If the barometer is 30.00 or below and falling slowly with northeast to southeast winds the storm will continue 24 to 48 hours. If the barometer falls rapidly the wind will be high with rain and the change to rising barometer with clearing and colder will probably come within 20 to 30 hours.
If the barometer is below 30.00 but rising slowly the clear weather will last several days.
If the barometer is 29.80 or below and falling rapidly with winds south of east a severe storm is at hand to be followed within 24 hours by clearing and colder. Under the same conditions but with northeast winds there will occur heavy snow followed by a cold wave.
If these promises do not always bear fruit it is because they will have been interrupted by an unseen shifting of the atmospheric weights. But the barometer will record them. A rapid rise may be checked in ascent and the instrument may fluctuate like a stock-ticker. Its tale is of very unsettled weather conditions and consequently no particular brand of weather will last for very long at a time.
A sudden rise of the barometer may bring its gale of wind as well as a sudden fall. But the tendency will be toward clearing and much colder.
A fall of the barometer on a west wind is not common. It means rain. A rise on a south wind means fair. A low barometer and a cold south wind mean a change to west with squalls for a while. On the other hand, a high barometer with warmer weather means a shift of the wind to southerly quarters and an imminent fall.
If the barometer rises fast and the temperature does, too, look for another storm. This is often noticed in summer.
There is a slight daily oscillation of the mercury, which, if other things are steady, registers highest at 10 A. M. and 10 P. M. and lowest at 4 A. M. and 4 P. M.
If this data confuses bear in mind the simple ordinary progress of the barometer in the usual storm: First, it will stand steady for a day or so at any point between 30.10 and 30.50. Then the glass will begin (for most storms) to fall gradually. As the center nears the fall hastens. After the lowest point has been reached a slight rise will be followed by another slight fall and then the final long rise will commence. The rain begins and ceases at different stages for different storms, depending upon the wind’s velocity and direction.
For every 900 feet of altitude the height of the mercury is about one inch less. Do not complain that your barometer is inaccurate if you are living up in the mountains and your readings are not the same as the weather reports which are reduced to sea level. All the figures given in this chapter are for sea level and if your house is 1900 feet above you must move the copper hand of your aneroid 1.95 inches from the pressure hand. If the pressure hand would read 28.05 the adjustable copper hand would read 30.00 which is the sea level reading.