Altimeter changes

rchamble

rchamble

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rschamblee
I am having trouble wrapping my head around this for some reason..

An altimeter change from 29.15 to 29.85 would equal how much altitude change? My thinking is your going from a lower atmospheric pressure to a higher atmospheric pressure which would mean you are going from higher to lower altitude which my answer would be 700 ft lower altitude but the answer is 700 ft increase in altitude, Why is this so??
 
Well, the old saying goes: "Going from low to high, watch the sky", meaning that when you go from low pressure to high pressure your altimeter was reading your altitude as being lower than you truly were. Hence, you might be closer to other aircraft/clouds/whatever than you previously thought.
 
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You are going from a higher pressure to a lower pressure in your question.

Think of it this way. Atmospheric pressure is highest at the lowest point on the planet. It has the most atmosphere pressing down on it...right? So...

Going from 29.15 to 28.85, you are going up in altitude...less pressure, less atmosphere...

Super clear, right? :mad2:
 
madriverman said:
You are going from a higher pressure to a lower pressure in your question.

Think of it this way. Atmospheric pressure is highest at the lowest point on the planet. It has the most atmosphere pressing down on it...right? So...

Going from 29.15 to 28.85, you are going up in altitude...less pressure, less atmosphere...

Super clear, right? :mad2:
Click to expand...
Your first sentence is incorrect. The OP is going from low to high so his altimeter is reading low. Assuming he unconsciously climbs to maintain whatever his desired level altitude is, he will be higher than his desired level altitude truly is because his altimeter is incorrectly set with a lower barometric pressure setting. Let's just say he thought he was maintaining a level 3000 ft MSL altitude with an incorrect 29.15 setting. Doing nothing else but resetting his altimeter to a correct 29.85 would change his altimeter to indicate his actual MSL altitude of 3700 ft MSL.
 
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29.15 is a lower atmospheric pressure than 29.85 correct? A lower atmospheric pressure is seen at higher altitudes correct? The higher you go in altitude the lower your atmospheric pressure gets because you have less weight pushing down on you.. so, when you are going from a lower atmospheric pressure (greater altitude) to a higher atmospheric pressure (lower altitude) why is the change not from higher altitude to lower altitude??
 
It makes sense, but it's not how the altimeter works. Read that link...pretty cool. And here's a brief description:

Flying from a Low to a High:

In this case, the barometric pressure is higher at our destination than it is at the airport we just left. Now, as we fly along, the actual barometric pressure corresponding to our cruising altitude is going to slowly increase. And, the density of the air at our (true) cruising altitude will slowly increase. But we will not know that the air density has changed due to increased barometric pressure. We will assume that the increase in density is due to a loss of altitude. After all, the indicated altitude is slowly decreasing.

So, what will we do? Adjust trim, engine power, or whatever so that we keep the altimeter indication from going down. The result is that we will climb to a true altitude that is greater than our initial cruising altitude. We will climb in search of the same air density we had been tracking all along. And all the while, our altimeter will read very nearly the same thing.

In the equation, if (K -- B) is constant and B increases because we're flying into an area with greater barometric pressure, the indicated altitude will decrease. This is saying the same thing. We will climb to maintain the same altimeter indication.

An old saying: "Low to high, toward the sky."

Flying from a High to a Low:

This is just the opposite of the situation described above. Flying into a low pressure causes the indicated altitude to slowly increase. If we maintain the same indicated altitude by descending, our true altitude will gradually decrease.

A person flying on instruments or at night in a region where there is high terrain or obstacles must be aware of this and be sure to update the altimeter setting frequently.

Another old saying: "High to low, look out below."
 
rchamble said:
29.15 is a lower atmospheric pressure than 29.85 correct? A lower atmospheric pressure is seen at higher altitudes correct? The higher you go in altitude the lower your atmospheric pressure gets because you have less weight pushing down on you.. so, when you are going from a lower atmospheric pressure (greater altitude) to a higher atmospheric pressure (lower altitude) why is the change not from higher altitude to lower altitude??
Click to expand...

