Pressure VS Altitude

[TacticalBoltKnob]

Hey, guys.

I am having trouble understanding the relationship between barometric pressure and altitude for calculating firing solutions. I understand the effect of altitude on pressure, but why does altitude matter for your firing solution if you have an accurate measure of the air pressure (B.P.)? I.E. is there some other aspect of altitude that affects external ballistics other than pressure?

 

[sandwarrior]

Think of it in steps.

1. You are at a physical altitude, i.e sea level, 1000 ft., 4000 ft.
2. You have a barometric pressure at that altitude that usually varies air density from actual altitude, i.e. a pretty cold day at 1000 ft. might seem a lot more like sea level. A storm moving in is 'low pressure'
3. To get "pressure altitude" turn your altimeter to 29.92 and that tells you what "relative altitude" you are at, or 'pressure altitude'.
Explanation of temperature on density. higher temp means higher altitude, lower temp means lower altitude.
Definition of air density: High density = high altitude, low density = low altitude.
4 'Pressure altitude' gets corrected to make density altitude by integrating the temperature factor.

 

[bigbear_98]

Hey, guys.

I am having trouble understanding the relationship between barometric pressure and altitude for calculating firing solutions. I understand the effect of altitude on pressure, but why does altitude matter for your firing solution if you have an accurate measure of the air pressure (B.P.)? I.E. is there some other aspect of altitude that affects external ballistics other than pressure?

As I understand it
Altitude doesn't matter if you have true station pressure which is pressure at your location. This can be had by setting reference altitude to 0 on your kestrel.

If you are using corrected pressure from a weather reporting station as they correct the pressure for altitude, You have to know your true altitude so the formula can be reversed to station pressure.

 

[KYpatriot]

Pressure is all that matters...but sometimes we tell our tools to give us the wrong info to use because we don't understand the settings. I recommend using station pressure if you have a kestrel...but your kestrel setup and the ballistic calculator setup must match!


Bigbear is right... Use station pressure, temperature, and humidity with reference altitude set to zero if you are using a kestrel and make sure your ballistic calculator is set up for station (actual) pressure. Once you have done that do not use pressure readings from public weather sources like the news or local airports because they will be reporting corrected sea level pressure and you will be missing high if you input corrected pressure into a calculator that is set up for station pressure. Most calculators have an option to use either, but You must have your ballistic calculator set up for one or the other it can't use both.

Summary of your two options:
If your ballistic calculator is set up for station pressure (you do this in shooter for example by toggling "pressure is absolute" to on), set reference altitude to zero in your kestrel and input the station pressure, temp, and humidity into your calculator. Notice the altitude window in shooter will be grayed out because it is not used. You are done.

On the other hand if your ballistic calculator is set up for corrected pressure (in shooter when the "pressure is absolute" toggle is off) then you must have a reference altitude in your kestrel, which the kestrel will use to correct the pressure to the sea level equivalent. You will then input that into your calculator, along with a reference altitude, and the usual temp and humidity. If your calculator is set up this way you may also use public sources for pressure readings like the local airport or local news, and you should be fairly close as long as your altitude is similar to the source, unless a front is moving through your area and that source is in the other side. If you have a kestrel there is no good reason to do this, since it requires the extra step of determining your altitude and entering it in both the kestrel and the calculator. Without a kestrel, using public wx info, this is what you must do.

 

[sandwarrior]

Something that throws people off when figuring this too is "low pressure" as in a storm moving in means a reduction in pressure and therefore relates to a higher altitude.

Air density is given in terms of lower or higher as it relates to altitude. Low air density means low altitude (high pressure), high air density means high altitude (low pressure).

The opposing use of expressions often leads to confusion.

Added:

In a better answer to your question. True/actual altitude doesn't matter except to give you a starting point to calculate pressure. Keep in mind how long this has been done by hand and not kestrel.

Without any tooling you can use some rules of thumb to help you calculate pressure roughly. These aren't exact but they get you on:

With each increase of 1000 ft. elevation you lose one inch of mercury, i.e. std is 29.92, then 1000 ft. el. is 28.92 Take .5 in. Hg off for a storm moving in, add .5 in. Hg when you see a front pushing the storm out.

For temperature, add 70 ft. per degree above standard (59*) (very rough guess). Or, subtract if temperature is lower

 

[_Windrider_]

Something that throws people off when figuring this too is "low pressure" as in a storm moving in means a reduction in pressure and therefore relates to a higher altitude.

