Range Report Density Altitude

[JRose]

Just out of curiosity, I noticed a huge difference in Dens Altitude today from last time I shot.

Here's the conditions:

<span style="text-decoration: underline">25 Sept 2009 </span>---------------<span style="text-decoration: underline">16 Oct 2009</span>
Temp: 86.2-----------------------Temp: 65.8
Humidity: 68%---------------------Humidity: 71.7%
Dew Point: 76.1------------------- Dew Point: 56.4
Baro: 30.02----------------------- Baro: 29.91
Elevation: 10ft---------------------Elevation: 10ft
<span style="font-weight: bold">Dens Alt: 2092</span>---------------<span style="font-weight: bold">Dens Alt: 653</span>
Sunny / clear----------------------Partial clouds / cold front

Is this drastic drop in D/A normal, or did I get my measurements flucked up?

Second question, how would this drop affect my shooting at distance? I keep hearing about the importance of density altitude, but haven't been told what role it exactly play on the bullet in flight.

 

[Greg Langelius *]

Re: Density Altitude

Don't know; looking at he Baros, would expect the D/A's to be the reverse.

Some, not a ton. Going to 6000ft would be a lot more significant change.

 

[JRose]

Re: Density Altitude

I'm using a Kestrel NV4500. I'm assuming this cold front we have coming through had something to do with it????

 

[Greg Langelius *]

Re: Density Altitude

Honestly, I shouldn't have replied. I'm really not an expert...

 

[sobrbiker883]

Re: Density Altitude

I'm just learning, but looking at the cheat sheet I have from a DTAC 175SMK reticle you'd have about a minute more EL needed at 800 and 1.5 or so at 1K.

I just started looking at the DA factor when I went to the NorCal match and DA dropped 2K+ from morning to early afternoon (as we went from short lines to 1K on paper). Really had me scratching my head until I asked my shooting buddy if he remembered what the DA did that day, then it made sense.

 

[Lindy]

Re: Density Altitude

If you want to check the Kestrel calculation, go here:

http://wahiduddin.net/calc/calc_da_rh.htm

The calculations are pretty close.

The primary determinant there is the air temperature.

As to the magnitude of the effect, bear in mind that a 1000 foot change in elevation is about equal to 1 inch of mercury for the air pressure, which is about 3 percent.

At short distances, that's not much effect, but at longer distances, it's considerable.

 

[sobrbiker883]

Re: Density Altitude

J-check your email!

 

[JRose]

Re: Density Altitude

So DA is mostly affected by air temp?

Thanks for the info Steve, I'll print it and read it tomorrow at work!

Sometimes I feel in over my head. I want to wrap my brain around this stuff.

 

[Lindy]

Re: Density Altitude

DA is most sensitive to barometric pressure, but in your case, the BP was about the same and you were at the same altitude - it was the temperature which had changed.

 

[JRose]

Re: Density Altitude

So in DA terms, what's considered a "big enough" fluctuation to worry about? If I were at RO shooting, and I had a similar fluctuation from Monday to Tuesday as I did here (roughly 1500), would I need to adjust my dope?

 

[sobrbiker883]

Re: Density Altitude

Past 600yds a 1.5-2K shift will have an effect you may want to look out for, like I stated wehad a 2K shift from morning to afternoon that had a lot of guys hitting a lot lower than they were planning on once we got to 800-100 yards.
F-Class and HP guys get spotters, so they can adjust on the fly, but when you shoot 5 shots with no feedback it can really crimp your style. The guys watching their DA line to line did a lot better than us "seat of the pants" dopers.

 

[Shady_Jay]

Re: Density Altitude

I use Brand Cole's DA cards with a Kestrel 4500 religiously.

It's good JuJu.

 

[JRose]

Re: Density Altitude

Where do us non-Texans find that good juju???

 

[427Cobra]

Re: Density Altitude

I calibrate my 4000 from 737 altimeters, then at the I tweek the alititude at the range from my GPS

 

[Shady_Jay]

Re: Density Altitude

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JRose</div><div class="ubbcode-body">Where do us non-Texans find that good juju??? </div></div>

I thought you said you were banned?

