“High to Low look out below”, we all know this one, but why.

7 Answers

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   John D. Collins on Jan 02, 2012

   High to low, lookout below can be used for altimeter pressure settings, or temperatures, or both.  As one climbs to higher altitude, the pressure drops, so a lower pressure results in a higher altimeter reading.  If you have your altimeter set to a higher pressure and you fly into an area of lower pressure, the altimeter will read higher than the actual baro altitude.  At least in the case of pressure, we have a means of correcting for it on the altimeter by changing the Kollsman window setting. 
    
   The baro altimeter altitude readout is based on a standard temperature lapse rate, with the temperature decreasing approximately 2 degrees C for each 1000 feet of elevation.  The baro altimeter has no correction for temperatures when they are not the standard values.  If you remember your high school physics, the formula for the ideal gas law is pV=NRT where p is pressure, V is volume, N is the quantity of moles of the gas, R is a constant and T is the temperature. If you solve  this for pressure, you will get that pressure is a function of temperature, with a higher temperature resulting in a higher pressure and a lower temperature results in a lower pressure.
    
   When an altimeter setting is determined at a particular location, one way of doing this is to set an  altimeter to the known field elevation and read the Kollsman pressure value.  This is the correct value for that location and corrects for both the pressure and temperature.  However, as you climb higher over the spot where the altimeter setting was determined, if the temperature is different than standared, a temperature induced error will start to be displayed on your altimeter.  The higher you go, the greater the error accumulates.  At a higher temperature than standard, the actual pressure will be higher and the altimeter will read lower than it should.  At a lower temperature, the actual pressure will be lower and the altimeter will read higher than it should.  Colder temperatures than standard will result in the altimeter reading above your actual altitude.  This can be a problem for clearance of obstacles or mountains at night or when you are IFR and can’t see them.  You can find “cold temperature” adjustments for MDH or DH on instrument approaches in the AIM.  In the north, Canada in particular, they have adjustments that add to the altimeter reading when flying approaches in cold temperatures.  Also, if you are an instrument pilot, you will notice that LNAV/VNAV approaches will have high and low temperature limits for the procedure.  This is because with Baro-VNAV equipment, a glidepath is calculated based on the altimeter altitude value.  This makes the glide slope vary with temperature, being too shallow when it is too cold and being too steep when it is too hot.
    
   There is a great tool you can play with that you can find at http://www.luizmonteiro.com/Learning_Alt_Errors_Sim.aspx 
   You can play with this to adjust the pressure and temperature and see what it does to the altitude readout.

   +20 Votes 22 Votes  2 Votes

2. 
   

   
   

   Brian on Jan 02, 2012

   John, and others, also have addressed this in some more detail in the blog section of this site. Visit the link below to read:
    
   http://www.askacfi.com/3518/barometric-altimeter-accuracy.htm

   +2 Votes 2 Votes  0 Votes

3. 
   

   
   

   Nathan Parker on Jan 02, 2012

   This can’t be understood without first understanding that pressure is caused by the weight of all the air molecules above the point where the pressure is being measured.
    
   Using this concept, if we heat a column of air, it will expand vertically, but this alone will not affect the pressure felt at the surface, since the weight of the column of air has not changed.  However, this is not true at points higher in the column of air.
    
   For simplicity’s sake, let’s assume the column of air is 1,000 feet high.  The pressure at 500 feet is due to the weight of air from 501 to 1000 feet.  If you double the temperature of the air, the column will expand to be 2,000 feet high.  At the altitude of 500 feet, you now have 3/4 of all the air above that point, so you will feel an increase in pressure due to the weight of the air from 501-2,000 feet. 
    
   The flip side of temperature increase is that the previous pressure felt at 500 feet will now lie above this point.  Since airplanes use pressure to determine altitude, an aircraft striving to maintain 500 feet will climb in order to follow this pressure level which now resides at a higher altitutde, hence “Clear the Sky”.
    
    

   +15 Votes 15 Votes  0 Votes

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   Sean McKenzie-Mardelli on Aug 01, 2013

   This makes sense, but seems to contradict From the Ground Up. In that reference, it says that cold air masses can produce barometric pressures higher than 31.00″. If PV = nRT, it seems to be that a low temperature (cold air) should produce a low barometric pressure.

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   DeltaFan on Jan 20, 2014

   Reading through the instrument flying handbook, I had some of these same questions and came across this topic. I too have the same question as Sean above about pressures. You always see weather maps with cold temperatures in high pressure areas and warm temperatures with low pressure areas. Yet, in Nathan’s response, warm temperatures create higher pressures (?). I am confused! Thank you for your help, DeltaFan

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   David Lane on May 01, 2015

   This is an old thread, but contains some serious errors that would confuse the student. While the ideal gas law shows that pressure increases with temperature, this is not the case for atmospheric pressure! Why? Because the ideal gas law pV=NRT refers to a fixed volume containing a fixed number of gas molecules, and we don’t have a fixed volume — the air mass moves around. The comment about weight of air is partly correct, think of pressure instead as the number of gas molecules in a given volume, or density (mass) rather than weight. A volume of air at low temperature is heavier because there are more gas molecules in it. There are more molecules in it because the lower temperature slows down their motion agitating each other and pushing other molecules out of the way. An altimeter measures the ratio of gas density between the ambient and a sealed volume of air, and both are at the same temperature — so a colder air mass will contain more molecules, creating a higher pressure than the fixed volume of air inside the altimeter’s aneroid bellows.

   In aviation, “high to low, look out below” does apply to high and low pressure, of course, and also to high and low temperature. As you fly into an area of lower pressure (without adjusting the altimeter setting) the altimeter will indicate higher than you really are, because the pressure at a given altitude is lower, the same as it would be if you were actually flying higher. Also, as you fly into an area of colder air, the actual pressure is higher because the air is colder, not vice versa.

   Refer to section 7-2 of the AIM. When you have an altimeter setting for a station near your route of flight, it will already be corrected for temperature and assumes standard lapse rate. But if you don’t, and you fly into an area much colder than where you were, the altitude will read higher than you actually are, because the colder air is more dense than it was where you got your last altimeter setting. For instance if you have an altimeter setting from Denver and you fly to Leadville where it’s -10C, your altimeter will be almost 500 feet off even if the actual atmospheric pressure doesn’t change. This is why instrument approach procedures are so picky about “NA when xxx altimeter setting not available.”

   +7 Votes 8 Votes  1 Votes

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   High to low, look out below… | Private Pilot on Sep 20, 2017

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