Does Pressure Altitude change with tempreture?

5 Answers

1. Best Answer
   
   

   
   

   John D Collins on Jul 15, 2014

   You are doing a good job of analyzing the effects of temperature on pressure and altimetry. The altimeter bases its readings on a model of the atmosphere where as you increase in altitude both the pressure and the temperature decrease at a standard rate. The atmosphere rarely matches the model as the model is one of average conditions. Pressure altitude is the altitude read by the altimeter when the pressure setting is adjusted to or fixed at 29.92 inches of mercury. Altimeter encoders use pressure altitude. In flight, we use either altitude corrected for the local barometric pressure (barometric altitude) or pressure altitude as appropriate. Here in the US, we are required to use barometric altitude when below 18000 MSL and pressure altitude when higher. The latter are termed Flight Levels as they are really not constant altitudes but rather based on constant pressure levels. We use Flight Levels because at the higher speeds typically flown at these altitudes, it is not practical to be constantly changing the altimeter setting. Outside of the US, many counties use Flight Levels because they don’t have practical means of measuring the local barometric pressure and they don’t have to worry about mountains or high obstacles (think the Bahamas).

   Because of the temperature and other effects, a safe altitude to avoid obstacles is set to 1000 feet above them as indicated on our barometric altimeter, but when we are in mountainous areas, we go to a 2000 foot standard. Our altimeters have no correction for temperature, just a correction for the barometric pressure. At the airport spot where the barometric pressure is determined, it matches the actual altitude and an aircraft near it with the barometric altimeter set accordingly will also be very close to the correct altitude. Also, at the point the altimeter setting is determined, the temperature effects are fully taken into account by the setting at that specific location. The further we are from that location, both in distance and altitude, the more any deviations from the standard average atmosphere will affect what the altimeter reads. It is not uncommon for the altimeter to be off by hundreds to even thousands of feet on very hot or cold days at cruise altitudes.

   In spite of the errors in the altimeter that are not compensated for, we all use the same model and therefore aircraft will remain separated vertically from one another while in flight. Also, since the error largely disappears as we approach the airport, instrument approaches and normal pattern work are not adversely affected by the temperature errors.

   One last point, a GPS derived altitude is much more accurate than a barometric altimeter and matches much closer to the true altitude. Frequently you can see this as a difference between what your altimeter reads in comparison to the GPS. The GPS is unaffected by temperature or pressure variations that don’t match with the model. This is why it is the preferred source for terrain and obstacle avoidance systems. But the barometric altimeter will keep you separated from opposite direction traffic whereas the GPS won’t, so don’t use the GPS to determine your cruise altitude.

   +21 Votes 22 Votes  1 Votes

2. 
   

   
   

   Jordan Cobb on Jul 15, 2014

   Mr. Collins,

   I must say, that is a very well articulated and complete answer. I figured someone would take my question eventually, but I never thought I would get a response as detailed as yours. This has greatly clarified my understanding of altitudes, temperature, and pressure. You pretty much summed up in four paragraphs what hours of reading could not. Thanks very much for taking the time. If you are ever in Lansing, I owe you a beer.

   +4 Votes 4 Votes  0 Votes

3. 
   

   
   

   Mark Kolber on Jul 16, 2014

   I was quite surprised to see the thumbs down on John’s answer. For that person, who is obviously looking for a simpler sounding answer, does it help to say:

   yes, pressure altitude takes temperature into account, but only to the extent of the temperature provided by the standard lapse rate. Density altitude represents pressure altitude as adjusted for by the difference between the standard lapse rate temperature and the actual environmental temperature.

   ?

   +4 Votes 4 Votes  0 Votes

4. 
   

   
   

   Drew on Jul 18, 2014

   I found this discussion to be helpful and had questions of my own that are related. I’d appreciate it if somebody could provide an answer.

   “Well, we also know that in a parcel of warm air, there are less molecules than there would be in a parcel of cold air of identical size. That being the case, would that air not weigh less, and there for exert less pressure on the air below it? Would temperature also affect pressure altitude as well?”

   1. Are there less molecules in a parcel of warm air compared to a parcel of cold air? I thought that the molecules in the warm parcel moved faster/had more energy, which shouldn’t affect the number of molecules in the parcel.

   2. Would anyone happen to know how exactly the stations correct for the temperature when they determine the altimeter settings?

   0 Votes 0 Votes  0 Votes

5. 
   

   
   

   Mike on Aug 27, 2014

   To answer your question:
   Temperature does effect atmospheric pressure and hence the measured barometric pressure used to correct the pressure altitude. However, on a hot day, the pressure should actually increase per newtons gas laws but the density will decrease drastically at the level of measurement. Remember that the atmosphere is miles deep and small temperature variations on the surface will not change the pressure much. Large storms that change the consistency and movement of air through significant proportions of the atmosphere will tend to change the pressure much more.

   Actuall molecules running in to the propeller is what effects performance, not the pressure. So, on a given day, if the temp is at 100 F and the barometer is close to the usual 29.92 (at sea level), this says nothing directly about how many molecules are in the air. You would need to calculate the density (or guess if you are close to standard levels). For us pilots, density altitude is much more handy because we can compare engine performance at the adjusted level with which we should already be familiar. This is not go without mentioning that an increase in pressure from another source (ie storm) would increase in density and effect performance. For precision in elevation we should use pressure, for precision with performance density.

   Another item. Water/humidity is a solid. Then why would performance decrease with its increase? Water has less mass in the space that it displaces oxygen and nitrogen, therfore, less effect on the wing. Think—two hydrogens and one oxygen versus two oxygen atoms. Hydrogen is much lighter.

   0 Votes 1 Votes  1 Votes