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Statistically speaking, at what altitude under 10k feet there are less chances for turbulence during a typical flight?

Personal experience is also welcomed.

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    $\begingroup$ This will no doubt be highly dependent on location. $\endgroup$
    – Jamiec
    Jan 18 at 8:38
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    $\begingroup$ And on conditions on a particular day. A smooth altitude today may be bumpy tomorrow, and today's rough altitude may be tomorrow's best ride. $\endgroup$
    – Ralph J
    Jan 18 at 10:34
  • $\begingroup$ Should this question's title be rephrased to either use AGL or MSL? $\endgroup$
    – Jeff B
    Jan 29 at 16:52

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There are 3 main sources of turbulence:

  1. Mechanical, from air flowing over the surface, like river rapids.
  2. Convective from solar heating of the surface (discounting other surface heat sources which are too minor to bother with).
  3. Mechanical, from shearing of airflows going in different directions above the surface.

Numbers 1. and 2. will be the vast majority of the turbulence you will encounter. All that is required is hilly terrain and winds in the case of 1., or sunshine in the case of 2.

Number 3 requires special conditions of air flows with sharply diverging directions and resulting shear effects, or "nocturnal jet streams" where a strong shear from a sudden change in velocity can exist at a few hundred feet over flat prairie topography.

Therefore, statistically, the vast majority of turbulence you'll encounter below 10000 feet originates at the surface. The height above the surface will be a function of lapse rate and relative humidity in the case of convection (it generally stops at the cumulus cloud base) or the wind strength and surface contour in the case of mechanical turbulence.

Since it originates from the surface one way or another most of the time, the majority of turbulence declines with altitude, and therefore the least likely altitude, below 10000 feet, to encounter it, is 9,999 feet. (Note that in mountainous terrain, 10000 feet AGL may not be enough to get you clear of mechanical turbulence, but the concept is the same).

And in day to day flying, "higher is smoother" is obvious.

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  • $\begingroup$ While the turbulence, including the thermally driven one, does "originate" at the surface (due to the surface heat/buoyancy flux), the maximum strenght is higher. The thermals are gaining velocity while accelerating in the boundary layer. The lapse rate is important, but mainly only to find out where the inversion is, the turbulence profiles are functions of w*, which is given by the inversion height and by the surface buoyancy flux. $\endgroup$ Jan 19 at 12:27
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    $\begingroup$ But the perceived intensity declines with altitude when you fly through them as they spread out and the shear zone at the margin gets wider. IOW the bumps you feel from thermals gets softer and softer as you go up. When soaring, they get much easier to stay in. $\endgroup$
    – John K
    Jan 19 at 14:38
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    $\begingroup$ Actually, the turbulent kinetic energy may have a maximum close to the ground, due to the horizontal velocity components. The variance of the vertical velocity component is what has a maximum at those 1/3 to 1/2. What you describe is quite possible. The effects on an airplane also depend on the length scales and time scales of the turbulence, not just on the variances only. $\endgroup$ Jan 19 at 16:24
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Upon further research I found a useful article which references an NCAR study and answers this question.

"Considering both in-cloud and out-of-cloud turbulence, flights between 8,000 and 12,000 feet will allow for the smoothest ride, on average" Turbulence Versus Altitude

enter image description here

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    $\begingroup$ What does this chart's horizontal axis measure? And is the vertical axis AGL or MSL? The linked article doesn't say. Neither does it name the actual study, so we can't see for ourselves. $\endgroup$ Jan 18 at 17:39
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    $\begingroup$ I'm also curious how much of this is reporting bias, or what the actual study might have done, if anything, to account for that. You could look at the graph and (I guess, depending on what the axes mean) conclude that 35,000ft is the most turbulent place to be, but surely there are more PIREPs reporting turbulence around FL350 because that's around where airliners fly and they're more likely to report bad ride conditions. $\endgroup$ Jan 19 at 8:24
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    $\begingroup$ @ZachLipton yes, those are PIREPs, I've updated the NCAR link to the actual pdf file. $\endgroup$
    – Gabe
    Jan 19 at 17:58
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Since turbulence is many times caused by terrain, one might say 9999 feet. However, turbulence can also be caused by passing fronts. This is why METARs list wind speed and direction at various altitudes.

