ASK THE METEOROLOGIST
“What are the different units of pressure and why?”
Nancy Wieber, Grants Pass
There are a few different units of atmospheric pressure, but the most commonly used by meteorologists is millibars. You may have also heard of inches of mercury, which is used for mercury barometers and is another fairly common unit of pressure. Nowadays, the internationally recognized unit of pressure is a Pascal. In meteorology, hectopascals are used less frequently than millibars. However, Pascals are actually considered “S.I. units,” meaning that is the standard unit used around the world to measure pressure.
So why do meteorologists so frequently use millibars instead? Well, it might help to know that millibars were introduced by a meteorologist, Sir William Napier Shaw, in the late 1920’s. Ever since then, it’s been the unit for pressure used in atmospheric science, and when hectopascals became the S.I. unit of pressure, meteorologists were slow to make the switch. Fortunately, as mentioned earlier, both hectopascals and millibars convert equally. That is, 1,000 mb = 1,000 hPa. So, for convenience’s sake, meteorologists still regularly use millibars rather than hectopascals.
Chief Meteorologist Kate McKenna