T.G.I.F. It’s time for our weekly shoot-the-shit topic. This week we continue where we left of with last Friday’s Station Pressure vs. Barometric Pressure topic.
What is a shoot-the-shit? It’s a social interaction during which participants can make any claim without factual support, insult or denigrate anyone they wish, use uncensored profanity – even if you are a boatswain mate or you can change the subject altogether. Unlike psychologists, psychiatrists and philosophers, all of whom are living proof that free market economies produce sufficient surpluses to support their ass, we don’t charge you!
So, let’s get on with barometric pressure and altitude…
Last Friday we talked about station pressure and barometric pressure. We said that to measure pressure we need a meter, like the Kestrel Weather Meter, and that to measure station pressure, we need to set the reference altitude to zero. We then went on to say that to measure barometric pressure, we set the reference altitude to the correct altitude at your location
Note the difference in pressure readings between the two pictures on the left. This difference is converted to altitude in weather meter software. In our case, the difference between station pressure and barometric pressure is 0.15InHg which translates to 131.4 feet. This value is a measure of the size of the column of air from your elevation to sea level, hence it translates to your altitude.
When I make an approach to an airport, I contact approach control, or UNICOM, and ask for the altimeter setting if it’s not being broadcast. Approach control won’t say 300 feet they’ll say 29.97. The 29.97 translates to a specific column of air above sea level. Your weather meter works very much like an aircraft altimeter.
Take a look at the altimeter on the right. You’ll notice the two small windows at the bottom of the altimeter’s face, the one on the right is pressure in INHg and the one on the left is in milibars. When approach control tells me altimeter at 29.97 they are providing the barometric pressure in INHg. I then rotate the knob in the lower right hand corner and set INHg to 29.97. Now the altimeter will correctly indicate elevation.
To arrive at a reasonable altitude reading from your weather meter, you need to set the reference to the barometric pressure for your area. Don’t expect the meter’s indicated altitude to exactly match that of a topographic map. Even GPS elevation values are often off by as much as 90 meters and more. The reason for the error stems from the fact that GPS receivers calculate position and elevation by measuring time differences between the signals from at least 4 satellites, and any anomaly (terrain, structures, atmospherics, forest canopies, etc.) will influence the position and elevation calculation generating the error.
For ballistic purposes, altitude – better stated as pressure altitude – is not a requirement. What you’ll need is temperature, wind speed, wind angle, station pressure, relative humidity and/or density altitude to arrive at an accurate firing solution.
As customary, we like hearing your thoughts and ideas.
Have a safe and relaxing weekend. Get some range time in with your buds and family!