Air density is given in terms of lower or higher as it relates to altitude. Low air density means low altitude (high pressure), high air density means high altitude (low pressure).

The opposing use of expressions often leads to confusion.

Added:

In a better answer to your question. True/actual altitude doesn't matter except to give you a starting point to calculate pressure. Keep in mind how long this has been done by hand and not kestrel.

Without any tooling you can use some rules of thumb to help you calculate pressure roughly. These aren't exact but they get you on:

With each increase of 1000 ft. elevation you lose one inch of mercury, i.e. std is 29.92, then 1000 ft. el. is 28.92 Take .5 in. Hg off for a storm moving in, add .5 in. Hg when you see a front pushing the storm out.

For temperature, add 70 ft. per degree above standard (59*) (very rough guess). Or, subtract if temperature is lower

pressure altitude = (standard pressure - your current pressure setting) x 1,000 + field elevation (or in our case, the firing position altitude)
density altitude = pressure altitude + [120 x (OAT - ISA Temp)].......ISA temp is 15C with a 2 degree lapse rate for every 1,000ft in elevation increase. So if you are at 6000 ft pressure altitude the ISA temp is 3C.

Standard Atmosphere is 29.92 inHg at Sea Level and 15C (59F)

Standard Lapse rates:

1 inHg for each 1,000ft INCREASE in altitude (as SandWarrior stated)
2C (3.56F) decrease for each 1,000ft INCREASE in altitude.

So if you are at 4,000' MSL, the standard atmosphere is 25.92 inHg and 7C.

BUT, I agree wholeheartedly with what KY said, use absolute pressure (or station pressure as it is sometiemes referred to) and you do not need to know the physical altitude you are at.

 

[TacticalBoltKnob]

Thanks guys. I was aware that the pressure on weather websites and the news is corrected to sea level (useless), and I have the formula to UNcorrect it, so to speak.

And my suspicions are confirmed that if you have absolute pressure (not corrected), then altitude is a non-factor.

 

[sandwarrior]

xx.....ISA temp is 15C with a 2 degree lapse rate for every 1,000ft in elevation increase. So if you are at 6000 ft pressure altitude the ISA temp is 3C.

Standard Atmosphere is 29.92 inHg at Sea Level and 15C (59F)

Standard Lapse rates:

1 inHg for each 1,000ft INCREASE in altitude (as SandWarrior stated)
2C (3.56F) decrease for each 1,000ft INCREASE in altitude.

..xx..

TacticalDillhole,

Thanks for catching that. Without a basis, the temp information I gave gives relatively little meaning. You gave standard temp. decline with altitude increase.

If you'll note that 1 Celsius converts to 1.8 F. Keeping with "rule of thumb" (no instruments), the temp I gave of 70 ft. per degree F, ( x 1.8 then x 2 {as given by TD} is approximately 252 ft.

However, the temperature increase at altitude is about 62.5 degrees, not 70. (I stand corrected) So what you are talking about is standard deviation in temperature with change of altitude, I'm talking change of density altitude with temperature change. which does affect the shot as does change in temp/alt.

 

[Ovrmydeadbdy]

Think of it in steps.

1. You are at a physical altitude, i.e sea level, 1000 ft., 4000 ft.
2. You have a barometric pressure at that altitude that usually varies air density from actual altitude, i.e. a pretty cold day at 1000 ft. might seem a lot more like sea level. A storm moving in is 'low pressure'
3. To get "pressure altitude" turn your altimeter to 29.92 and that tells you what "relative altitude" you are at, or 'pressure altitude'.
Explanation of temperature on density. higher temp means higher altitude, lower temp means lower altitude.
Definition of air density: High density = high altitude, low density = low altitude.
4 'Pressure altitude' gets corrected to make density altitude by integrating the temperature factor.

I'm sorry, you are wrong on the air density definition.... I teach people to fly helicopters and the higher you go, the LESS dense air gets.

Also, the higher you go the lower the temperature gets.....

Temp and pressure are inversely proportionate, usually unless of course there is a temperature inversion... PM me if you want a better more thorough explanation.

Think of why people kick footballs further in the NFL in Colorado, compared to San Diego.... The air is less dense in Colorado.

Not measuring penises here, I just want the right info out there for people to read here on the hide!




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[_Windrider_]

.........Temp and pressure are inversely proportionate, usually unless of course there is a temperature inversion... PM me if you want a better more thorough explanation.

