Density Altitude and trajectory

[*AimSmall*]

Ive been playing with JBM ballistic quite a bit latley and never realized how dramatic affect DA had on your trajectory. I ran the numbers of sierra 175gr hpbt doing 2588 out of my 20" ar10 and it shows going transonic at around 1100yards. Elevation of 1400'
DA 28.68
61 deg

If I go 3 hours up the road the alttitude changes to 6990 and the DA is 23.36 @33 deg. It says I dont go transonic until just under 1800 yards. Is this right? Did I do this right? I am new to long range and still trying to learn this.

I sure appreciate all the knowledge and help this sight has been. I appreciate the online training amd the Everyday Sniper podcast. Thankyou

 

[tnichols]

DA is huge. Playing with a ballistic calc as you're doing will help you to understand the limitations of your system. Now when you see questions posed about "effective range for a 308" posted here, DA really makes a huge difference. What a guy in Vegas can do with a 24" 308 is a LOT different than what I can do with a 20" 308 in Iowa.

The worst DA I've shot in here is -3400'

 

[ShtrRdy]

What you're calling Dendity Altitude is actually Air Pressure. Density Altitude is a number that combines Air Pressure, Temperature, and Humidity.

As far as going sub-sonic, you have to begin to be concerned when the bullet begins the Trans-sonic region. This is a little higher than Mach 1.

 

[shoot_steel17]

I just want to bring one thing up. You mentioned that your density altitude is 28.68. DA is a reference altitude.

Your ballistic calculator needs to know the air density. Factors of air density are temperature, pressure, and humidity. When calculating pressure you can use barometric pressure which is the pressure corrected to sea level, and must be used with altitude, or station pressure which is the pressure measured where you stand.

Another method of calculating the air density is to use Density Altitude. The ballistic computer takes a reading on the factors mentioned above, and sets them to a standard day. The ballistic computer will give you an altitude that you are "simulating" shooting at on a day in standard conditions. DA is good to keep track of as the constant changing components of air density can be confusing, but with DA it's easy to recognize when the atmosphere is getting thinner or thicker. When using DA just make sure to include temperature in your calculations, as the speed of sound changes with the air temperature, and should be accounted for. One can also go down the rabbit hole of temperature and muzzle velocity as well. Maybe it was just a typo and if it was my bad, just wanted to bring it up because sometimes its easy to mix them up. Hope this helps.

 

[*AimSmall*]

What you're calling Dendity Altitude is actually Air Pressure. Density Altitude is a number that combines Air Pressure, Temperature, and Humidity.

As far as going sub-sonic, you have to begin to be concerned when the bullet begins the Trans-sonic region. This is a little higher than Mach 1.

I thought the air pressure is the "barometric pressure"? The BP is 30.27 inHg. 38 deg F, 26 deg dewpoint and an elevation of 6990. Making the density alttitude of 23.36? I dont have a kestrel so im just inputing the numbers into an online DA calculator.

 

[shoot_steel17]

Making the density alttitude of 23.36?

It takes pressure, temperature, and humidity and gives you an altitude. 23.36 is a pressure measurement. Should say something like 2000 ft which is simulating shooting at 2000 ft on a standard day.

 

[PinesAndProjectiles]

It takes pressure, temperature, and humidity and gives you an altitude. 23.36 is a pressure measurement. Should say something like 2000 ft which is simulating shooting at 2000 ft on a standard day.

^This. My Kestrel only outputs DA as a whole number. 23.36 seems rather low for pressure, though. Where are you located?

Here in Nebraska, I'm usually 28.8 or 28.9.

 

[*AimSmall*]

I just want to bring one thing up. You mentioned that your density altitude is 28.68. DA is a reference altitude.

Your ballistic calculator needs to know the air density. Factors of air density are temperature, pressure, and humidity. When calculating pressure you can use barometric pressure which is the pressure corrected to sea level, and must be used with altitude, or station pressure which is the pressure measured where you stand.

