Understanding wind/gust effect with altitude

Discussion in 'Pilot Training' started by aeronav, Nov 27, 2020.

  1. aeronav

    aeronav Pre-Flight

    Joined:
    Jul 26, 2018
    Messages:
    43

    Display name:
    AeroNav
    Low time PPL, there is once aspect of wind effect i am still not able to fully understand. This question can also serve as a refresher to other private pilots.
    The scenario is as follows:
    I am taking off in a relatively windy/gust day say 13knots/G20 knots, after rolling out and during the climb I feel that the plane being kicked around by wind gust, I reach my cruising altitude of 4000 ft, the flight becomes much less bumpy and more comfortable, then I look at my instruments to see that wind speed up here is around 32 knots (which is understandable given that wind speed is lower at the surface due to friction with trees, buildings ,etc.). then again I descend to 2000 ft for the approach (assuming it's the same airport/same wind conditions) and I am getting kicked around again, but the wind speed is lower than it was at 4000, maybe 17 knots here at 2000ft.

    So the question becomes, if the wind speed is higher at 4000 ft and lower at 2000 or 1000 ft, why is it alot more quite up at 4000ft and bumpy at 2000 ft?
    The only thing I can think of is that at 2000 ft it is the gust that is kicking the plane, because it is the gust factor that is actually higher, while gust factor is low at 4000 ft, but that's just my unproven theory.

    Other bonus question:
    How is it that airlines flying at high flight levels almost always have greater ground speed, regardless of wind direction (we already know wind speed can reach over 100 knots at flight levels)

    Thanks
     
  2. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

    Joined:
    Jul 31, 2007
    Messages:
    20,926
    Location:
    Catawba, NC

    Display name:
    FlyingRon
    It's that same friction and it's ununiform application that makes the air choppier down there. Your airplane has no clue if you're in a 100-knot steady wind or in dead calm. Indeed, I've had tailwinds of that magnitude at higher altitudes in rock smooth air. What makes the choppiness are changes in wind velocity and direction. Some of that comes from thermal lift (rising air over hotter areas and sinking air over cooler), where you pass in and out of the list zones. Others are caused by the wind changed due to being redirected by terrain and friction with it. Still others are caused by the instability around the edges of fronts.

    Here's a good article: https://www.weather.gov/source/zhu/ZHU_Training_Page/turbulence_stuff/turbulence/turbulence.htm
     
  3. aeronav

    aeronav Pre-Flight

    Joined:
    Jul 26, 2018
    Messages:
    43

    Display name:
    AeroNav
    Makes perfect sense, thanks for the article link.
     
  4. Hang 4

    Hang 4 Cleared for Takeoff

    Joined:
    Aug 18, 2017
    Messages:
    1,300

    Display name:
    Hang 4
    One other factor, there might well have been an inversion. Thermic activity below it, not pushing through the inversion so smooth above. All the mechanical factors already mentioned as well. Quite common to get very smooth above an inversion, and quite common to be bumpy below it.
     
    aeronav likes this.
  5. PaulS

    PaulS Touchdown! Greaser!

    Joined:
    May 29, 2007
    Messages:
    11,920
    Location:
    New England

    Display name:
    PaulS
    All kinds of things going on lower, hills, mountains, heat from the sun, all churning things up. Higher you get away from that so usually it's smoother even though the wind speed is higher, unless the atmosphere is churned up from fronts, weather or whatever.

    The forecasts are pretty good for things like winds and turbulence. I generally don't worry about airmets for turbulence unless things are really turned up or the wind is really blowing (flying an SR-22). In lighter aircraft I'm a little more concerned. Generally I won't put up with being banged around unless I want to get somewhere. If I'm concerned by a weather forecast I'll look for pireps from similar aircraft.

    Bonus question, airliners are designed to go fast up high, the air is thinner, which means true airspeed and indicated airspeed are further and further apart as you go higher. They also have to worry about turbulence up there, it's part of flying. As far as the speed goes, it's the same for our airplanes, only on a smaller scale. Most airplanes have a sweet spot at which a certain altitude results in the best true airspeed. It depends on a few factors and can generally be determined by looking at the charts in the POH.
     
    aeronav likes this.
  6. William Pete Hodges

    William Pete Hodges Pre-takeoff checklist PoA Supporter

    Joined:
    Mar 6, 2020
    Messages:
    265
    Location:
    Virginia

    Display name:
    Petehdgs
    This may not be a very scientific answer but here goes:

    The wind near the ground and interacting with it causes the air to tumble as it slows down, just like water tumbles around a big rock in current. The tumbling wind is bumper than a smooth wind, even if it is slower.

    Second, all aircraft can travel faster at higher altitudes because the air is thinner, but I disagree they are always faster even in a headwind. The correct answer here is it depends...
     
    aeronav likes this.
  7. ebetancourt

    ebetancourt Line Up and Wait

    Joined:
    Feb 12, 2010
    Messages:
    608
    Location:
    Middle Tennessee

    Display name:
    Ernie
    Small bonus comment: Winds at altitude can easily be above 200Kts in some areas. N Pacific comes to mind.
     
    aeronav likes this.
  8. aeronav

    aeronav Pre-Flight

    Joined:
    Jul 26, 2018
    Messages:
    43

    Display name:
    AeroNav
    I remember the navigator in Strategic Air Command telling Jimmy Stewart "we've bumped into a 150 knot headwind"