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
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.
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![nortliwest and northeast; if he is within an area of high pres- sure, and on the northwest side of its center, he should lind the wind blowing from some point in the qnadrant between southwest and southeast. When no well-marked central areas are actually within the limits of the map, the winds still unite with the lines of equal pressure in indicating such areas as existing near by. Thus, during the summer, for instance, the southwest and southeast winds of the western plains indicate the low ]3ressure that exists at that season in the Missouri Yalley and northward. When the pressure over a considerable area is very uniform, then minor local influences (such as cannot generally be exactly located by the limited number of reports that are at present received) aliect the gentle winds that then exist. On such occasions local differences of temperature and moisture, alfect- ing as they do the local pressures, give rise to light winds, extending over only a few square miles, but which still tend to obey the general laws given in the above table. Vertical as well as horizontal systems of winds, depending upon the disturbances of equilibrium continually taking place in the region of the clouds, always exist in connection with the ordinary horizontal gales; these are, in fact, a most prominent feature of tornadoes and water-spouts. The system of winds above given is not only that which all observations show to exist, but also that which follows from the mathematical theory of the motions of fluids on the earth's surface, as developed by Ferrel and others, which, as has been already stated, proves that all bodies, whatever be their direc- tion of motion on the earth's surface, in the northern hemi- sphere are deflected, or trend toward the right hand in their onward jjrogress. This deflection toward the right, small as it is originally, increases with the diameter of the storm, and seems, by its cumulative force, to determine the general direc- tion of the rotation of the West India cyclones, as well as that of the much weaker storms that pass over our continent; in all which it is evident that the air actually has a sinuous spiral motion inward toward the central area of lowest pressure, and at the center upward from the earth's surface; on the outer edge of the tornado and cyclones, a downward current is evi- dent. This same principle (Ferrel's Law) also decides the deflection northward of the afternoon sea-breezes, and south- ward of the evening land-breezes, as experienced along our 2](https://iiif.wellcomecollection.org/image/b21070374_0025.jp2/full/800%2C/0/default.jpg)
