Yeah. That's what I am thinking. As the altitude increases the pressure decreases (because of reducing air density) but the temperature also decreases (increasing air density) but not necessarily at the same rate.
Please let a scientist clarify this before I talk myself into a hole.
In the example given, air will cool between 1.5 and 3 degrees /1000' of altitude (depending on it's saturation level), so a rise of 4000" altitude will equate to a temperature drop of between 6 and 12 degrees Celsius.
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