SEATTLE - A barometer measures atmospheric pressure, which can help give you a rough idea of how the weather will be changing in the next 24 hours.
Sea-level barometer readings usually range from about 29.70 inches of mercury to 30.20 or so, with 29.92 being the average reading.
Pressure also drops with altitude too, so reported pressures are converted to sea-level to factor out the difference in altitude from individual reports. You can convert your observed pressure to sea-level by subtracting .06 inches for every 50 feet in altitude.
But it's not so much the actual number reading as the trend the readings are taking. A steadily falling barometer usually signals a storm is approaching, while a rising barometer normally means sunnier skies ahead. Although very low readings (under 29.50 inches) can suggest a strong storm approaching.