WEATHER FORECASTING WITH A BAROMETER
The barometer will give a much better guide to weather forecasting if account is taken of the rate at which the pressure rises and falls as well as the information on the dial.
Most barometers have two pointers for this purpose: one is set by hand to align with the other, the moving pointer, each night (or whenever is convenient). In the morning, the change in pressure can be observed by the difference between the two pointers and, by using the information given below, you can make your own local weather forecast. This is for the northern hemisphere, for the southern hemisphere, reverse north and south.
A Steady Barometric Pressure
With dry air and seasonable temperatures, indicates a continuance of existing weather.
A Rising Barometric Pressure
A rapid rise indicates unsettled weather.
A gradual rise indicates settled weather.
A rise, with dry air, and cold increasing in summer, indicates wind from the north;
and, if rain has fallen, better weather is coming.
A rise, with moist air and a low temperature, indicates wind and rain from the north.
A rise with a southerly wind indicates fine weather.
A Falling Barometric Pressure
A rapid fall indicates stormy weather.
A rapid fall with westerly wind indicates stormy weather from the north.
A fall with a northerly wind indicates storm, with rain and hail in summer, and snow in the winter.
A fall with increased moisture in the air and the heat increasing, indicates rain and wind from the south with the possibility of thunder.
A fall with dry air and increasing cold in winter indicates snow.
A fall after very calm and warm weather indicates rain and squally weather with the possibility of thunder.
Using winds and a barometer to make forecasts
Here are some general statements of wind-barometer indications that are generally applicable:
"When the wind sets in from points between south and southeast and the barometer falls steadily, a storm is approaching from the west or northwest, and its center will pass near or north of the observer within 12 to 24 hours, with wind shifting to the northwest by way of south and southwest.
"When the wind sets in from points between east and northeast and the barometer falls steadily, a storm is approaching from the south or southwest, and its center will pass near or to the south of the observer within 12 to 24 hours, with winds shifting to northwest by way of north. The rapidity of the storm's approach and its intensity will be indicated by the rate and amount of the fall in the barometer.
"As a rule, winds from the east quadrants and falling barometric pressure indicate foul weather, and winds shifting to the west quadrants indicate clearing and fair weather, but again there are exceptions and in some parts of the country these rules do not apply."
The following table generally summarizes wind and barometer indications. The amateur forecaster should modify the table in accordance with his or her own observations. The following show the wind direction, the barometer reduced to sea level and the character of the weather indicated:
SW to NW, 30.10 to 30.20 and steady - Fair with slight temperature change for 1 to 2 days.
SW to NW, 30.10 to 30.20 and rising rapidly - Fair, followed within 2 days by rain.
SW to NW, 30.20 and above and stationary - Continued fair, with no decided temperature change.
SW to NW, 30.20 and above and falling slowly - Slowly rising temperature and fair for 2 days.
S to SE, 30.10 to 30.20 and falling slowly - Rain within 24 hours.
S to SE, 30.10 to 30.20 and falling rapidly - Wind increasing in force, with rain within 12 to 24 hours.
SE to NE, 30.10 to 30.20 and falling slowly - Rain in 12 to 18 hours.
SE to NE, 30.10 to 30.20 and falling rapidly - Increasing wind, and rain within 12 hours.
E to NE, 30.10 and above and falling slowly - In summer, with light winds, rain may not fall for several days. In winter, rain within 24 hours.
E to NE, 30.10 and above and falling rapidly - In summer, rain probably within 12 to 24 hours. In winter, rain or snow, with increasing winds, will often set in when the barometer begins to fall and the wind sets in from the NE.
SE to NE, 30.00 or below and falling slowly - Rain will continue 1 to 2 days. SE to NE, 30.00 or below and falling rapidly - Rain, with high wind, followed, within 36 hours by clearing, and in winter by colder.
S to SW, 30.00 or below and rising slowly - Clearing within a few hours, and fair for several days.
S to E, 29.80 or below and falling rapidly - Severe storm imminent, followed within 24 hours, by clearing, and in winter by colder.
E to N, 29.80 or below and falling rapidly - Severe northeast gale and heavy precipitation; in winter, heavy snow, followed by a cold wave.
Going to W, 29.80 or below and rising rapidly - Clearing and colder.
