Pressure and Altitude Question

[WoooPigSooie]

I'm having trouble wrapping my head around something so thought I'd ask the board.

I know that the lapse rate for atmospheric pressure is 1'' decrease in pressure for every 1000' increase in altitude. However, I also know that if I lower my altimeter from 30.11 to 29.92 my indicated altitude is now 150' lower. Why? Applying the lapse rate if my pressure lowered then shouldn't my altitude raise?

I'm having trouble marrying these two together.

 

[Capt. Geoffrey Thorpe]

You are at sea level and barometric pressure is 30.11 - you set that value in the window and your altimeter sez 0 MSL.

You have a really long checklist. While you are sorting things out, the actual barometric pressure has dropped to 29.92. What does your altimeter say?

+150 MSL. It reacts to the reduced pressure by indicating a higher altitude.

Turning the setting down to the actual sea level pressure of 29.92 gets you back to 0 AGL.

Low barometric pressure makes your altimeter read high, high pressure makes it read low (at least until you re-adjust).

 

[JeffDG]

And if you have a pressure change that dramatic, you may want to reconsider flying, because the weather's gonna suck somethin' terrible.

 

[WoooPigSooie]

You are at sea level and barometric pressure is 30.11 - you set that value in the window and your altimeter sez 0 MSL.

You have a really long checklist. While you are sorting things out, the actual barometric pressure has dropped to 29.92. What does your altimeter say?

+150 MSL. It reacts to the reduced pressure by indicating a higher altitude.

Turning the setting down to the actual sea level pressure of 29.92 gets you back to 0 AGL.

Low barometric pressure makes your altimeter read high, high pressure makes it read low (at least until you re-adjust).

Ok, got that. When I turned the setting from 30.11 back to 29.92, that resulted in a 150' decrease in the indication. Why, when lapse rate says that as pressure lowers altitude raises?

 

[WoooPigSooie]

And if you have a pressure change that dramatic, you may want to reconsider flying, because the weather's gonna suck somethin' terrible.

Agreed. I'm just dealing with theory right now studying for the oral, so I'm not sure, "I know it's a sign a crappy weather ahead so I won't fly," will be the complete answer the DPE will accept.

 

[Capt. Geoffrey Thorpe]

Ok, got that. When I turned the setting from 30.11 back to 29.92, that resulted in a 150' decrease in the indication. Why, when lapse rate says that as pressure lowers altitude raises?

Lower pressure indicates higher altitude. If you change altitude the pressure drops and the altimeter goes up. If the actual barometric pressure drops the indicated altitude goes up when it should not. Dialing in a lower pressure setting brings the altitude back down to correct for the error caused by the baro decrease.

 

[N801BH]

I'm having trouble wrapping my head around something so thought I'd ask the board.

I know that the lapse rate for atmospheric pressure is 1'' decrease in pressure for every 1000' increase in altitude. However, I also know that if I lower my altimeter from 30.11 to 29.92 my indicated altitude is now 150' lower. Why? Applying the lapse rate if my pressure lowered then shouldn't my altitude raise?

I'm having trouble marrying these two together.

High to low = look out below....

Low to high = clear blue skies.....

 

[N801BH]

I'm having trouble wrapping my head around something so thought I'd ask the board.

I know that the lapse rate for atmospheric pressure is 1'' decrease in pressure for every 1000' increase in altitude. However, I also know that if I lower my altimeter from 30.11 to 29.92 my indicated altitude is now 150' lower. Why? Applying the lapse rate if my pressure lowered then shouldn't my altitude raise?

I'm having trouble marrying these two together.

Also , remember, that trick is NOT linear..... Other wise you would be in a total vacuum at 30,000 feet ... ya need to go about 180,000 feet higher to get that...

 

[Norman]

Ok, got that. When I turned the setting from 30.11 back to 29.92, that resulted in a 150' decrease in the indication. Why, when lapse rate says that as pressure lowers altitude raises?

I believe you've got that backward. When you lower the value in the Kolsman window it will read a higher altitude.

 

[AggieMike88]

One of the best ways to remember how a change of altitude affects temperature and pressure is a visualization of a mountain.


