Decoding Barometers: The Importance of Understanding Pressure Changes


If you just look at a barometer once, it’s pretty much as satirist Ambrose Bierce described. The needle on the front of that old school brass barometer points to “Fair,” and when you look outside, sure enough, it’s not a bad day. When it says “Rain,” often enough, it’s raining or about to.

That doesn’t sound very useful. And the truth is, it isn’t helpful to just look at one reading. What makes barometers useful as a forecasting device is the trend in the readings. Are they going up or down over time, and how quickly? And to know that, you need to pay a little more attention to what they’re doing. And you need to know why the readings are changing and what that portends.

Welcome to Weather 101. Over the next few months we’re going to tackle some weather fundamentals to help boaters get a more solid understanding of marine weather, and how to look at a bewildering array of data to determine if you want to go out in your boat, and what to expect when you’re there.

What is a barometer?

A barometer measures atmospheric pressure. It’s a measurement of the weight of the atmosphere directly over where the barometer is sitting.

The earth’s atmosphere is about 60 miles thick around the surface of the planet. It’s a thin layer relative to the size of the planet, but air has weight and square inch of the surface has pressure from that invisible column of air over it. And that is atmospheric pressure.

But atmospheric pressure isn’t uniform. The earth isn’t completely flat, for starters. If you’re on the top of a one-mile tall summit, you have one mile less atmosphere over your head, and a barometer will show a slightly lower reading as you climb.