National Geographic : 1890 Apr
National GeographicMagazine. frequently commented on by the forecast officials of the Signal Service, that storms passing over the United States were in gene ral preceded by an increase in moisture, but unfortunately little effort had been made on the part of previous investigators to de termine any quantitative relation between the actual humidity and the amount of precipitation or its relation to the storm move ment. It has long been regretted that the direct relations of this to other meteorological phenomena were not more fully defined. During the past year Captain James Allen, of the Signal Office, has endeavored to apply the results of his investigations and theories to the practical forecasts of storm conditions. Captain Allen has carefully studied the relations of the potential energy of the surface air, as represented by the total quantity of heat it contained, to the movement of storm centres and the extent of accompanying rain areas. In his first investigations the potential energy per cubic foot was estimated as follows : Supposing the air to have been originally 32° and the moisture in it as water at 32°, the total quantity of heat applied to reduce to the state of vw 32 ) observation will be A=(t-32) + Q in which A is total heat per unit volume ; t is the temperature of the air, Q the total heat of vapor, and the specific heat of air at constant volume being taken as one-sixth (.168). From Regnault's formula we have Q = 1091.7 + .305(t-32). For the mechanical equivalent we have J= '772A. If we divide J by the pressure estimated in pounds per square foot, it will give the height through which the pressure can be lifted if all the heat is spent in work by expanding the air. An approximate expression for the upward velocity V may be obtained from Torrecelli's theorem from which we have V'=2gh, h in this case being being the height through which the pressure would be lifted if all the heat is spent in work. The theory has been that the storm centre will move over that section of the country where V is the greatest, and that the time of occurrence and amount of rain have a relation of conformity to the changes in Q and its actual amount. Auxiliary charts were also made showing for each station the following following values of Q: 1st. Highest Q not followed by rain in 24 hours. 2d. Greatest plus change in Q not followed by rain in 24 hours.. 3d. Lowest value for Q followed by rain in 12 hours.