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You are flying at FL340 (250 hPa) on the northern hemisphere. The wind is geostrophic and there is a cross wind from the right all the time. Your true altitude will:
  • A
    decrease.
  • B
    increase, only if the temperature at your flight level is rising on your route.
  • C
    decrease, only if the pressure at the surface is decreasing on your route.
  • D
    increase.

Refer to figure.

We know pressure systems act in the different hemispheres:

Northern Hemisphere:
High pressure areas (anticyclones) turn in a clockwise direction.
Low pressure areas (cyclones) turn in an anti-clockwise direction.
If you stand with your back in the wind, the low-pressure area is on your left-hand side.

Southern Hemisphere is the other way around:
High pressure areas (anticyclones) turn in an anti-clockwise direction.
Low pressure areas (cyclones) turn in a clockwise direction.
If you stand with your back in the wind, the low-pressure area is on your right-hand side.

We know the following about pressure and temperature:

  • When an aircraft is in a lower pressure area, the altimeter will overread. The aircrafts true altitude will be lower than indicated.
  • When an aircraft is in a higher-pressure area, the altimeter will under read. The aircrafts true altitude will be higher than indicated.
  • When an aircraft is in a colder airmass than ISA, the altimeter will overread. The aircrafts true altitude will be lower than indicated.
  • When an aircraft is in a warmer airmass than ISA, the altimeter will under read. The aircrafts true altitude will be higher than indicated.
In the case of this specific question: The question states we are flying in the Northern Hemisphere.
Cross wind from the right means that the wind is blowing from the right to the left.
If we imagine ourselves standing with our back into the wind, we will now have the low-pressure area to our left (where we came from) and the high-pressure area on our right (where we are flying to).
So, we will be flying to a higher-pressure area and because of that the altimeter will under read.
The aircrafts true altitude will be increasing flying to the higher-pressure area.

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