The key is that the altimeter setting corrects for pressure changes so you get the right altitude readout.

If you're on the ground at sea level in standard conditions, your altimeter will read 0 if set to 29.92. Let's say the atmospheric pressure gets higher. Now, you're still on the ground and you know you're at sea level. Your altimeter will read a negative altitude. But, you're not below sea level! So, you adjust the altimeter to a higher pressure setting. The altimeter rolls higher as you increase the setting. It's correcting for a higher pressure setting.
 
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rchamble said:
I am having trouble wrapping my head around this for some reason..

An altimeter change from 29.15 to 29.85 would equal how much altitude change? My thinking is your going from a lower atmospheric pressure to a higher atmospheric pressure which would mean you are going from higher to lower altitude which my answer would be 700 ft lower altitude but the answer is 700 ft increase in altitude, Why is this so??
Click to expand...

29.92--> 0 ft (29.92 always = 0)

29.82--> 100 ft lower than 29.92 (-100)

29.12--> 800 ft lower than 29.92 (-800)

Moving from .12 --> .82 is an increase of 700 ft with respect to 29.92

Conversely:

30.92--> 1000 ft higher than 29.92 (+1000)

30.12--> 200 ft higher than 29.92 (+200)

29.92--> 0 ft (29.92 always = 0)

Moving from 29.92 --> 30.92 is an increase of 1000ft

The rules of "high to low--look out below" and "low to high--clear sky" are intended to be applied when flying from one of those conditions to another without changing your altimeter. Once you sweeten up your altimeter those rules no longer matter, you are reading an accurate altitude.
 
rchamble said:
I am having trouble wrapping my head around this for some reason..

An altimeter change from 29.15 to 29.85 would equal how much altitude change? My thinking is your going from a lower atmospheric pressure to a higher atmospheric pressure which would mean you are going from higher to lower altitude which my answer would be 700 ft lower altitude but the answer is 700 ft increase in altitude, Why is this so??
Click to expand...

Multiply the difference in the Kollsman window by 1,000. That's the change in altitude.

29.15 - 29.85 = -0.7
-0.7 x 1,000 = -700
 
wlawr005 said:
29.92--> 0 ft (29.92 always = 0)

29.82--> 100 ft lower than 29.92 (-100)

29.12--> 800 ft lower than 29.92 (-800)

Moving from .12 --> .82 is an increase of 700 ft with respect to 29.92

Conversely:

30.92--> 1000 ft higher than 29.92 (+1000)

30.12--> 200 ft higher than 29.92 (+200)

29.92--> 0 ft (29.92 always = 0)

Moving from 29.92 --> 30.92 is an increase of 1000ft

The rules of "high to low--look out below" and "low to high--clear sky" are intended to be applied when flying from one of those conditions to another without changing your altimeter. Once you sweeten up your altimeter those rules no longer matter, you are reading an accurate altitude.
Click to expand...

This helps make it make more since, so 29.92 is your datum plane or your starting point per say (which we know is standard condition).. I guess its just the way an altimeter works, when going up in atmospheric pressure your altitude is going up also, that just seems backwards to me when in reality your atmospheric pressure goes down with an increase in altitude..
 
rchamble said:
I am having trouble wrapping my head around this for some reason..

An altimeter change from 29.15 to 29.85 would equal how much altitude change? My thinking is your going from a lower atmospheric pressure to a higher atmospheric pressure which would mean you are going from higher to lower altitude which my answer would be 700 ft lower altitude but the answer is 700 ft increase in altitude, Why is this so??
Click to expand...

This helps make it make more since, so 29.92 is your datum plane or your starting point per say (which we know is standard condition).. I guess its just the way an altimeter works, when going up in atmospheric pressure your altitude is going up also, that just seems backwards to me when in reality your atmospheric pressure goes down with an increase in altitude..
Click to expand...

I've seen a few people dancing around the answer, but the fundamental problem is that you're looking at the question backwards.

You are not going from a lower pressure to a higher pressure.

