The forecaster first observes a cold wave in the northern Rocky Mountain region, in the form of an intense High. It will travel southeastward to the center of the continent, and often to the Gulf if it is preceded by an active Low that is located on a low latitude, as the latter will draw southward the frosty air of the High; after that the course of the storm will be more nearly eastward. Now it is of rare occurrence that a cold wave gains entrance to any considerable area of our territory without warning, but in the early days of the Weather Bureau they too often reach Iowa, or States farther east, without any notice whatever. It was then discovered that a certain type of weather map preceded such failures of the forecaster. One who is interested in gaining early knowledge of the approach of a cold wave to the United States should watch not only for the appearance of abnormally high barometer readings, from the stations of the Canadian Northwest, or from Montana and North Dakota, but especially for a crescent-shaped Low, with one horn of the crescent touching Lake Superior and the other extending into the middle Rocky Mountain region, at about Colorado. This Low will appear to be an innocent affair; there may be a small secondary Low in each end of the crescent, and no High of any importance in the northwest, for which one ordinarily would look in anticipating a cold wave. But when this crescent-shaped Low appears on the morning weather map, a High of marked intensity invariably will develop with great suddenness over Montana and North Dakota and bring a cold wave to the Middle Mississippi Valley before the next morning, if the time of year be winter.
Do not forget that the Low is as important as the High in causing a cold wave, for the High that brings the cold air must follow in the track of the Low and will be attracted by the latter in proportion to its lowness, as indicated by the isobar inclosing the center of the Low. A cold wave will reach the Gulf only if the preceding Low originate in Texas; it will be confined to the Ohio Valley as the limit of its southern influence if the preceding Low originate in Colorado; and it will only skirt the northern border of the United States and the Lake region if the Low begin in Montana.
More and more is man applying science to commerce and industry. When the weather map, which was unknown but little more than half a century ago, indicates the formation of a heavy body of cold air in the extreme northwest, the chief official forecaster at Washington is on the alert; he orders special observations every few hours from the Weather Bureau stations directly within and well in advance of the cold area, and as soon as he becomes satisfied that a cold wave is on its way, the previously arranged system of disseminating warnings is brought into action, and by telegraph, telephone, flags, whistles, bulletins, and other agencies, the people in every city, town and hamlet, and many in the stock and farming regions, are notified of the advancing cold twelve to twenty-four hours before it reaches them.
Chart 6.—Cold Wave Zones, March to November. Amount of fall and verifying limit.
[Charts 6] and [7] show how the Weather Bureau defines a cold wave. There must be a fall of sixteen degrees, eighteen degrees, or twenty degrees within thirty-six hours and a certain degree of coldness must be reached. The charts show that what is a cold wave in the Gulf region is far from one in the northwest.
Chart 7.—Cold Wave Zones, December, January, and February. Amount of fall and verifying limit.
Chart 8.—Lowest Temperatures in the United States, 1871-1913.