P-N wrote: “[...] regardless whether the change in internal balance between electrostatic potential and molecular KE contributes 10% or 0.1% of the real temperature drop [...]”
OK, I found a source that may be the origin of you figure.
“Joule performed his experiment with air at room temperature which was expanded from a pressure of about 22 bar. Air, under these conditions, is almost an ideal gas, but not quite. As a result the real temperature change will not be exactly zero. With our present knowledge of the thermodynamic properties of air [9] we can calculate that the temperature of the air should drop by about 3 degrees Celsius when the volume is doubled under adiabatic conditions.” Wikipedia
Volume doubles at about 5km above sea level. So, the ratio is about 3/50. There is a 3°C drop from electrostatic potential energy increase, and the remaining 47°C drop is the part of molecular EK reduction that you can’t account for when you assume conservation of total internal energy.
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