Another method of calculating the air density is to use Density Altitude. The ballistic computer takes a reading on the factors mentioned above, and sets them to a standard day. The ballistic computer will give you an altitude that you are "simulating" shooting at on a day in standard conditions. DA is good to keep track of as the constant changing components of air density can be confusing, but with DA it's easy to recognize when the atmosphere is getting thinner or thicker. When using DA just make sure to include temperature in your calculations, as the speed of sound changes with the air temperature, and should be accounted for. One can also go down the rabbit hole of temperature and muzzle velocity as well. Maybe it was just a typo and if it was my bad, just wanted to bring it up because sometimes its easy to mix them up. Hope this helps.

So when Im putting in pressure in JBM its not absolute pressure?

 

[tnichols]

Barometric pressure is corrected for altitude (height above sea level) its what is commonly used by pilots, weather folks on the news, etc... Absolute pressure is not corrected for altitude. Most calcs can use either, BUT, you have to make sure the calc knows which you're using. It's commonly in the settings portion of the calc.

 

[*AimSmall*]

^This. My Kestrel only outputs DA as a whole number. 23.36 seems rather low for pressure, though. Where are you located?

Here in Nebraska, I'm usually 28.8 or 28.9.

Im in AZ.

 

[ShtrRdy]

Air Pressure can be thought of as Barometric Pressure or Absolute/Station Pressure. Barometric Pressure is referenced to Sea Level, so if you use Barometric Pressure you have to enter at what altitude.

Absolute/Station Pressure is the raw Air Pressure where the reading is taken.

Density Altitude is a combination of Air Pressure, Temperature, and Humidity. It's basically a "altitude" number under Standard Conditions. Put another way it's the Air Density at the altitude which is at Standard Conditions.

 

[tnichols]

I see in the screenshot from JBM, it says "Pressure Is Corrected" which if I was a betting man, it wants BP. In your second example, you may have made and entry error as that pressure is extremely low, like hurricane low, that is if it's BP

 

[PinesAndProjectiles]

That's strange I've never seen DA be given as anything other than an altitude. I'm normally located in Southern California and DA is normally around 1500 - 2000 ft where I shoot if I remember correctly.

My bad. BP is usually 28.9 or 28.9
DA ranges from -400 to 4000.

 

[shoot_steel17]

I see in the screenshot from JBM, it says "Pressure Is Corrected" which if I was a betting man, it wants BP. In your second example, you may have made and entry error as that pressure is extremely low, like hurricane low, that is if it's BP

I agree with tnichols because it's asking for your elevation and altimeter setting, which would be your BP. Then it is calculating your absolute pressure and DA for you.

 

[*AimSmall*]

To be honest I don't use JBM, but you should be able to input either absolute pressure, or barometric pressure with an altitude, and temp and humidity. Or you can input DA and temp. It is probably just calculating the DA for you after input pressure, temp, and humidity.

Lol so i was all screwed up. This would be more like it then?

 

[*AimSmall*]

So pressure would be BP. And when it asks if alttitude is corrected, if yes, that would be DA?

 

[*AimSmall*]

I thought I heard Frank say that your pressure drops 1" for every 1000ft above sea level?

 

[shoot_steel17]

So pressure would be BP. And when it asks if altitude is corrected, if yes, that would be DA?

That looks good to me if your putting in BP, and you would say yes because BP is corrected to sea level. If you input station or absolute pressure you would say no. DA is calculated for you after you input the pressure temp and humidity.

 

[*AimSmall*]

I see in the screenshot from JBM, it says "Pressure Is Corrected" which if I was a betting man, it wants BP. In your second example, you may have made and entry error as that pressure is extremely low, like hurricane low, that is if it's BP

You are right for sure haha thanks for the help.

 

[shoot_steel17]

I thought I heard Frank say that your pressure drops 1" for every 1000ft above sea level?

That's right so if the pressure is 29.92 inches of mercury and you increase in elevation by 1000 ft then it will be about 28.92 inHG

 

[tnichols]

That's right so if the pressure is 29.92 inches of mercury and you increase in elevation by 1000 ft then it will be about 28.92 inHG

You may be mixing BP with SP there. Frank uses SP I believe.

 

[*AimSmall*]

You may be mixing BP with SP there. Frank uses SP I believe.

This makes sense. And I was using SP as BP Thankyou all for the clarification

 

[shoot_steel17]

You may be mixing BP with SP there. Frank uses SP I believe.

You're right, that is BP not SP, my bad.