Start by imagining that you live by the sea and decide to go visit Colorado Pike's Peak. One of the way's to get to the top is to ride the cog railway. As you climb upwards, you notice the temperature dropping, and even in the late spring, it's cold enough for their to be snow on the ground. So you went from low to high, and it got colder, right?

And once you're at the top, you notice that any exertion gets you out of breath faster than when you do your daily jog on beach. Beach, low, Pike's Peak, high, and lower pressure. So from low altitude to higher, the pressure went from "high" to "lower".

There is a YouTube video out there that does a better job explaining this visualization... I will see if I can find a link and share it.

 

[WoooPigSooie]

I believe you've got that backward. When you lower the value in the Kolsman window it will read a higher altitude.

If you change the setting from 30.11 to 29.92 the approx. change in indication is 10' lower.

 

[WoooPigSooie]

One of the best ways to remember how a change of altitude affects temperature and pressure is a visualization of a mountain.


Start by imagining that you live by the sea and decide to go visit Colorado Pike's Peak. One of the way's to get to the top is to ride the cog railway. As you climb upwards, you notice the temperature dropping, and even in the late spring, it's cold enough for their to be snow on the ground. So you went from low to high, and it got colder, right?

And once you're at the top, you notice that any exertion gets you out of breath faster than when you do your daily jog on beach. Beach, low, Pike's Peak, high, and lower pressure. So from low altitude to higher, the pressure went from "high" to "lower".

There is a YouTube video out there that does a better job explaining this visualization... I will see if I can find a link and share it.

I understand that. I'm not asking my question very well, although I think Capt. Thorpe has helped, I'm still chewing on that answer.

Here's the question: If atmospheric pressure lapse rate is 1'' decrease in pressure for a 1000' increase in altitude, why does turning my dial to a lower pressure setting indicate a lower altitude?

 

[N801BH]

I understand that. I'm not asking my question very well, although I think Capt. Thorpe has helped, I'm still chewing on that answer.

Here's the question: If atmospheric pressure lapse rate is 1'' decrease in pressure for a 1000' increase in altitude, why does turning my dial to a lower pressure setting indicate a lower altitude?

Do you even know how an altimeter works??

 

[Norman]

If you change the setting from 30.11 to 29.92 the approx. change in indication is 10' lower.

Earlier, you said 150 ft.

 

[kevmor99]

Here's the question: If atmospheric pressure lapse rate is 1'' decrease in pressure for a 1000' increase in altitude, why does turning my dial to a lower pressure setting indicate a lower altitude?

You're using the knob to recalibrate your altimeter, since normally the altimeter is set to the pressure in the kollsman window, the needle moves up with outside pressure dropping and down with outside pressure increasing. When you move the knob you're adjusting the mechanisms in the reverse way to calibrate.

1" = 1000" ft. so:

31.11 - 29.92 * 1000 = approx. 190'

 

[vkhosid]

I understand that. I'm not asking my question very well, although I think Capt. Thorpe has helped, I'm still chewing on that answer.

Here's the question: If atmospheric pressure lapse rate is 1'' decrease in pressure for a 1000' increase in altitude, why does turning my dial to a lower pressure setting indicate a lower altitude?

Can we agree on the fact that as altitude goes up, the static air pressure goes down? If so, then what you're basically doing when you're turning the knob to indicate a "lower pressure setting" is telling the plane that the pressure is actually less than what the static port is measuring. Sooooo, relative to the actual pressure that the plane is getting from the static port (the ACTUAL static air pressure), you're saying the pressure is lower, annnnnnnnnnd......as we agreed with the first sentence, the lower the pressure, the higher the altitude.

 

[CharlieTango]

If atmospheric pressure lapse rate is 1'' decrease in pressure for a 1000' increase in altitude, why does ...

Where do you get that?

I live at 8,000' and the pressure is currently 29.75. 150 miles west we find sea level and the pressure there is 29.62. if we were 8" lower then SFO our pressure would be 21.75, kinda low no?

 

[RotorDude]

I understand that. I'm not asking my question very well, although I think Capt. Thorpe has helped, I'm still chewing on that answer.

Here's the question: If atmospheric pressure lapse rate is 1'' decrease in pressure for a 1000' increase in altitude, why does turning my dial to a lower pressure setting indicate a lower altitude?