One correction is that they are not inversely proportional but rather directly related. Think about the earth as a crock pot or an oxygen tank for a diver. When that O2 is under extreme pressures, the temp increases. the same thing happens to the atmosphere in high pressure situations. as temp increases, so does pressure and as pressure increases so does temperature. The opposit is true when the temp or pressure decreases. But in the world of meteorology, these pressure changes are relative, meaning when we talk about a low pressure system or a high pressure system it means only that the new air moving in is lower or higher relative to the one its replacing. Blah Blah Blah.

Absolute pressure and Temp. Done deal. Thats all you need. And if you have your GPS, you can use the altitude readout, the absolute pressure and temp and you can derive pressure altitide and density altitude using the equations and standard lapse rates i gae you above.

As for penis measurements.......Mine may not hit bottom, but it will sure scrape the sides.

 

[Ovrmydeadbdy]

One correction is that they are not inversely proportional but rather directly related. Think about the earth as a crock pot or an oxygen tank for a diver. When that O2 is under extreme pressures, the temp increases. the same thing happens to the atmosphere in high pressure situations. as temp increases, so does pressure and as pressure increases so does temperature. The opposit is true when the temp or pressure decreases. But in the world of meteorology, these pressure changes are relative, meaning when we talk about a low pressure system or a high pressure system it means only that the new air moving in is lower or higher relative to the one its replacing. Blah Blah Blah.

Absolute pressure and Temp. Done deal. Thats all you need. And if you have your GPS, you can use the altitude readout, the absolute pressure and temp and you can derive pressure altitide and density altitude using the equations and standard lapse rates i gae you above.

As for penis measurements.......Mine may not hit bottom, but it will sure scrape the sides.

Please no one listen to this nonsense!!
Let's get down to the molecular level..... When molecules heat up, they move faster, thus making them more distant from each other, thus making the molecules per any given volume LESS..... Meaning, the HIGHER the temperature, the LESS the pressure.... Inversely measured.

Funny penis joke though!

Next question please! This is my favorite subject in aviation, which a bullet is a smaller form of an airplane! Drag, lift, Magnus theory are relevant to aviation and shooting bullets.....


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[Ovrmydeadbdy]

One correction is that they are not inversely proportional but rather directly related. Think about the earth as a crock pot or an oxygen tank for a diver. When that O2 is under extreme pressures, the temp increases. the same thing happens to the atmosphere in high pressure situations. as temp increases, so does pressure and as pressure increases so does temperature. The opposit is true when the temp or pressure decreases. But in the world of meteorology, these pressure changes are relative, meaning when we talk about a low pressure system or a high pressure system it means only that the new air moving in is lower or higher relative to the one its replacing. Blah Blah Blah.

Absolute pressure and Temp. Done deal. Thats all you need. And if you have your GPS, you can use the altitude readout, the absolute pressure and temp and you can derive pressure altitide and density altitude using the equations and standard lapse rates i gae you above.

As for penis measurements.......Mine may not hit bottom, but it will sure scrape the sides.

Also, when oxygen is under high pressure is is extremely COLD!!! Please just let this go! Once again. HIGH pressure= low temperature.....


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[sandwarrior]

I'm sorry, you are wrong on the air density definition.... I teach people to fly helicopters and the higher you go, the LESS dense air gets.

Also, the higher you go the lower the temperature gets.....

Temp and pressure are inversely proportionate, usually unless of course there is a temperature inversion... PM me if you want a better more thorough explanation.

Think of why people kick footballs further in the NFL in Colorado, compared to San Diego.... The air is less dense in Colorado.

Not measuring penises here, I just want the right info out there for people to read here on the hide!




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My apologies, lets be clear, High density altitude is high altitude or thin air. Low density altitude is low altitude or thicker air. Correct me if that is not how it expressed.

Density altitude is often shortened to 'air density'.

The same term applies, high or low, relating to altitude.

The point being in aviation (I've been in it now for 22 years) is it's always expressed one way so the listener is supposed to be able to understand. Just like why anyone in aviation can understand #1 or #2 engine, depending on what aircraft they are talking about. Why everything either forward or behind the pilot is given from the perspective the person describing the issue is behind the item described and L-R is always given from a 'looking forward' position.

High air density and low air density are given in terms of altitude above mean sea level. Not how thick or thin the air is. But, that's just aviation. Because, you now (being in aviation I'm surprised) have given your own definition of it.

