The seaman distinguishes between waves of two kinds, but there is also a scale of nine degrees, ranging from "calm" to "precipitous sea" (in which the waves are forty feet or more in height) for estimating sea disturbance, and another one, recently adopted by international agreement, for estimating swell. Whenever tropical hurricanes are likely to be about the navigator keeps a lookout for an increase in the swell, which often gives the first warning of one of these storms, as the storm waves travel much faster than the storm itself. Of all atmospheric phenomena observed at sea, the waterspout is probably the most interesting.
A waterspout is a tornado occurring over a body of water; that is, a local whirl in the atmosphere, revealing its presence by the formation of a more or less funnel-shaped cloud, due to the condensation of moisture where the air expands, and therefore cools, in the core of the whirl. As this moisture comes from the air and not from the sea, it consists of droplets of fresh water, such as we observe in a fog; but at the base of the column a good deal of salt-water spray is dashed upward. Contrary to popular belief, the spout is merely a cloud, and never a solid column of water.
Waterspouts occur over lakes and rivers, as well as over the ocean, but they are commonest over the seas of low latitudes, where a dozen or more are sometimes seen at one time. A spout rarely lasts more than a few minutes. Waterspouts are much less violent than tornadoes, but are quite capable of swamping small vessels. Sailors formerly fired cannon at waterspouts in order to dissipate them, and perhaps sometimes still do so, but this procedure is perfectly useless.
Nobody has profited more than the sailor by the discoveries of modern science concerning the winds and the weather. Formerly all storm winds were supposed to blow in a straight line. The fact that storms are really great traveling whirlwinds, though previously recognized by a few meteorologists, did not become common knowledge until toward the middle of the nineteenth century. The rules of "storm strategy," which enable the marine to avoid the more dangerous regions in these gyratiing wind systems, have gradually developed since that period.
When the sailor had only his personal observation to guide him in locating a storm center and determining its movements the process was fraught with much uncertainty, but nowadays he generally has radio reports from other vessels and from points on land to aid him so that he is in comparatively little danger of running into a gale without ample warning.
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