Because you are not changing the altitude by turning the dial. You are entering a correction to make the altimeter read correctly.
Imagine you are parked at sea level, with everything standard. Your altimeter compensation (aka Kollsman window) is reading "29.92" and the altimeter is showing 0 feet MSL.
Now imagine that a low pressure system rolls in. The air around you gets thin, and your altimeter starts "climbing", even though you are still parked on the ramp. Let's say it climbs to 200 feet and stops, as the pressure outside stabilizes. Now, you know you are at sea level, but your altimeter indicates 200 feet high, so you want to correct it. The way to do that is to set in the Kollsman the outside pressure, which is 29.72 (.2" below standard). When you turn the dial to that correction value, your altimeter reading is lowered to the correct value, 0 in your case. So again, you are not "raising" or "lowering" the "altimeter", you are doing it to the altimeter correction, which then brings the altimeter reading into the correct value for the ambient pressure.

 

[WoooPigSooie]

Earlier, you said 150 ft.

Sorry, typo. I just realized I've been typing 29.92 when I've been meaning 29.96. So indicated altitude would be 150' lower.

 

[MAKG1]

Where do you get that?

I live at 8,000' and the pressure is currently 29.75. 150 miles west we find sea level and the pressure there is 29.62. if we were 8" lower then SFO our pressure would be 21.75, kinda low no?

Umm, your pressure really is closer to 21.75.

The Kollsman window uses pressure corrected to sea level.

The evidence -- just how long does it take you to boil spaghetti up there? The boiling temperature of water is a fairly strong function of pressure. At 8000 ft pressure altitude, it's under 200 F.

 

[N801BH]

Where do you get that?

I live at 8,000' and the pressure is currently 29.75. 150 miles west we find sea level and the pressure there is 29.62. if we were 8" lower then SFO our pressure would be 21.75, kinda low no?

...

Corrected or absolute pressure??

 

[WoooPigSooie]

Where do you get that?

I live at 8,000' and the pressure is currently 29.75. 150 miles west we find sea level and the pressure there is 29.62. if we were 8" lower then SFO our pressure would be 21.75, kinda low no?

PHAK, Chpt 3, page 3-2. Standard press & temp for your altitude is 22.22 and temp -.9C, so you're in a non-std press & temp.

 

[Norman]

Where do you get that?

I live at 8,000' and the pressure is currently 29.75. 150 miles west we find sea level and the pressure there is 29.62. if we were 8" lower then SFO our pressure would be 21.75, kinda low no?

Corrected to sea level I presume.

 

[RotorDude]

Where do you get that?

I live at 8,000' and the pressure is currently 29.75. 150 miles west we find sea level and the pressure there is 29.62. if we were 8" lower then SFO our pressure would be 21.75, kinda low no?

The "pressure" you quote is not the real outside barometric pressure, but the pressure correction value for the Collsman window, which helps the altimeter compensate for non-standard pressure. On average, your pressure correction value should be about 29.92" (regardless of where you live), so 29.75 is not far off.

 

[WoooPigSooie]

Because you are not changing the altitude by turning the dial. You are entering a correction to make the altimeter read correctly.
Imagine you are parked at sea level, with everything standard. Your altimeter compensation (aka Kollsman window) is reading "29.92" and the altimeter is showing 0 feet MSL.
Now imagine that a low pressure system rolls in. The air around you gets thin, and your altimeter starts "climbing", even though you are still parked on the ramp. Let's say it climbs to 200 feet and stops, as the pressure outside stabilizes. Now, you know you are at sea level, but your altimeter indicates 200 feet high, so you want to correct it. The way to do that is to set in the Kollsman the outside pressure, which is 29.72 (.2" below standard). When you turn the dial to that correction value, your altimeter reading is lowered to the correct value, 0 in your case. So again, you are not "raising" or "lowering" the "altimeter", you are doing it to the altimeter correction, which then brings the altimeter reading into the correct value for the ambient pressure.

I read this similar to Capt. Thorpe's reply. The altimeter correction is just a function of the mechanics that the engineers designed for correction, and the lapse rate is a meteorological event.

 

[RotorDude]

I read this similar to Capt. Thorpe's reply. The altimeter correction is just a function of the mechanics that the engineers designed for correction, and the lapse rate is a meteorological event.

That's exactly right.

 

[NickC99]

That's exactly right.

I was going to give a similar reply, but your explanation is better.