The wikipedia info is nice indeed. In particular the image below was very informative for me (fluid dynamics simulation, wind left to right, mountain in the middle – better description in this wiki article). I never realised mountain waves depart the range at a vertical angle of 45 degrees. Agree it fits with what I saw – mountain wave it is. I considered pressure variations as well but that doesn’t explain the GS changes.
gallois wrote:
Could the answer be much more simple.
IAS is a result of the difference between total pressure as collected at the pitot and static pressure at a port out of the wind.
Pt -Ps =Pd (dynamic pressure)
As you travel through different air masses and different wind directions the Pd is going to change.
I’m not sure that’s a simpler explanation…
In any case, if the air masses have so different density that the IAS will change by 20% or more, then you would have a very noticeable frontal situation. It can’t be windshear either as that would only have a momentary effect.
Speed fluctuations like this can only be caused by up/downdrafts. Either waves or thermals. Waves don’t have to be mountain waves either. You frequently see weak but noticeable waves in and near inversions.
