orwellian
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Outside airplane temperatures are expressed in degrees Celsius. At sea level, the standard temperature is (supposedly) 15ºC (59ºF) everywhere, poles or equator... and the standard lapse rate (cooling off when going up) is 2ºC per 1000 feet of altitude increase... Reaching the altitude of 36,000 feet, the lapse rate (cooling off going up) stops - it is called the TROPOPAUSE. At that point, the outside temperature is supposed to be -56ºC and will stay at that value even if you go further up... Deviations from the temperatures are expressed as standard plus or standard minus temperatures... I have seen deviations of 20ºC warmer or colder than standard.
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Using the explanation above, at 35,000 feet, a typical altitude for an airliner, the temperature is supposed to be -54ºC... I tell people who do not seek a full explanation that the "OAT" (outside air temperature) for a jet airliner in cruise is "minus 55ºC"... But I have seen it as cold as -65ºC or maybe colder.
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Baggage compartments receive same air temperature and pressurization as the passenger cabins, but compartment walls are not isolated like passenger cabins. I have never seen an airplane with a temperature gage (thermometer) for the baggage compartment, so I can only volunteer to say that it gets cold in a baggage compartment, and that I have seen frozen water liquids, upon arrival of long flights at cold temperatures. To answer your question, it could probably be slightly below freezing, shall we say 25ºF or -5ºC at the end of a long flight.
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I was a 747 captain and 747 pilot instructor in classrooms, simulators and actual aircraft. I also instructed 747 flight engineers. Previously, I had flown 707, 727 and DC8 type aircraft as captain and instructor as well, passenger and cargo. I flew worldwide, and retired a week before my 65th birthday, the airline pilot age limit.
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You owe me a Quilmes lager (or two) for the briefing. That is much less than my classroom lectures rate...