...yes we know as you go lower the air normally gets thicker. And, as you go higher the air normally gets thinner...But, that is not how it is expressed in aviation. Thus why in post #5 that I explained that there is confusion with the differences in terminology. Since the shooting world has recently moved to density altitude, like aviation, it ought to be expressed that way.

Now, temperatures affect on air pressure. As temperature rises at a given altitude the air thins out. As it decreases (again, at a given altitude) the air pressure gets thicker. That is of course in open atmosphere. In contained atmosphere (like inside a sub or a pressurized plane) the opposite is true.

Regarding oxygen, TacticalDillHole is right. As pressure increases so does temperature. And if you want to make statements about it getting super cold, you might look up how to make liquid oxygen. The temperature increases as the oxygen pressure increases. The machines that make liquid oxygen then remove the latent heat. The pressure is taken past what oxygen turns to liquid. But it remains a gas. When the heat is removed, the oxygen is then allowed to expand slightly and this cools it even further turning it from a gas to a liquid. Much like steam when it hits something cold turns from a gas to a liquid.

Oh, and here's a good pilot joke: How many pilots does it take to change a light bulb? A: One, he holds the bulb in place and the whole world revolves around him.

 

[Ovrmydeadbdy]

My apologies, lets be clear, High density altitude is high altitude or thin air. Low density altitude is low altitude or thicker air. Correct me if that is not how it expressed.

Density altitude is often shortened to 'air density'.

The same term applies, high or low, relating to altitude.

The point being in aviation (I've been in it now for 22 years) is it's always expressed one way so the listener is supposed to be able to understand. Just like why anyone in aviation can understand #1 or #2 engine, depending on what aircraft they are talking about. Why everything either forward or behind the pilot is given from the perspective the person describing the issue is behind the item described and L-R is always given from a 'looking forward' position.

High air density and low air density are given in terms of altitude above mean sea level. Not how thick or thin the air is. But, that's just aviation. Because, you now (being in aviation I'm surprised) have given your own definition of it.

...yes we know as you go lower the air normally gets thicker. And, as you go higher the air normally gets thinner...But, that is not how it is expressed in aviation. Thus why in post #5 that I explained that there is confusion with the differences in terminology. Since the shooting world has recently moved to density altitude, like aviation, it ought to be expressed that way.

Now, temperatures affect on air pressure. As temperature rises at a given altitude the air thins out. As it decreases (again, at a given altitude) the air pressure gets thicker. That is of course in open atmosphere. In contained atmosphere (like inside a sub or a pressurized plane) the opposite is true.

Regarding oxygen, TacticalDillHole is right. As pressure increases so does temperature. And if you want to make statements about it getting super cold, you might look up how to make liquid oxygen. The temperature increases as the oxygen pressure increases. The machines that make liquid oxygen then remove the latent heat. The pressure is taken past what oxygen turns to liquid. But it remains a gas. When the heat is removed, the oxygen is then allowed to expand slightly and this cools it even further turning it from a gas to a liquid. Much like steam when it hits something cold turns from a gas to a liquid.

Oh, and here's a good pilot joke: How many pilots does it take to change a light bulb? A: One, he holds the bulb in place and the whole world revolves around him.

We are saying the same thing on the pressure.
I agree with you on oxygen in an oxygen high pressure bottle!!! But we do not shoot in high pressure bottles..... If you increase the pressure of any gas in a pressure bottle, the temp will rise!!

We are getting way off topic.....

Let's shoot some shit!!


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[_Windrider_]

Please no one listen to this nonsense!!
Let's get down to the molecular level..... When molecules heat up, they move faster, thus making them more distant from each other, thus making the molecules per any given volume LESS..... Meaning, the HIGHER the temperature, the LESS the pressure.... Inversely measured.

Funny penis joke though!

Next question please! This is my favorite subject in aviation, which a bullet is a smaller form of an airplane! Drag, lift, Magnus theory are relevant to aviation and shooting bullets.....


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A density altitude greater than pressure altitude reduces the engines thrust and a wings lift thus requiring longer takeoff, amongst other things, due to decreased performance but I thought we were talking about something completely different here.

I was referencing the relationship of pressure (higher or lower) and temp from ISA, maybe I wasn't clear on that. Going from sea level up through the standard lapse rates, temp decreases and pressure decreases every 1000' ft right? Isn't that a direct relationship? Density changes come into play when the temp is non-standard (pressure altitude corrected for non standard temp).

