Speaking of Science Column May 03, 2009
Under pressure? It’s all your frame of reference
In our day-to-day communications we blithely speak about topics of interest. Such conversation is usually general in nature, but when we begin to make measurements and then write about them, we must get more particular about how we express ourselves.
These factors often deal with the units of measurement or the frame of reference.
In reality, these considerations pertain to most subjects.
As mentioned in a previous article on pressure, a great variety of topics related to pressure are important for us to understand and explain phenomena or processes that affect our lives. In preparing this article, I learned something new that I’d like to share with you as an example of this.
You recall that atmospheric pressure was explained as the weight of a square-inch column of air reaching from that spot up to outer space.
At sea level the weight is about 14.7 pounds for that square-inch column. In different units, this would be the same as the weight of a square-inch column of mercury about 30 inches tall or a 33.9-foot tall column of water.
We’ve all seen the TV weather forecaster refer to the term “barometric pressure.”
Simplistically, this term signifies measurement of atmospheric pressure by the use of a barometer. He/ she might report the pressure as “29.92 and rising.”
A rising barometer indicates that the air pressure is increasing and improved weather is probably on the way. The number represents the height (in inches) of a column of mercury in an evacuated tube inverted in a dish of mercury with the weight of the atmosphere acting on the mercury.
But now it gets interesting, because the forecaster’s report of barometric pressure is not what would be read with a barometer outside the National Weather Service facility at the airport.
Are you scratching your head yet? It turns out that different professions just report the same data differently.
Like me, you might be baffled to read on the National Weather Service Web site that the barometric pressure at Grand Junction or Leadville is about 30 inches of mercury, approximately the same value you’d read for Los Angeles, thousands of feet lower in elevation.
What’s going on here?
Weather services around the world publish what is called the Sea Level Pressure for each station.
A mathematical calculation (see http://www.shodor.org/metweb/session3/sealev1calc.html) converts the measured barometric/ atmospheric pressure at the given elevation to a value that would be observed if that instrument were at sea level (and at a temperature of 15 degrees Celsius).
This manipulation allows the weather service to more effectively compare pressures between station locations to aid their weather forecasting.
Before I realized this, I expected to read the pressures published by the National Weather Service to be about 25 inches of mercury for Grand Junction and about 21.5 inches of mercury for Leadville. These are the values one would determine from just considering the weight of the air above a station.
Values such as these would be of vital importance to someone designing an air-supported building, such as a tennis facility.
The designer of an air-supported building is interested in the pressure that will be acting on the exterior of the building in order to specify the proper fan size to inflate the building to its proper shape and then maintain it as economically as possible.
As you can see, different conclusions might be drawn between those communicating with each other if they are not careful to clearly indicate the units and the frames of reference they are using.
Scientists and engineers continually strive to assure that such confusion is minimized by paying close attention to considerations such as these. I find that to be a challenging and satisfying aspect of being an engineer.
Allan Conrad is a retired project manager for the Jet Propulsion Laboratory in Pasadena, Calif. He has volunteered for eight years at the Western Colorado Math & Science Center and is a math tutor at Grand Junction High School.