The pressure you're flying at is constant if you're maintaining indicated altitude. What's happened is that you have flown from an area of low pressure to an area of higher pressure while you yourself have maintained a fixed pressure altitude. Essentially you are still but the measurement scale moved around you.

So, think about it this way -- as you moved from a low surface pressure region to a high surface pressure region, the difference in pressure between your altitude and the ground has increased. That means you are higher than you think you are. Pretending for a minute that ground is at sea level and the altimeter reading is 29.00. You are probably chugging around the atmosphere at, say, 26.00 inHg and assuming you're about 3000 ft up. Suddenly you are informed that the ground is now experiencing a pressure of 30.00. You've been maintaining 26.00 by holding your altimeter steady. Now you're 4000 ft up.
 
If you are sitting on the ground and the elevation dial of the altimeter is reading lower altitude than your airport, you grab the knob and move the elevation UP, the number in the barometer window of the altimeter goes up also. THEY GO UP TOGETHER! (and vice versa).

Now, if you set it correctly and fly to an airport where the barometric pressure is lower than where you took off from, that is HIGH TO LOW LOOK OUT BELOW.

If you THEN set it to the lower reading barometer the elevation will go down, again they go down together. So you WERE flying at say 1000' indicated and you adjust it down to say 800' indicated which is the correct altitude.
 
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rchamble said:
This helps make it make more since, so 29.92 is your datum plane or your starting point per say (which we know is standard condition).. I guess its just the way an altimeter works, when going up in atmospheric pressure your altitude is going up also, that just seems backwards to me when in reality your atmospheric pressure goes down with an increase in altitude..
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Meh, not really. You are confusing an altimeter setting with altitude.

Setting the altimeter tells it where zero is, nothing more...nothing less. Set it wrong and it will be wrong...set it correctly based on atmospheric conditions, it will be right.

Changing the setting from 29.12 --> 29.82 will make the altimeter move +700 feet no matter where you are on the face of the planet or what altitude you're at. It's the correlation between altimeter setting and atmospheric conditions that let's us determine our correct altitude. Basically, you tell the thing where zero (0 feet Pressure Altitude) is on the ground and then forget about it until ATC tells you something different.

Example: ATIS says altimeter is 30.42, field elevation is 300MSL. You spin in 30.42 and BAM!...you see 300MSL on the altimeter. Ten minutes later you get bored and spin in 30.92...what do you get? (If you said ~800MSL, you are paying attention). Is it right? No...it's reading 500 feet high (you're setting is 500 feet higher than it needs to be...look out below!)
 
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rchamble said:
I am having trouble wrapping my head around this for some reason..

An altimeter change from 29.15 to 29.85 would equal how much altitude change? My thinking is your going from a lower atmospheric pressure to a higher atmospheric pressure which would mean you are going from higher to lower altitude which my answer would be 700 ft lower altitude but the answer is 700 ft increase in altitude, Why is this so??
Click to expand...

I was having a similar problem with this and also started a thread on it a while back. For me I couldn't reconcile 2 things; why when I moved the knob in the kohlsman from 29.15 to 29.85 did I see an indicated increase in altitude of 700', and that a 1'' increase in atmospheric pressure equates to a 1000' decrease in altitude (pressure lapse rate). Those two truths seemed at odds.
For the me light came on when I started thinking about it like this. The altimeter adjustments are just a function of the mechanics that the engineers designed for correction, and the pressure lapse rate is a meteorological event that gets measured by the instrument.
 
Thanks for all the comments, y'all have cleared this concept up for me!! I HAVE SEEN THE LIGHT, thanks!!
 
Cpt_Kirk said:
Multiply the difference in the Kollsman window by 1,000. That's the change in altitude.

29.15 - 29.85 = -0.7
-0.7 x 1,000 = -700
Click to expand...
Thank you for this. Super simple, easy to remember, makes sense.
 
rchamble said:
I am having trouble wrapping my head around this for some reason..

