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The worst effects of seeing are due to turbulence and thermals in the densest part of the airmass above you - ie the first 1000-2000 metres.
Second comes wind shear - while you may think it’s calm on the ground yet your scope shows poor seeing, the cause can be wind at 30+ knots just 50 or 100m above you.
Lastly inversion layers, where a layer of cold air sits above a warmer layer under - and often the wind direction in each layer is at 90 degrees. The warm air tries to rise into the colder air in big bubbles but the cross-wind knocks the top of these and turbulent “rotor” is the result - rather like an invisible breaking wave rolling over as a tube. The effect on swing is terrible.
FWIW inversion layers are often formed at cloud base, typically 1500-2000 metres above sea level.
The significance of the jet stream is overrated IMHO - the air up there is too thin and well mixed.
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