The warning signal, to attract attention, will be a long blast of from fifteen to twenty seconds' duration. After this warning signal has been sounded, long blasts (of from four to six seconds' duration) refer to weather, and short blasts (of from one to three seconds' duration) refer to temperature; those for weather to be sounded first.
One long......... Fair weather.
Two long......... Rain or snow.
Three long........ Local rains.
One short........ Lower temperature.
Two short........ Higher temperature.
Three short........ Cold wave.
Interpretation of Combination Blasts
One long, alone..... Fair weather, stationary temperature.
Two long, alone..... Rain or snow, stationary temperature.
One long and short .... Fair weather, lower temperature.
Two long and two short . . . Rain or snow, higher temperature.
One long and three short . . Fair weather, cold wave.
Three long and two short . . Local rains, higher temperature.
By repeating each combination a few times, with an interval of ten seconds between, possibilities of error in reading the forecasts will be avoided, such as may arise from variable winds, or failure to hear the warning signal.
Canadian signals (Fig. 3)
Fig. 3. — Canadian storm warnings.
No. 1, gale at first from an easterly direction.
No. 3, heavy gale at first from an easterly direction.
No. 2, gale at first from a westerly direction.
No. 4, heavy gale at first from a westerly direction.
The night signal corresponding to Nos. 1 and 3 is a red light. Night signal corresponding to Nos. 2 and 4 is a white light above a red light.
Barometer and Wind Indications (W. M. Wilson)
The mercurial barometer is the instrument used for all observations when great accuracy is required, but an aneroid barometer is more convenient, less liable to injury, and will answer all practical purposes.
Attention need not be given to the legends fair, changeable, stormy, etc., that usually appear on the face of the instrument, because changes in pressure are much more important indications of approaching weather than the actual pressure at a given time.
To forecast the weather accurately, the force and direction of the wind should always receive equal consideration with the changes of pressure as indicated by the barometer.
The following general statements may aid in showing the relation of wind, pressure, and weather: —
The atmosphere may be compared to an ocean of air that rests upon the earth just as the water rests upon the bed of the oceans. There are great currents of air in the atmosphere, just as there are great currents or rivers of water in the oceans.
Storms are eddies in the atmosphere, and float along in the currents or rivers of air very much like the eddies often seen floating on the surface of a river.
All of the United States and Canada, except the southern part of Florida, lies at the bottom of a great river of air that flows from west to east around the world with the north-pole at the center. It is called the circumpolar whirl. And as the storms in this latitude are eddies in the north-circumpolar whirl, they float along from west to east in the current of this river of air.
The air always whirls about the center of every storm-eddy in the same direction — counter-clock-wise in the northern hemisphere and clock-wise in the southern hemisphere. Therefore, if a storm-eddy in the latitude of the United States is approaching, the winds will first be from a southerly direction, and when the center of the storm has passed, the wind will come from a northerly direction.