High to Low Look Out Below

Today we are going to take a quick look at what happens when you fly your plane from an area of high pressure to an area of low pressure and when flying from hot air to cold air.

The altimeter error is what we are going to pay special attention to and why it is important to be aware of the changes as we fly across country.

Change in Pressure

If we take a look at the picture below we can see when going from an area of high pressure to an area of low pressure our aircraft will be lower than what it indicates if we do not change the sub-scale setting on our altimeter. This happens because as the area pressure decreases you try and keep the plane reading the required altitude (in this case 5000 ft) and therefore willingly descend. So to keep you plane at the correct altitude above the ground you would have to change the sub-scale to the lower setting, in this case from 1018 to 1016.

HIGH TO LOW LOOK OUT BELOW

High to Low Look Out Below

Change in Temperature

Similar to the above scenario, when we fly from a hot area to a cold area our aircraft will be lower than what it indicates if we do not change the sub-scale setting on our altimeter. A good way to think about this is if you were in a elevator “floating” above a column of air which can only expand/contract vertically, when that air cools down the column shrinks and therefore the elevator sinks. If you had an altimeter in the elevator it will still be reading the same, giving you the impression that you are at the same altitude where in fact you are lower.

HOT TO COLD DON’T BE BOLD

Hot to Cold don’t be Bold

I hope this simple explanation has helped. Please leave any comments below if you don’t understand or have any points to add for others benefit.

2 Comments on “High to Low Look Out Below”

  1. Thanks for this article! I understand how changing the altimeter setting fixes the high pressure to low pressure scenario. And I understand that hot air expands/cold air retracts. But does updating the altimeter setting fix the hot air to cold air scenario, keeping you at the correct altitude MSL? and how?

    1. Hi Jessica, sorry for the delay in reply, my internet has been down.. Things I have to deal with in the jungle;)
      So to answer your question: Yes, changing the altimeter setting will correct the altitude error. This works because the hot to cold are essentially changes in pressure. In a closed system, when the temperature of a mass of air is increased it will expand, resulting in an increase of pressure. Therefore you can assume hot is has a higher pressure than cold air. So when you look at HOT to COLD you can replace it with HIGH to LOW.

      This is a general rule and works in a closed system, however the earth is not a closed system and there may be exceptions.

      I hope this helps you understand a little better.

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