Working under pressure...

from Click2houston.com

I had this email from a viewer a few months ago:

Can you elaborate some more on the meaning of a hurricane/storm pressure intensity and its effect? You usually have good examples of comparing one to the other.

Regards,

Deo

This is easier to understand than you might think so let’s take one issue at a time!

First, air has weight. At sea level, where we all basically live here in Southeast Texas, that weight is 14.7 pounds per square inch (rounding up to 15 pounds PSI). You actually know this because you fill your car tires with around 35 to 40 PSI so that the air inside the tire (35 pounds PSI) will be heavier than the air outside the tire (15 pounds PSI). Otherwise, those tires would be low to flat!

You have also felt the weight of air, as air weighs less the higher up you go. Perhaps your ears have popped riding an elevator quickly since the air weighs a little less 30 floors up. The air INSIDE your body (yes, we breathe in the atmosphere) would weigh more than what is outside at 30 floors so that difference makes your ears pop! And in an airplane this can be painful! We start at the tarmac with those 15 pounds PSI, but once we get to 35,000 feet that air pressure is only 3.1 pounds PSI! Hopefully, the cabin is well-pressurized so you don’t feel it too much, but usually on the way down my ears start to really hurt. Now you understand why those babies cry so loudly on ascent and descent!

So now that you get that air has weight, let’s talk about how to lower the air pressure and that answer is be creating wind. Think about it: if the air is moving quickly then it is being dispersed, so with less of it, then it will weigh less.

Back to airplanes: when they take off those flaps go down creating more curvature so that the air goes faster over the top of the wing. That faster air has lower pressure. So underneath the wing the air has higher pressure and that is what helps give the airplane lift to take off. This is called Bernoulli’s principle and what you might not know about the Bernoullis is they had six kids and they were ALL geniuses....their kids, too. Smart family.

Mother Nature also creates wind, as you know, and when a tropical storm or hurricane begins spinning (thanks to the spin of the Earth) those winds spiral in toward the center and become faster. This is conservation of angular momentum which is very simple: when the distance from the outside to the inside is decreased, the object spins faster. For instance, an ice skater brings their arms inward to get that extra fast spin. If you’ve played tetherball, then you know as the ball gets closer to the pole it gets faster. So the closer to the center means faster.

How does this answer our original question? The faster winds at the eye of a hurricane create lower pressure. When we talk about pressure intensity, we are simply saying that the pressure is so low because the winds are so fast. That’s important because the storm surge is wind-driven and the winds themselves are going to be even more destructive. In any storm, the lower the pressure, the more dangerous.

Frank

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About the Authors:

KPRC 2's chief meteorologist with three decades of experience forecasting Houston's weather.

Amanda Cochran is an Edward R. Murrow award-winning journalist. She specializes in Texas features, consumer and business news and local crime coverage.