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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![subside and dissolve when tliey cease to be fed by rising currents of moist air: the thickness of the cumuli from base to peak is less in cold dry weather than on warm moist days. The cirrus clouds are probably formed independently by the radiation of heat outward into the highest regions of the atmosphere, in which case they are comi^osed of snow-flakes, or of spiculte of ice; and they are also formed of the remnants of the storm- clouds, in which case they are generally composed of warmer vapor. The strong winds that attend areas of low barometer give rise, through the influence of friction, &c., as before stated, to ascending strata of moist air, in which, by expansion or cool- ing, as the case may be, are produced the scud and rain cloud of which there is a fine example in the easterly rains of the Atlantic coast. This scud-cloud, which is at first like a cumulus of irregular shape, subsequently spreads into broad sheets of stratus and nimbus. Two or more layers of clouds almost invariably coexist wher- ever extended rain-storms prevail, the upper layer stretching far in advance of the lower, but descending and merging into the lower over the area on which rain is falling most abundantly. In the rear of this area cumulus clouds are abundant. A gen- eral survey of the map will show that cumuli or the cirri first mentioned in the preceding sentence are not inconsistent with fair and clear weather, as these terms are popularly used. An increased accumulation of large cumulus clouds may become cloudy weather, but does not generally presage the extended storms of winter. The cirrus of the second class, sometimes called cirro-stratus, almost always precedes at some distance any extensive rain-storm, whether of winter or summer. The stratus will generally be found to be reported in connection with threatening weather at the different stations. The classification of clouds into cumulus, cirrus, &c., as ori- ginally given by Howard, is indicated on the accompanying plate. GENERAL ATMOSPHERIC CURRENTS. Without undertaking in these suggestions to discuss the general system of circulation of the atmosphere, it is necessary to call to mind a few general facts. The trade-winds on the surface of the earth, as is well known, steadily blow from the northeast and southeast in the res])ective hemispheres toward the meteorological torrid zone, which is a narrow belt where calms and rains prevail at all seasons, and the uniformity of these winds is only disturbed, as in the Indian Ocean, by the](https://iiif.wellcomecollection.org/image/b21070374_0034.jp2/full/800%2C/0/default.jpg)
