Reading the Weather

by Thomas Morris Longstreth

But the weather is for everybody. To be sure the sunrise that talks so confidentially to the hunter of the coming day does not exist for the commuter. But the coming day does, even though the things it means are essentially different. To the hunter with his seasoned clothes and well-earned health a rain is only of concern in so much as it affects the business of the day; personally it is of small moment. But to the commuter what does the weather mean? Dollars and cents, of course. His business goes on, but to his person one unexpected shower == the cost of pressing a suit; one thorough soaking == one doctor’s bill. For you cannot expect the man to throw off a chill who can quiet his conscience on the matter of daily exercise by watering the geraniums and reading the newspaper.

Weather wisdom is necessary for the hunter; for the commuter it pays.

The hunter had to rely on local weather signs. The commuter can go him one better by investing $10 (how finance will creep in!) in a little aneroid barometer. The local weather signs were good for twelve hours at the longest. The barometer is a faithful instrument that adds another twelve hours to a man’s knowledge. Half a day, or even a day before any local sign of changing wind or growing cloud appears the barometer is on the job. It will register in Philadelphia the news of a disturbance approaching the Mississippi. So sensitive is it that it is the slave to every wave of the great air ocean.

The barometer gauges for the eye the amount of atmosphere that is piled above one. If the amount is normal and at sea-level the instrument will measure 30.00 inches. This air pressure is equivalent to a column of water 30 feet high. As this would make unwieldy prognosticators the scientists use mercury instead, which requires a column less than three feet long. And for general purposes this is supplanted by the handy little aneroid (which means “without fluid”). This is so fixed that the pressure of the air influences the upper surface of a vacuum chamber, balanced perfectly between this pressure and a main spring. This action is transmitted to an index hand moving across the dial marked into fractions of inches after the manner of the recognized standard, the mercurial barometer.

When the warm moist light air of a cyclone invades a locality the pressure is partially removed, the vacuum chamber is not pressed so hard and the dial hand or the mercury subsides. When the cold, dry, heavy air of the anticyclone lumbers in more pressure is applied and the mercury, or the dial hand, climbs. So a falling barometer means a storm, a rising one fair weather.

That is a generality that glitters. If that were all there was to it weather officials would have a sinecure. But each cyclone varies in size, intensity, and rate of progress. Some do not advance for days. Therefore there has grown up a pretty large body of information as each storm has had to be watched and the barometric movements recorded. The most important variations follow:

Remembering that 30.00 inches is sea-level normal, if the barometer is steady at 30.10 or 30.20 the weather will remain fair as long as the steadiness continues, and on the turn, if the fall proceeds slowly with the wind from a westerly direction fair to partly cloudy weather with slowly rising temperature will follow for two days.

If the barometer rises rapidly from 30.10 the fall will be equally rapid and rain or snow may be expected within a couple of days. Since the depressions of the atmosphere tend to a certain regularity about the center of the storm it follows that the reactions will follow the actions in similar manner,—a long rise portending a long fall and a variable glass meaning unsettled conditions.

The barometer does not rise with wind from an easterly direction unless a shift is imminent. In winter the air is so much colder over the land than over the sea that the air brought in by an easterly wind is soon condensed. Consequently with winds from the south or southeast, even if the barometer is 30.20 or 30.10 and falling slowly rain usually arrives (and rain of course is meant to include snow whenever the mercury is below the freezing point) within 24 hours. If the fall is rapid there may be precipitation within 12 hours, and the wind will rapidly increase and the temperature rise.

If the wind is from the east or northeast and the barometer 30.10 or above and falling slowly it means rain within 24 hours in winter. In summer if the wind is light rain may not fall for a day or so. If the fall is rapid in winter rain with increasing winds will often set in when the barometer begins its fall and the wind gets to a point a little east of north.