Air density

Africa Pilot

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Hi everybody,

I wonder if you all can help with a problem that has stumped a student and me:

How can it be true that an increase in temperature has the effect of decreasing indicated altitude, while at the same time high temperature increases density altitude?

The crux of the question is this: Does an increase in temperature increase or decrease ambient pressure? The altimeter seems to think that there is an INCREASE in pressure, whereas the fact that density altitude increases w/temp. points to a pressure DECREASE.

I must be missing something obvious.

THANKS!
 

moeron

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Keep in mind, when you increase the pressure the temp. increases. I have been peplexed by this question also.
 

tarp

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"High to Low, look out below", "Low to High, nothing but Sky".

This little pnemonic got me through every one of my written tests.

If High Pressure or High Temperature, your altimeter will "indicate" that you are lower than you actually are (i.e. indicated is lower than true altitude).

If Low Pressure or Low Temperatures, your altimeter will "indicate" that you are higher than you actually are (i.e. indicated is higher than true altitude).

Pressure and temperature are directly proportional or act the same.

The scientific way to think about this is a can of soda going up and down in the airplane. On the ground we are used to the soda pressing against the aluminum walls of the can with close to normal pressures. However, when we take the soda up in the air the pressure on the outside of the can is reduced and the contents want to come out. The same thing is happening on the bellows inside of the altimeter. As the pressure or temperature rise outside of the bellows, the altimeter thinks it is going lower and lower. As the pressure and/or temperature lowers outside the bellows, the altimeter thinks it is rising.

Science is funny - remember to think of things in a "closed" system and then in an "open" system. We take a parcel of air and close is a jar or an altimeter case. Heat the air up and what happens - the molecules want to get farther apart and they then exert greater pressure against the walls of the container. Take the same amount of air in an "open" system like the atmosphere and the only result is that the air molecules push further apart - "no walls". The aircraft trying to run on less dense air acts like it is at a "higher altitude", running out of the needed density of air molecules.

Balloons and soda cans - that's what science is all about - with an ocassional bicycle wheel thrown in.

Happy flying.
 

BigFlyr

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RE: How can it be true that an increase in temperature has the effect of decreasing indicated altitude, while at the same time high temperature increases density altitude?

That stuff always confuses the hell out of me because of the wording...If you look at what you're saying..."an increase temperature decreases indicated altitude"... That's true only if you're maintaining true altitude, which you dont, since you, as the good pilot you are, or even your autopilot, will make subtle corrections to MAINTAIN your "indicated" altitude. Therefore when going into warmer air you're riding higher in the sky due to the hotter more spread out air molecules (thicker atmosphere), but you never noticed it! The opposite is true when entering an area of lower temperature or pressure. The airplane while MAINTAINING "indicated" altitude is always riding on temperature and pressure gradients that will bring it closer to and further away from the earth. Got it? :confused:
 
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avbug

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In flight, high temperature will cause an increase in true altitude, while maintaining the same indicated altitude.

Density altitude increases with any temperature above standard, because quite simply, the air is less dense. The density of the air is then equivilent to the pressure altitude on a standard day at a higher elevation; density altitude.

While maintaining a constant indicated altitude in flight, imagine a model airplane riding on a balloon which is sitting on a table. If the air in the balloon is colder, it contracts, and the airplane on the balloon will be lowered; it will be closer to the table. The pressure in the balloon remains the same, and thus the "indicated altitude" for the airplane remains the same...but the true altitude has decreased as the airplane has flown into an area of colder air.

If the air in the balloon is heated, the balloon expands, and soon the model airplane is higher above the table. True altitude has increased but the internal pressure remains the same. (relatively speaking...the balloon isn't a perfect example).

Imagine holding the model airplane at the same true altitude however. This would occur parked on the ramp. Experience a drastic temperature change, and all else being equal, you're going to have to reset the pressure on the altimeter (altimeter setting) in order to maintain the same indication.

As was stated before, simply remember the phrase, "When flying from hot to cold, or high to low, look out below."

I have seen an altimeter be off by a thousand feet when flying in mountainous terrain across a front. Between temperature changes and pressure changes, it really can make a big difference. On one occasion I saw a change that big, it was on a trip less than 50 miles in length.
 

Africa Pilot

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Thanks a bunch

Well, that has really cleared things up for me, especially Tarp's distinction between "open" and "closed" systems.

Thanks to all for the clarification. What a great thing this internet is sometimes.
 

FlyinBrian

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Even though we have beat the topic to death, I thought I'd simplify a bit...

Q: How can an increase in temperature result in PA decrease and a DA increase?

A: Increased temperature results in greater pressure an less density.
 

alimaui

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Science Schmience

Of all the explanations, tarps makes the most sence. Open system vs closed system.
 
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