Physics
It takes much longer to cook food in the hills than in the plains, because

in the hills the atmospheric density is low and therefore a lot of heat is lost to the atmosphere
n the hills the atmospheric pressure is lower than that in the plains and therefore water boils at a temperature lower than 100oC causing an increase in cooking time
in the hills the humidity is high and therefore a lot of heat is absorbed by the atmosphere leaving very little heat for cooking
due to low atmospheric pressure on the hills, the water boils at a temperature higher than 100oC and therefore water takes longer to boil

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Physics
Inside an aeroplane, flying at a high altitude,

normal atmospheric pressure is maintained by the use of air pumps.
normal humidity and partial vacuum are maintained.
the pressure is the same as that outside.
the pressure inside is less than the pressure outside.

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