Black lines connect places having equal barometric pressure; red lines connect places having equal temperature; arrows point in direction wind is blowing; figures at end of arrows show wind velocity, when it is more than light.
○ clear; ◓ partly cloudy; ● cloudy; R rain; S snow.
HIGH indicates center of anti-cyclone, or high-pressure area; LOW indicates center of cyclone, or low-pressure area.
Large figures show average temperature in each quadrant of cyclone.
Shading shows precipitation area of last 24 hours.
On [Chart 5] a line of arrows extends from the storm center westward to Wyoming, where the storm originated. A small cross inclosed by a circle marks its western extremity. Another cross located near Cheyenne shows where the storm center was located twelve hours after its origin. A third cross gives it location near Des Moines twenty-four hours after it started eastward. It was here that we began the study of this storm on [Chart 3]. A cross near Chicago indicates the distance traveled by the center during the third twelve hours, and [Chart 5] shows its progress during the fourth twelve-hour period. When the storm was central at Cheyenne the danger warnings for mariners were displayed at all ports of the Great Lakes, as the forecaster knew that in accordance with general laws the storm must move toward the east. When it was centered at Chicago, danger warnings were displayed on the Atlantic coast from North Carolina to Maine, as it was known that long before the storm reached the ocean the in-rush of wind toward the storm center would cause a dangerous on-shore gale and the breaking of heavy seas on the shore line. All craft that could be reached with the danger signals made safe in port, except the great ocean liners, which are of such strength as to safely withstand almost any storm. A special set of observations ordered by the Washington office of the Weather Bureau from its stations in the region of the storm, and well in advance of it, kept the chief forecaster informed as to the progress of the cyclone, and before the storm center reached the coast the danger signals communicated to mariners the fact that the winds would soon shift to northwest as the center of the disturbance passed out to sea.
The reader’s attention will now be directed to the red lines on [Chart 5]; they pass through places having the same temperature, but for simplicity the readings of temperature, whereby these lines were located, are omitted from the printed chart. Observe the line marked 40°; it passes across southern New England to western New York, but when it reaches the center of the storm it encounters the cold northwest winds blowing into the storm on its west side and is forced southward to Texas.
[Charts 3], [4], and [5] give a graphic history of one severe winter storm. In summer such general storms do not often occur. They are frequent in spring and fall, but of higher temperature and less severity than in winter. In summer Lows drift sluggishly across the continent; the barometer at the center of the cyclone is usually not more than two to four tenths of an inch below the pressure of the Highs, and the rain, instead of falling in a broad sheet, as shown by the shading of charts 4 and 5, falls in numerous sporadic outbursts, each of which is but a few square miles in area, their combined surfaces usually covering only a part of the region over which passes the Low.
Cold Waves and the Speed of Storm Movement. Highs and Lows drift across the continent from the west towards the east at the average rate of about six hundred miles per day, or about thirty-seven miles per hour in winter and twenty-two miles in summer, the first at about the rate of an express train, and the second approximating the speed of a freight. The Highs are attended by dry, cool, and settled weather. By a vortical action at their centers they draw down the cold air from great altitudes above the clouds. In winter, when vortical action is vigorous, they may reach upward to an altitude of seven miles. Air starting downward from this region has a temperature of some 70° below zero. We know this from the records secured by sending aloft free balloons carrying automatic thermometers. ([Chapters II] and [III].) This air heats by compression because in its downward movement it is continually leaving more and more air above it to exercise pressure upon it. It gains about twenty degrees with each mile of descent, and if there were no other factors to the problem it would be hot air when it reached the surface of the earth instead of cold air. But early in its descent it gains such heat as to melt and evaporate the ice spiculæ floating at the height of the fleecy cirrus clouds; then it evaporates and clears away the moist clouds lower down and finally creates such diathermancy (the capacity to transmit heat without absorption; see [Chapter V]) that the heat lost by radiation to a clear sky causes what we call a “cold wave”, and this notwithstanding the heat of compression.