The tropical hurricanes are the most violent and most dreaded of all the storms that sweep over any portion of our continent, but fortunately for dwellers on the land, are confined for the most part to the sea, since the atmospheric conditions over the land lead to their loss of energy, although in rare instances they may be re-enforced by uniting with a cyclonic storm, as happened in the case of the Galveston hurricane, and thus continued after reaching the land. The destructiveness of the hurricanes at sea has been greatly lessened in recent years, not only on account of the general use of steam as a motive power for vessels instead of the wind, but because meteorologists can designate the time when they are likely to occur and the best method of sailing
away from them if encountered. Since the establishment of the United States stations for observing and reporting the atmospheric conditions on the West India islands, the approach of a hurricane can be foretold and warning given to navigators and others of the coming danger.
EVAPORATION
An important element in climate is the amount of moisture the air contains. The absolute amount of water vapour in a given volume of air is of interest in this connection, but what is of still greater importance is the ratio of the amount of water vapour present to that which the air might contain, or what is termed the relative humidity. The relative humidity, providing the actual amount of vapour present remains unchanged, depends upon the temperature of the atmosphere. For this reason, the warming of an air-current, as the trade-winds, for example, in which the water vapour present may, previous to the warming, have approached saturation, causes it to have a still greater capacity, and hence decreases the relative humidity.
The winds in passing over the land may be either cooled or warmed, and hence their influence on evaporation is continually changing; but the mean rate of evaporation from an open water body can be determined for a definite time, say a year, for various localities, and thus afford a means of comparison between one region and another. Observation of the mean annual evaporation for various stations, mostly within the United States, have been made, and the result shown by lines drawn through places where the rate is the same. A map showing this data, on which the figures indicate the depth of evaporation in inches, is here presented. The systematic study of evaporation, and especially the part played in it by plants, has scarcely more than been begun on this continent, and important results concerning its influence on atmospheric conditions are to be expected. The subject is also of great importance in reference to agriculture, the prevention of frost, etc.
Fig. 27.—Depth of evaporation. After T. Russell.
LITERATURE