Barometric Air Pressure 101

Susie and I woke up to quite a storm yesterday morning — the fall rains have finally arrived (and we can only hope they stick around a bit longer this year).

Either way, I was rubbing some goop on Susie’s sore arm before she got up — I’m not the only one in our household whose body is rebelling after over seventy years of abuse — and mentioned that the air pressure had fallen 0.15 inches of mercury (inHg) in less than an hour. Her entirely reasonable question was “Is that a lot?” My equally reasonable answer was “Yep” (my vocabulary often struggles, especially before the sun comes up).

This led to a short discussion of air pressure: what’s “standard,” and what qualifies as really low. This in turn led to the obvious follow-up queries: What’s the lowest; Does a low mean rain is coming; How is air pressure measured; What Bozo came up with a scale that starts at 29.92? If this sounds like a lot to clarify before the first cup of coffee you are right, so I mumbled through some bromides (I mumble exceptionally well, especially without any caffeine to lubricate my neurons), and promised to do some research and get back to her with the straight stuff.

So I did, and figured I might as well share some of my research with you, too.

First a definition so we’re all on the same page. To quote the Maximum Weather Instruments website in answer to the question: What is Atmospheric Pressure?

Essentially, atmospheric pressure is the force exerted at any given point on the Earth’s surface by the weight of the air above that point. In short: the air that surrounds the Earth creates atmospheric pressure and this pressure is determined by the collective weight of air molecules. Air molecules at higher altitudes have fewer molecules pressing down on them from above and therefore experience lower pressure, while lower molecules have more force or pressure exerted on them by molecules piled on top of them and are more tightly packed together.

Whew. A short version may help: Even though air molecules are invisible, they have mass, and therefore weight. We all understand that if you pile a load of rocks on your body it’s gonna hurt. More rocks and it hurts more. Fewer rocks and the weight is less. Thanks to gravity pulling the air towards the earth, it works exactly the same as you go up and down in the atmosphere: higher has fewer molecules above you so the pressure is less. Lower is the opposite.

Measuring air pressure isn’t all that tough. While this has obviously been upgraded in these modern days of digital everything (and is further complicated by the U.S. using one scale and nearly everybody else using another), the original device was pretty simple. As shown in the image above, a barometer (also called a manometer) doesn’t need much more than a U-shaped tube, a fluid (commonly mercury, just like in a thermometer), and a ruler.

The operation is pretty basic: the atmosphere presses on the open end of the tube and pushes the mercury either farther up or not as far. The air pressure can be read (in this case in inHg) directly off the ruler.

As you would expect, modern technology has improved on this primitive design, to the point that we can now measure and report barometric air pressure on our smartphones and Fitbits.

There are several units that can be used when reporting atmospheric pressure. Let’s start at sea level with what is called “standard atmospheric pressure” (which uses the symbol “atm”). With that forming our basis, 1 atm (at sea level) equals:

• 29.92 inches of mercury (abbreviated “inHg”)
• 14.7 pounds per square inch (lb/in², or psi)
• 1013.35 millibars (mb)
• 101,325 pascals (Pa)
• 1,013.25 hectopascals (hPa), which also equals 1 mb

For the sake of simplicity, I’m going to focus on inches of mercury and millibars, and ignore anything to do with pascals (maybe because it rhymes with rascals).

So why is the pressure at sea level used as the standard? As we already know, as you go to higher elevations the amount of air molecules above you continues to get less and less, so the atmospheric pressure gets lower and lower. The opposite holds as you go below sea level. I used to ask on my final exam at the high school something along the lines of: “You are at the top of Mt. Everest and your buddy is in Death Valley. Each of you are boiling water to make Ramen for lunch. Whose water boils first, and whose noodles taste better? And, of course, explain why.” (No pressure, but most of them got it.)

So why do we care about air pressure? A partial list…

There are many things that we deal with every day that are in some way based upon (or at least related to) air pressure. Many of us puzzle over the unexpected headaches that can crop up at the strangest times, until we hear that the pressure has either skyrocketed or plummeted. Either extreme — especially if it happens fast enough — can certainly set my head to throbbing.

But there is so much more. A few obvious examples include the altimeters on your wall or phone, stair counters on your Fitbit, and reliable estimates of the strength of a hurricane.

This last one is actually pretty important, especially to those portions of the country at the mercy of the severe storms that are becoming ever more frequent. This includes the Gulf Coast and the Atlantic Seaboard, and — as we recently learned from Helene —at least as far inland as North Carolina.

Hurricanes probably have some of the earth’s lowest air pressures as commonly measured in a natural setting. A hurricane reaches Category 5 status when the barometric pressure in the eye drops below 27.17 inHg (920 mb), and this is just the beginning of the Cat 5 level — the pressure continues to drop from there. The lowest pressure ever recorded in an Atlantic hurricane was 26.04 inHg (882 mb), which was measured in the eye of Hurricane Wilma on October 19, 2005. Since “standard” pressure is 29.92 inHg (1013.35 mb), this represents quite a drop!

Tornadoes are (probably) even lower — the lowest air pressure is estimated to be around 880 mb. The largest pressure drop ever recorded within a tornado was a 100 mb drop, measured by a probe placed ahead of an EF-4 tornado in Manchester, South Dakota on June 24, 2003. Unfortunately, the device failed to continue functioning once the tornado reached its location. But this is probably to be expected: the incredible wind speeds and resulting destruction literally destroys any measuring device placed in a tornado’s path, and a spotter plane surely cannot penetrate the eye to get an accurate reading while it is active.

Speaking of airplanes, they too rely on an accurate reading of air pressure; in this case to ensure a safe landing. This holds for both private and commercial; fixed wing and rotor. No matter what your political bias, I imagine we would all agree that it’s best that aircraft of any size or flavor stay in the air until it’s time to land… but where is the land?

I never had a commercial pilot’s license, but did fly single-engine planes back in my salad days. I vividly remember the information that the tower would ALWAYS give when I called in to tell them I was entering the pattern, and was requesting permission to land. There were two critical bits of data that the air traffic controller would always, without fail, supply: wind velocity (speed and direction), and the altimeter setting — any pilot absolutely has to know what the current pressure is so their altimeter will give them an accurate reading. (As an aside, this is what Colonel Stuart uses to send his message in Die Hard 2. If you haven’t seen it, check it out — it is a very powerful scene!)

Anyway, I could go on and on with examples of how barometric air pressure impacts our lives, and how important it is to have accurate and reliable measuring and reporting. With that said, I would like to encourage all of us to show some respect for the governmental agencies tasked with this most important of duties — NOAA and the National Weather Service come to mind — and would urge all of us to extend the respect — and critical funding — so they can remain in the service of ALL of us.