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.

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    have to cousicler the contrary influences exerted by areas of dry air, such as are found on the eastern slope of the Eocky Mountains. When a storm-center is driven into such a region, clouds and weaker winds may be expected, and the storm will gradually die out, unless moist air lies beyond. 11. Besides the forces attracting the areas of low pressure, we have also to consider the presence of any area of high barometer; this, by causing a system of decidedly strong- winds to circulate around it in the anti-cyclonic direction, may drive the storm-center (whether it be a small tornado, or an extensive cyclone) before it, so as to undergo quite a change in its jDath. 12. The general path pursued, day after day, by a storm- center, is, indeed, the resultant of a system of pressures, some of which arise from the pressures of areas of high and low barometer, some from areas of moist or heated air, some from winds, and others from the rotation of the earth on its axis. These are the important controlling influences; such others as may arise from lunar tides in the air, &c., are considered at present as of inferior prominence. 13. V/hether the observer determines the probable movement of a given storm-center by means of these general considera- tions or not, he cannot safely neglect the indications of his own barometer. The experience of the past fifty years has borne uniform, testimony to the pre-eminent value of the indications of this instrument, taken in conjunction, of course, with the wind and weather. 14. If it be known that a center of low pressure is in the neighborhood of the observer, and he stand facing it, he will find the wind blowing from some point on his left toward some point on his right: and conversely if he stand with his left hand toward the direction from which the wind comes, he will face the region of lowest pressure. It is better to use, not the direction of the local wind, but that from which the lotc clouds are moving; the ve7y Mgli clouds should not be used. 15. If, while facing the low pressure, he finds the wind steady, and his barometer falling, then the central area is advancing directly toward him, and, so long as this continues, he may expect the wind to increase until the barometer reaches its lowest; then a lull will take iflace, followed by strong- winds from the opposite quarter, which will continue while the ])arometer is rapidly rising, but subside as it rises more slowly.