The practical use of meteorological reports and weather-maps / Office of the chief signal officer, Division of telegrams and reports for the benefit of commerce.

  • United States. Army. Signal Corps.
Date:
1871
Catalogue details

Licence: Public Domain Mark

Credit: The practical use of meteorological reports and weather-maps / Office of the chief signal officer, Division of telegrams and reports for the benefit of commerce. Source: Wellcome Collection.

Provider: This material has been provided by the Francis A. Countway Library of Medicine, through the Medical Heritage Library. The original may be consulted at the Francis A. Countway Library of Medicine, Harvard Medical School.

    23/96 (page 15)
    THE WINDS AND THEIR LAW. Whether considered as the indices, or as the causes of com- ing changes of weather, no phenomenon is more important than that of the winds. Upon the direction and force of the winds, some meteorologists lay very great stress in every attempt at storm forecasting; and, in order to determine these, it is necessary to draw the Isobars. Assuming the lines of equal barometric pressure to be drawn on the Weather-Chart, it is at once perceived that, in well- understood accordance with the laws of mechanics, the atmos- phere must be x)ushing from the regions of higher to those of lower xjressure. The resulting movement of the air, modified by the forces of inertia and friction, and by the rotation of the earth and local obstructions, is converted into the local winds whose directions are indicated by the arrows upon the maps, and whose velocities are given in miles per hour. These winds may be called local winds, as distinguished from the general winds in any section, and from the great currents of air to be hereafter spoken of; the general winds appear to be primarily dependent upon the existence and position of the areas of low and high xjressure; the great currents, spreading, as they do, over whole continents and encircling the earth, are largely influenced by, if not entirely dependent upon, the earth's axial rotation. If the earth were not in rotation on its axis, the winds would uniformly blow in straight lines outward from the center of every area of high barometer toward the surrounding localities of lower barometer. Observation, however, has long since clearly shown, that in this hemisphere, within any area of high pressure, the winds will be found to be not only blowing away from the cen- ter, (outwards,) but also to be deflected toward the right hand as they move forward. Observation has also shown, with equal clearness, that in this hemisphere, within any area of low pres- sure, the winds will blow toward the center, (inwards,) and will also be deflected toward the right hand as they move forward. This deflection to the right has been demonstrated by Mr. Wm. Ferrel, of Cambridge, Mass., to be a mathematical neces- sity arising from the influence of the earth's diurnal rotation which causes everything moving on its surface to deflect slightly to the right in the northern hemisphere, and to the left in the