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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    Service Bulletins, not iu the absolute quantity in wliicli it is found at any given place, but as a percentage of full saturation, or Tvliat, in the language of meteorologists, is expressed by the term Relative Humidity. This must not be confounded with absolute humidity, which is a very different thing. For, sup- posing the temperature of the air at a given place to be iOo and fully saturated with aqueous vapor, and then to be sud- denly raised to 50° without any addition being made to its store of vapor, its absolute humidity would in each case be exactly the same, but in the former case the weather would, in popular language, be very damp, and in the latter case very dry. In the former case the relative humidity (or humidity, as it is often simply called) would be very high, i. e., 100 per cent.; iu the latter very low, i. e., 50 per cent. Watery vapor dissolves in air very much as salt dissolves in water, and as the salt is deposited iu crystals whenever the water becomes fully saturated, so, whenever the air becomes fully saturated with vapor, the latter is deposited on the earth in the form of mist, dew, and rain, if the temperature be high, or as frost, hail, or snow crystals if the temperature be low. One cubic foot of air, having a temperature of 50°, and under a uniform barometric pressure of 30.00 inches, and fully satu- rated, will hold 4.28 grains of water according to Glashier's tables. If, under these conditions, the temperature or the pressure of the air is lowered, there will result a deposition of a portion of the water, and that either in the form of fog, dew, rain, frost, or snow and hail. On the other hand, if there is an increase in the temperature or the pressure, the air becomes capable of holding a larger quantity of vapor, and ceases to be fully saturated. Eelative humidity expresses the proportion of vapor actually contained in the air compared with what the air could contain. By denoting full saturation by 100 per cent,, and absolute dryness by zero, the relative moisture of the air at the different stations can be indicated on the map by the proper percentage. [This relative humidity is obtained from the Tables of Eelative Humidity, (pp. 59, 60,) where the practical process is fully ex- plained.] This table is directly applicable to such stations as are less than 1,000 feet above the sea. A correction of consid- erable amount is needed for mountain stations. The absolute quantity of moisture in the normal condition of the atmosphere decreases with ascent above the earth's surface, but the law of decrease in cloudy and falling weather is, of