general circulation represented by a correctly determined mean of a number of vectors, such as X 2 and Y 2 . Hence such means determined for the various districts of the country have been designated 'normal storm tracks,' and are shown for May in chart XVII.