Small Met Q
Joined: Oct 2002
Posts: 97
Likes: 0
From: UK
Q: An aircraft is en-route at FL180 in the northern hemisphere; wind is from the left, what happens to the oat when heading 360deg?
My apologies. Did what students alway do; did not read the question! I "assumed" (aka 'did not check') a change of heading onto 360. It is a straight forward thermal wind problem.
The loss of altitude could be explained using the usual 3 columns of air, warm, ISA and cold. Even if the surface pressures were the same the change in temperature at FL180 would give a wind coming from the left.
Using 96T/P, ISA+20 and ISA-20 and average pressure (9000ft) 756.5hPa I find that the aircraft is at a true altitude of 16178ft in the colder air compared with 18463ft in the warm air. A more accurate answer could be find by using that long winded formula but I don't have log tables.
All mistakes are due to it being too early in the morning and no caffeine fix.
My apologies. Did what students alway do; did not read the question! I "assumed" (aka 'did not check') a change of heading onto 360. It is a straight forward thermal wind problem.
The loss of altitude could be explained using the usual 3 columns of air, warm, ISA and cold. Even if the surface pressures were the same the change in temperature at FL180 would give a wind coming from the left.
Using 96T/P, ISA+20 and ISA-20 and average pressure (9000ft) 756.5hPa I find that the aircraft is at a true altitude of 16178ft in the colder air compared with 18463ft in the warm air. A more accurate answer could be find by using that long winded formula but I don't have log tables.
All mistakes are due to it being too early in the morning and no caffeine fix.
Joined: Mar 2001
Posts: 120
Likes: 0
From: Uk
Sent Clown
(again, potential met. students read with caution, I dont want to mess with your head here)
You have a point of course, but this mixing is only really prevelent in the lower atmosphere and even if you mix a cold air mass, it's still a cold air mass if you get my meaning.
Mixing due to air rising and falling does of course happen and will change the temperature even at high altitudes, but to give you a demonstration, imagine a chessboard. The whole board representing about 20 square miles.
For an unstable atmosphere only 2 random squares on the chess board with have air that is rising and/or falling and therefore adiabatically changing the environmental temperature. But again, thats only 2 squares over the whole chess board.
Downstairs is a different story altogether, surface friction will effectively mix ALL the squares, thereofre mixing the adiabatic rates together and add in varying terrestrial radiation and surface albedoes and you get a very complicated set of process which control near surface ELR's and near surface temperatures.
So you are right, but you need the bigger picture.
In relation to the question from the student the overall effect of the cold air is that it lowers the pressure upstairs.
Pugzi
(again, potential met. students read with caution, I dont want to mess with your head here)
You have a point of course, but this mixing is only really prevelent in the lower atmosphere and even if you mix a cold air mass, it's still a cold air mass if you get my meaning.
Mixing due to air rising and falling does of course happen and will change the temperature even at high altitudes, but to give you a demonstration, imagine a chessboard. The whole board representing about 20 square miles.
For an unstable atmosphere only 2 random squares on the chess board with have air that is rising and/or falling and therefore adiabatically changing the environmental temperature. But again, thats only 2 squares over the whole chess board.
Downstairs is a different story altogether, surface friction will effectively mix ALL the squares, thereofre mixing the adiabatic rates together and add in varying terrestrial radiation and surface albedoes and you get a very complicated set of process which control near surface ELR's and near surface temperatures.
So you are right, but you need the bigger picture.
In relation to the question from the student the overall effect of the cold air is that it lowers the pressure upstairs.
Pugzi
