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 Observations covering more than twenty years show that winter storms of the United States advance at the rate of a little more than 700 miles per twenty-four hours; summer storms cover about 500 miles. These figures differ from the values obtained by the British Meteorological Office, 576 miles and 474 miles per day respectively. The progress of the cyclone is merely the velocity of the general drift of air, and this varies in different latitudes, and at different times.

Inasmuch as the storm tracks of the different types are fairly regular in position, and the velocity of progress is known, it is not difficult to forecast the position of a storm from day to day; that is, a storm center which is over Cincinnati may be expected to reach Philadelphia or New York at about the same hour on the following day. Fast express trains run at a rate of speed that rarely varies; the cyclonic storm moves also at a fairly uniform speed. The express train does not ordinarily leave its steel-bound track; in this respect it differs from the cyclonic storm which occasionally does swerve from its expected track to the confounding of the forecaster. This is likely to happen about once in five times.

Let us suppose that a storm of the Alberta type, after reaching the Great Lakes, takes a dip southward and passes off the coast somewhere near Cape May, instead of following a predicted course across New York. In the eastern part of the United States practically all forecasts north of Cape Hatteras will be upset. Instead of rain, central New York and Massachusetts will have clear or partly cloudy weather. Baltimore and Washington will have cloudiness, easterly winds and rain, instead of clear or partly cloudy skies.

Not only may a cyclonic storm swerve from its predicted track; it also may fail to produce the rain or the snow which, according to popular tradition, constitutes the storm. As a matter of fact, the rain and the snow are merely an incident in a cyclonic movement. The essential feature of cyclone mechanics is the updraught. Now, in its progress if the cyclone invades an area of very dry air, the updraught may not be cooled to the temperature of condensation; in such a case there will be no precipitation. All lows are not rain storms or snow storms in the ordinary meaning; but practically all the rain and snow that fall on large areas accompany winter lows.