An altimeter change from 29.15 to 29.85 would equal how much altitude change? My thinking is your going from a lower atmospheric pressure to a higher atmospheric pressure which would mean you are going from higher to lower altitude which my answer would be 700 ft lower altitude but the answer is 700 ft increase in altitude, Why is this so??
Click to expand...
Without reading all the replies, I predict that this will cause confusion because people can read it two different ways.

1. You fly into an area of higher pressure without being aware (and without changing your altimeter) — your pressure altitude is the same, but your actual altitude is now 700 ft higher.

2. You note that the altimeter setting has increased, and change your altimeter to compensate — your pressure altitude is now 700 ft lower, but your actual altitude is the same.

In the first case, you followed the pressure slope gradually upwards; in the second case, you didn't.
 
When I was taking instrument training, I flew one day after a big low pressure system had moved in. My altimeter setting was 28.85. I believe that was the lowest setting I can remember ever flying in. (yes it was pretty windy that day) I finished the lesson, and then flew home to my private strip where the field elevation was 1500 ft msl. The next time I flew was about a week later. Our private strip did not have any weather reporting, so when I would get in the plane, I always set the altimeter to the field elevation. The barometric pressure was very high that day. in fact it was 30.45, which was one of the highest I could ever remember flying in at the time. When I went to set my altimeter, which was still set from the extremely low pressure day, it showed that I was sitting 100 ft below sea level. I obviously was not at that altitude. Our strip was still at 1500 ft msl.

In simple terms without moving my plane, I had gone from a low pressure system to a high pressure system. Because I had not reset my altimeter yet, It showed that I was 1600 ft below what I actually was. in order to just get the altimeter to show the field elevation, I would have had to clear 1600 ft in the air. (low to high, clear the sky)

If the reverse were true and I would have flown on a day when the barometric pressure was at 30.45 and then came back a week later and the barometric pressure would have been 28.85, that would be going from a high to low. Since my altimeter would have been reading 1600 ft above what I was actually at, an attempt to fly a reading of 2500 ft on the altimeter without resetting it would have put me 500 ft below the ground. ( hence, high to low, look out below)
 
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While a useful memory jogger for some, I've found that "low to high, clear the sky/high to low, look out below" etc. tends to be confusing for many people.

I've had success having learners visualize how the "legacy" instrument works. The altimeter really just measures atmospheric pressure and presents it as an altitude. The altimeter itself is sealed, connected via plumbing to a static port. There's a little aneroid inside which expands or contracts based on changes in pressure. If pressure drops inside the chamber, the aneroid expands. If the pressure becomes greater, it contracts. As it does so, gears rotate the hands of the display to indicate altitude. That's it!

At the dawn of aviation, altimeters couldn't be quickly and easily calibrated for changing barometric pressures, which was a real problem. So in 1936 Paul Kollsman invented a way to do that. The sensitive altimeter was born and to this day pilots use the "Kollsman window" to set their altimeters to local pressure, in measurements of hectopascals or inches of mercury.

With that in mind, here's an artificial scenario: freeze frame an airplane in flight. Local pressure is 29.92". Altimeter indicates 2,000 feet.

Let's use our global weather control panel to lower the local pressure from 29.92" to 29.82" with that airplane still frozen in flight.

Inside the airplane, within the sealed chamber of the altimeter itself, pressure dropped by a tenth of an inch of mercury. As a result, the aneroid expanded. The gears turned. The indicated altitude increased by 100 feet.

If we unfreeze the frame and let the airplane fly, the pilot suddenly sees 2,100 feet indicated on the altimeter. There are two ways to make the instrument indicate the desired altitude, 2,000 feet again. Option a) is to obtain a local pressure setting. The pilot rotates the knob so that the Kollsman window indicates 29.82" instead of 29.92". Voila. Now the altimeter indicates 2,000 feet on the nose again. Option b) is to descend 100 feet. Of course doing so would result in the aircraft's true altitude being 1,900 feet even though the altimeter itself indicates 2,000.

Since not all pilots choose option a), this is where "high to low, look out below" comes from.
 
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