Flying near a wind shear interface can buy some fairly serious turbulence, along with some inclement weather. Lower altitudes may be better after a cold front passes.

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  • $\begingroup$ Most turbulence during the day will be thermally driven, not mechanically. In that case the maxium is not at the surface, but somewhere between 1/3 to 1/2 of z_i. $\endgroup$ Jan 19 at 12:22
  • $\begingroup$ @VladimirFГероямслава that will depend alot on the prevailing wind and terrain, but, yes, on a calm day fair weather cumulus will pop up and dominate the local wind flow. As for thermals "accelerating" we can certainly see that if there is condensation. Let's hope Camille G expands on the graphs presented in one answer. $\endgroup$ Jan 19 at 14:28
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In the daytime, most turbulence in flattish terrain comes from atmospheric convection in the atmospheric boundary layer. The convective boundary layer can be cloud-free or cloud-topped (cumulus or stratocumulus).

If clouds are present, turbulence strongly increases inside them, due to turbulence generation by the buoyancy flux due to the released latent heat due to water vapour condensation. For stratocumulus it is quite a homogeneous layer, but for cumulus it si very concentrated to the cloud cores.

Above the boundary layer, we usually assume laminar flow without turbulence. That is not completely true of course, as internal gravity waves travel there and they could break, but that usually happens much higher, even in the stratosphere.

All this is for fair weather situations, mostly anticyclonic. When atmospheric fronts form, the clouds systems form with them and a lot of turbulence inside them. But in that case the height dependence will be complicated and and probably will differ case to case.

In general, the air above the boundary layer should be mostly turbulence-free. Statistically, the BL top will be up to 2-3 km in summer, less in winter.

During the night a stable boundary layer forms. It is much shallower, in hundreds of meters. Near its tops we often see a region of air velocities that are significantly higher than the geostrophic flow (supergeostrophic). This region is called a low-level jet. The shear there is associated with mechanical turbulence, but in general, the turbulence there will be much larger during the day in the convective boundary layer.

I repeat, above the typical boundary layer heights of 2-3 km the chance of turbulence should be quite similar. This may be affected by topography in mountainous regions and if you are considering an airport located in a deep mountain valley, you certainly can expect an elevated maximum of turbulence at the layer of the mountain ridges, but for flattish terrain it should be quite similar above the BL.

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This question has no answer because for each altitude 𝑎 < 10000, there is an altitude 𝑎′ such that 𝑎 < 𝑎′ < 10000. Each such 𝑎′ is smoother than 𝑎.

Delegate the tasks of getting from 𝑎 to such a 𝑎′ exactly, measuring the actual turbulences, and getting the accurate statistics to the student pilots.

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    $\begingroup$ While this may be theoretically thrue, it's not very helpful as an answer :) $\endgroup$
    – Jpe61
    Jan 19 at 14:59
  • $\begingroup$ @Jpe61 The ability to delegate is undoubtefully a useful and HELPFUL skill. The original poster should exercise it! As well as thinking properly about a question before asking. $\endgroup$
    – AlMa1r
    Jan 19 at 18:01
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    $\begingroup$ What exactly do you mean by delegating in this context? The fact that you have, probably just, learned to formulate stuff in a fancy matter does not make you eligible to come on ASE and be a smartass. $\endgroup$
    – Jpe61
    Jan 19 at 22:00
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    $\begingroup$ You're claiming that the least-turbulent altitude is always infinitesimally close to 10000 ft. e.g. at any local minimum in turbulence below 10k, there's always a higher altitude with strictly less turbulence. If you're already at or above a local minimum, climbing an infinitesimal amount will be slightly more turbulent, so the only way your statement could be true for all $a$ is if there's another smaller minimum higher up. This contradicts all the other answers but you present no evidence, and your answer doesn't explain this practical implications of your invariant. $\endgroup$ Jan 20 at 20:27
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    $\begingroup$ I'm not one of your student pilots. You show me; it's your answer making these claims. $\endgroup$ Jan 22 at 13:42

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