How to Measure With a Barometer

Use a barometer to measure air pressure.

A barometer is used to measure air pressure. Air pressure occurs because the molecules in the air push down on surfaces, objects and other molecules. There is a lot of space between air molecules, and this means that air can be compressed and create high pressure. Meteorologists use a barometer to determine the changes in air pressure to forecast the weather. Low air pressure is a sign that storms and bad weather are coming. High air pressure is a sign of nice, sunny weather.

Things You'll Need

• Aneroid barometer

Instructions

Barometer Measurements

1. Place the barometer in a shaded location on a level surface. You can also hang the barometer from the wall. Direct sunlight will affect the barometer reading and should be avoided.

2. Record the current barometer reading by recording the number that the needle is pointing at. Move the barometer pointer to cover the needle. This makes it possible to determine in what direction the air pressure is changing. Gently tap the needle as sometimes the needle can get a little stuck. Tapping the barometer will unstick the needle.

3. Read the barometer once or twice a day at the same time each day. Record the number that the needle is pointing to. Compare this number with the previous reading or look where the pointer is located. If the barometer reading drops then bad weather is coming. If the barometer reading increases, then the weather will improve. You can also see what word the needle is closest as some barometers will have stormy, sun, cloudy and other weather terms on it. This is the weather that will occur over the next 24 hours, not necessarily right when you take the barometer reading.

Aneroid Barometer

1. An aneroid barometer utilizes the aneroid cell. This cell is made of a copper or beryllium alloy and does not collapse because of a spring in the center. When the air pressure changes, the cell will contract or expand. When the pressure is low, the cell will expand, and when the pressure is high, it will contract.

2. Levers that connect to the cell will move when it contracts and expands. These levers will cause the needle on the face of the barometer to move.

3. The units of measurement are in millibars, as this refers to one of the first barometers, a mercury barometer. The millibar refers to the pressure needed to move mercury up and down a tube, the "milli" referring to the distance the mercury moves.

Tips & Warnings

• The actual barometer reading is not nearly as important as comparing the barometer reading to the previous day. It is the change in air pressure that is important.

References

• NASA - For Kids Only: How Air Pressure Signals Changes in the Weather
• Weather Scope: Make and Use a Barometer to Measure Air Pressure
• Home Weather Station Guides: How to Use a Barometer