Question about Barometric Pressure

Discussion in 'Flight Following' started by George Foster, Jul 23, 2021.

  1. George Foster

    George Foster Filing Flight Plan

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    Just finished a mountain flying and a back country flying course in CO. I am surprised how much the pressure changed between reporting stations that aren't very far apart, often between 15 and 50 miles. The weather is clear, no fronts, stable air - light winds for a couple of days, no frontal activity on the weather charts. Yet, here are the altimeter readings of a few nearby stations and airports: 30.16, 30.55, 30.61, 30.65, 30.75. Anyone know why the pressure differs so much between reporting stations in the Rocky's? My instructor wasn't sure. He thought it might have to do with the Continential Divide.
     
  2. TommyG

    TommyG Pattern Altitude

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    I don’t much of anything about mountain flying, but I was guess that a CFI teaching mountain flying would know why such differences exist.
     
  3. 455 Bravo Uniform

    455 Bravo Uniform En-Route

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    Every 100 ft change in elevation the pressure changes by 0.1

    Go figure out the field elevations and see if that explains it.
     
  4. Wheels

    Wheels Pre-takeoff checklist

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    Were the airports all the same distance above sea level? A difference in elevation will cause the difference in pressure.
     
  5. Lindberg

    Lindberg Final Approach

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    An elevation change will cause a change in pressure, but elevation alone does not affect the altimeter setting.
     
  6. dmspilot

    dmspilot Final Approach

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    That isn't true. The altimeter setting is not actual sea level pressure, it's whatever the altimeter needs to be set to in order to read field elevation. It sounds like a distinction without a difference but in reality errors due to non-standard atmosphere add up at high elevations.

    For example right this moment in Denver the altimeter setting is 30.22 but the SLP is 1013.9mb which is 29.94"Hg.

    Conversely, right now in KMIA the altimeter is 30.03 while the SLP is 1017.0mb which is...30.03"Hg.
     
    Last edited: Jul 23, 2021
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  7. SloRoam

    SloRoam Pre-Flight

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    This is covered in quite a few books about mountain flying, and is a fantastic way to predict wind in various gaps because wind will follow those pressure gradients.

    I'm not sure I can explain it as well as the books, but in essence it has to do with air parcels having a hard time flowing from one valley to another, especially when they need to climb through an inversion, or stable air that acts almost as a lid to stop air from flowing over the top of one valley to another..

    In the valley where I live, it's easy to visualize the air wants to flow from the next valley over, but it would have to flow up over a pass, which would cause it to have to lift a great deal, and there's not a lot of room in that pass for it to flow. This causes a pressure difference between one valley to the next, and the resulting wind.
     
  8. Lindberg

    Lindberg Final Approach

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    But that is not "elevation alone."
     
  9. sarangan

    sarangan Pattern Altitude

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    Elevation changes the barometric pressure, but altimeter setting is the equivalent pressure at sea level. In other words, it is the pressure that would exist at sea level if you artificially extend the atmosphere down to sea level, assuming standard lapse rate.

    The reason for wild changes in altimeter settings in mountains is because of large pressure gradients. Even if two airports are at the exact same elevation, if they are separated by a mountain range, the air between the two airports may not equalize as compared to a flatter terrain. Because the altimeter has to indicate the same elevation at both airports, you need something else to compensate for that difference. That's what the altimeter setting does.
     
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  10. dmspilot

    dmspilot Final Approach

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    No, it isn't, as I just explained.
     
  11. dmspilot

    dmspilot Final Approach

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    Elevation and nonstandard atmosphere, and i think we all understand that nonstandard atmosphere changes the altimeter setting. So what is your point?