Example: I'm at 1000' according to the ole' Garmin and the kestrel says my absolute pressure is 30.29 at 22 degrees C. What's my pressure altitude and density altitude?

pressure altitude = [(standard pressure - your current pressure setting) x 1,000] + field elevation

PA = [(29.92 - 30.29)(1000)] + 1000
= (-0.37)(1000) + 1000
= -370 + 1000
PA = 630

Now plugging that into:

density altitude = pressure altitude + [120 x (OAT - ISA Temp)]

DA = 630 + [120(22-14)]
= 630 + (120(8))
= 630 + 960
DA = 1590

So here we have higher pressure than standard with less dense air due to a higher density altitude caused by higher than standard temp.

My reference point is the deviation from standard atmosphere, or ISA. I think I left that out.

Perhaps my examples above were flawed, but I think we can agree that we were both right.

To the OP, sorry for any confusion. I should have left well enough alone, sometimes you can't help yourself and end up making it worse.

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[_Windrider_]

We are saying the same thing on the pressure.
I agree with you on oxygen in an oxygen high pressure bottle!!! But we do not shoot in high pressure bottles..... If you increase the pressure of any gas in a pressure bottle, the temp will rise!!

We are getting way off topic.....

Let's shoot some shit!!


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Oh thank god. Ignore my last post, you responded while I was doing mine and putting the kiddos to sleep.

I've been flying for 19 years. I still botch up the easy stuff.

Another pilot joke, how do you know who the pilots are at a party? Wait 2 minutes, they will tell you.


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[Ovrmydeadbdy]

A density altitude greater than pressure altitude reduces the engines thrust and a wings lift thus requiring longer takeoff, amongst other things, due to decreased performance but I thought we were talking about something completely different here.

I was referencing the relationship of pressure (higher or lower) and temp from ISA, maybe I wasn't clear on that. Going from sea level up through the standard lapse rates, temp decreases and pressure decreases every 1000' ft right? Isn't that a direct relationship? Density changes come into play when the temp is non-standard (pressure altitude corrected for non standard temp).

Example: I'm at 1000' according to the ole' Garmin and the kestrel says my absolute pressure is 30.29 at 22 degrees C. What's my pressure altitude and density altitude?

pressure altitude = [(standard pressure - your current pressure setting) x 1,000] + field elevation

PA = [(29.92 - 30.29)(1000)] + 1000
= (-0.37)(1000) + 1000
= -370 + 1000
PA = 630

Now plugging that into:

density altitude = pressure altitude + [120 x (OAT - ISA Temp)]

DA = 630 + [120(22-14)]
= 630 + (120(8))
= 630 + 960
DA = 1590

So here we have higher pressure than standard with less dense air due to a higher density altitude caused by higher than standard temp.

My reference point is the deviation from standard atmosphere, or ISA. I think I left that out.

Perhaps my examples above were flawed, but I think we can agree that we were both right.

To the OP, sorry for any confusion. I should have left well enough alone, sometimes you can't help yourself and end up making it worse.

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I agree with everything you said!
I think your DA would be 1710

ISA @1000= 13 Celsius. Assuming standard lapse rate.
DA=630+(120(22-13))
=630+(120(9))
=630+(1080)
=1710


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[_Windrider_]

I agree with everything you said!
I think your DA would be 1710

ISA @1000= 13 Celsius. Assuming standard lapse rate.
DA=630+(120(22-13))
=630+(120(9))
=630+(1080)
=1710


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I interpolated a bit if 2c per 1000 then 1c per 500 right? That where I got the 14 (it's standard temp at the pressure altitude)


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[Ovrmydeadbdy]

But at that field elevation (1000ft) the temp should be 13 to meet the ISA standard....regardless of PA.


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[_Windrider_]

But at that field elevation (1000ft) the temp should be 13 to meet the ISA standard....regardless of PA.


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You are correct. My mistake. I double checked and you should use ISA T your elevation not pressure altitude. Thx.

No more 0200 posts!


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[sandwarrior]

In spite of the 0200 post I think it's an excellent example of why you need to go through it. The situation you gave is not uncommon. If the OP were to simply look at PA he would raise his POA. When in fact when PA is converted to DA he should lower his POA.

My point in this is, you don't call it higher pressure or lower pressure, you call it higher altitude or lower altitude