Weather Watch

What's a millibar? The answer lies with the father of modern weather forecasting

An old barometer.
An old barometer. Special to The Tribune

Marlene called KVEC morning news last week and asked an excellent question: What is a millibar?

To start this story, Sir Isaac Newton, who discovered gravity, which helps to produce atmospheric pressure, once wrote in a letter: “If I have seen further it is by standing on ye shoulders of Giants.”

In other words, those who came before us allowed others to make advancements in our meager understanding of the universe.

And so it was with Norwegian physicist Vilhelm Bjerknes, the father of the modern weather forecasting.

He introduced the unit of a millibar, but before he did, he was an assistant to Heinrich Hertz in Bonn, Germany, the birthplace of Beethoven, who the unit of frequency — cycles per second — was named “hertz” in his honor. This unit is used for electromagnetic and mechanical frequencies like sound. Ironic, that Beethoven would lose his hearing while composing some of his most significant works.

Later in life, Bjerknes explained the interaction between thermodynamics and fluid dynamics. In 1919, a team of Norwegian meteorologists, led by 57-year-old Bjerknes, came up with the theory of weather fronts, an analogy for World War I battlefronts. Fronts are long boundary lines that can stretch for hundreds or even thousands of miles across the Earth between two air masses of different densities, usually caused by temperature or humidity differences. They often bring precious rain. He realized that with enough current atmospheric data, such as pressure, temperature and humidity, you could use mathematical formulas to predict the weather.

To express air pressure, he used the bar, which is a metric unit of pressure; one bar is slightly less than the Standard Atmospheric Pressure at sea level, which is defined as 1.01325 bar or about 14.7 pounds per square inch. The millibar (mbar) is equal to one-thousandth of a bar, or 1,000 dynes per square centimeter.

Even though it’s a metric unit, is not approved as part of the International System of Units (SI). In fact, professional organizations are deprecated it.

Even though it’s not an approved SI unit, weather forecasters in the United States love using it because it’s convenient and familiar. I still refer to air temperature in Fahrenheit and not Celsius and rainfall in inches and not millimeters.

The millibar is slowly being phased out for hectopascal (kPa), which is numerically equivalent to millibars, but you have to move the decimal point one place to the left. For example, 1013.25 mbar is equal to 101.325 kPa. In the United States, many barometers measure pressure in inches of mercury, or “inHg,” although it’s considered archaic and is also fading away.

Which leads to the question, what causes air pressure?

The strong invisible force of gravity accelerates trillions of air molecules toward the Earth’s surface and produces weight. The weight of the air is what we feel as pressure. Imagine a 1-square-inch column of air measured from the top of the atmosphere down to sea level — it would weigh about 14.7 pounds. All the air surrounding the Earth would weigh nearly 5,600 trillion tons.

We usually don’t notice it because our bodies maintain an internal pressure that balances the external pressure. But swift changes in elevation can cause us to detect atmospheric pressure changes in our ears when they “pop,” which is caused by your inner ear trying to equalize the pressure with that of the outside air. Typically along the Central Coast, high pressure produces clear and dry weather, while low pressure can create rain.

▪ ▪ ▪

In 2017, nearly 80 percent of the electricity that PG&E provided to our customers came from sources that are renewable and/or emit no greenhouse gases.

John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at or follow him on Twitter: @PGE_John.
Related stories from San Luis Obispo Tribune