That it is at a temperature of 40° Fahr., and this ratio continues throughout the thermometric scale. Whether air at an average high temperature, like that in Florida, will contain this excess of moisture, depends only on the proximity of water whence the vapor may he obtained.
The figures of relative humidity recorded by the Signal Service at Jacksonville only indicate the ratio in per cent which the humidity of the air at the time bears to saturated air at the same temperature. But the weight of vapor in saturated air at that temperature is not given, therefore the absolute value of this percentage, so far as the records are concerned, is unknown. The basis for the calculation is, however, easily obtainable—the weight of vapor in troy grains, contained in saturated air at different temperatures (Glaisher's Table)—and will be found to increase in an ascending scale from zero. The following record of the Signal Service in Cincinnati, Ohio (Sergeant R. B. Watkins, U. S. A., observer), well illustrates the subject:
From the above table we see that on a day of high temperature and low relative humidity there was nearly six times more vapor in the air than on a day of low temperature and high relative humidity.
By way of comparison we here insert a table showing the mean annual and winter temperature, also the relative humidity and absolute humidity, of three winter stations. It shows the average quantity of vapor in the air at Jacksonville, throughout the year, to be twice as great as at St. Paul, Minnesota, and four times more during the winter season:
- The temperature of these places is from the "Smithsonian Temperature Tables." The relative humidity is calculated from data of five years 1877-1881 inclusive supplied the writer by the Chief Signal-Officer, General W. B. Hazen, Washington, D. C. The absolute humidity is calculated from "Table X" of "Smithsonian Meteorological Tables," by Guyot, giving weight of vapor in grains troy, contained in a cubic foot of saturated air between 0° and 105° Fahr.