Page:The National Geographic Magazine Vol 16 1905.djvu/336

"The value that should be given each tenth of an inch increase in pressure measured outwardly from the storm center is of vital importance to the correct working of the system, and is arrived at by constructing a number of charts similar to that presented in chart XVI. From an inspection of this figure it is apparent that if the value given each tenth of an inch increase in pressure be decreased the resultants X₁ and Y₂ will decrease in length, and the angle between the vectors X₁ and Y₂ will decrease, but the vectors themselves will become unequal—that is, X₂ will increase and Y₂ decrease in length. (The working of the system is dependent on the assumption that the general circulation, as represented by the vectors X₂ and Y₂, is fairly constant.) If the value given the pressure effect be increased, the resultants X₁ and Y₁ will likewise increase, and the vector X₂ will become shorter and Y₂ very much longer, and, at the same time, the angle made by these lines will increase, from which it follows that a value of one centimeter for each tenth of an inch increase in pressure best meets the requirements in this case."

To determine the future course of a storm in the month of May, for instance, resolve the pressure forces about the storm center into their resultant, then take for the future direction and distance of translation of the storm the resultant between this force and the force illustrated on chart XVII as the upper-air drift or normal storm track as follows:

In which the low is central near New Orleans, a b representing the pressure resultant, or line along which the low will be forced if acted upon by pressure gradient only, and b c the normal storm track, or the distance and direction of movement of the low as the result of upper-air drift alone, and a c the track that the storm center will follow. Hence, instead of the storm moving due south into the Gulf as the result of pressure, or northeast to southwestern Virginia, as chart XVII shows is the place to which upper-air drift will carry it, it moves due east to Jacksonville, Florida.

The four conditions essential to the formation of tornadoes are usually as follows: (1) A cyclone or area of low pressure, the center of which is to the north or northwest, with a barometric pressure not necessarily much below the normal; (2) a temperature of about 70 degrees on the morning map; (3) a great humidity, and (4) that the time of year be March 15 to June 15. These conditions may and often do exist separately; one or two of them may be found coexisting; but so long as the third be absent, tornadic formation is not likely to occur.

The number of these storms is not increasing; the breaking of the virgin soil, the planting or the cutting away of forests, the drainage of land surfaces by tiles, the stringing of thousands of miles of wire, or the laying of iron or steel rails have not materially altered the climatic conditions or contributed to the frequency or intensity of tornadoes. As well might one by the casting of a pebble expect to dam the waters of the Mississippi as attempt the modification or restriction by the feeble efforts of man of those tremendous forces of nature that surround our earth and control our storms and climate. To be sure, as towns become more numerous and population becomes more dense, greater destruction will ensue from the same number of storms.