A barometer is a mechanical instrument that measures barometric pressure. Barometric pressure is the amount of pressure that exists in the atmosphere surrounding a specific location. The amount of barometric pressure that exists in the surrounding atmosphere can be an indicator of approaching weather patterns. You can read a barometer by monitoring the barometric pressure readings that are produced by the barometer and comparing them to the average barometric pressure for your area.
Determine the average barometric pressure for your area before you attempt to read a barometer. The average atmospheric pressure at sea level is approximately 29.9 inches of mercury, or 1,013 millibars. According to the U.S. National Weather Service Forecast Office (NOAA), pressure drops approximately one inch of mercury for every 1,000 feet of altitude that is gained, or one millibar for every 8 meters that are gained. After the average barometric pressure is determined, set the needle on your barometer to the average pressure reading by manipulating the adjustment mechanism located on the device. This should be done on a clear day when there are no approaching weather fronts. Compare the readings from your barometer with a reliable barometric reading from a dependable source, such as the weather service in the country where you live.
Before you read a barometer, you should tap it gently. This will provide for a more accurate reading, because it forces existing or residual pressure out of the barometer. The needle on the barometer might change after you tap it.
Take note of the direction that the needle moves. If the needle returns to the same millibar on the barometer, it is likely that the weather will remain steady until there is a subsequent change in the barometric pressure. Additionally, you should not read a barometer that has been exposed to direct sunlight or air conditioning for an extended period of time. Abnormally warm, cold or damp conditions might distort the reading.
You can read a barometer by monitoring it throughout the day and watching for changes in the millibar readings. An increase in the pressure reading above the average barometric pressure for your area is an indicator of high pressure or mild weather. A decrease in the millibar reading indicates that rainy or stormy weather is approaching. Swift decreases in the pressure are a sign of a fast-moving storm front. The most important thing to remember when you read a barometer is that the reading does not necessarily apply to the current weather conditions; rather, it is a predictive device that indicates approaching weather patterns.