 

[Shady_Jay]

Re: Density Altitude

Call me tomorrow.

 

[JRose]

Re: Density Altitude

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Captain Kick-Ass</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JRose</div><div class="ubbcode-body">Where do us non-Texans find that good juju??? </div></div>

I thought you said you were banned? </div></div>

I'm too awesome to stay banned. Frank set me free.


I'll give you a ring-a-ling tomorrow.

Thanks everybody!

I got some reading to do tomorrow.

 

[Lindy]

Re: Density Altitude

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">If I were at RO shooting, and I had a similar fluctuation from Monday to Tuesday as I did here (roughly 1500), would I need to adjust my dope?</div></div>

Depends on how far you're shooting.

Keeping in mind that a 1500 foot change in density altitude corresponds approximately to an inch and a half change in barometric pressure, stick that change in your favorite ballistic program, and see how it changes your dope at various distances.

For that matter, stick in the parameters you initially posted in this thread, and see how that changes your dope at various distances.

<span style="font-style: italic">Don't get all hung up on the concept of density altitude.</span> DA is just a single-number calculation of air density, using the same inputs of temperature, pressure, and humidity that every ballistic program uses as inputs.

And if you're tweaking the reference altitude in the barometric pressure window of the Kestrel, you probably shouldn't be doing that. Read:

http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/Baro/

And for those who want to make their own DA table, you can read this:

http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/DA/

And if you want to save yourself some trouble, JBM will do all the work for you:

JBM DA Trajectory Cards

 

[BOLTRIPPER]

Re: Density Altitude

i have seen guys miss shots using day old dope due to a subtle DA shifts at distance.....its a match spoiler if you are not up on weather changes.

 

[Lindy]

Re: Density Altitude

Once upon a time at a Rifles Only match, I was running the 1000 yard stage. After everyone had shot, and after watching shot after shot hitting short, I stood up on the tower, and asked, loudly, "Did anyone check the barometric pressure?"

IIRC, it was 30.21 inches. There were several slapped foreheads, groans, and one guy who answered, "Yeah - after I shot."

It's not the highest I've seen down there at 72 feet above mean sea level. One cold winter day, I saw 30.76 inches.

 

[JLM]

Re: Density Altitude

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Captain Kick-Ass</div><div class="ubbcode-body">I use Brand Cole's DA cards with a Kestrel 4500 religiously.

It's good JuJu. </div></div>

Brand's cards are quite handy I must say.

 

[desertHK]

Re: Density Altitude

I am a firm believer in the DA stuff. Just to give you an example, where I shoot, depending on the baro, temp, and all of that stuff, the DA can change up to 2K easily. So a 1000 yards shot last week only needed 24 MOA come up, becomes a 27 MOA come up. That's why I make my own DA table for my 308 and 338LM. This is even more obvious when you go beyond 1200 yards. I took Lindy's advise and make up the DA from -4000 to 10,0000 for all the ranges for that particular cartridge. I then use my Kestrel to get the DA. Refering back to my table that's whay I use for that particular day for my elvation doping. The wind is another factor though.

 

[Pete E]

Re: Density Altitude

Sorry if this a dumb question, but if you input temp, actual altitude, and real time air pressure into a balistic program, won't it calculate any correction needed as part of its normal calulation? Or are PDA style Balistic Calculators not allowed at matches?

Regards,

Peter

 

[427Cobra]

Re: Density Altitude

Yes

 

[marc357]

Re: Density Altitude

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Pete E</div><div class="ubbcode-body">Sorry if this a dumb question, but if you input temp, actual altitude, and real time air pressure into a balistic program, won't it calculate any correction needed as part of its normal calulation? Or are PDA style Balistic Calculators not allowed at matches?

Regards,

Peter</div></div>

Sure,

A PDA will do all that, but-

(1) It's a battery-powered electronic device, which means it, or its batteries will go tits up when you need them the most.

and

(2) It's slower. I have laminated cards for every 1000" of D/A, all I have to do is flip to the right card, dial my dope, and shoot. Bang. I'm done, grabbing my shit and moving out, while other folks are still fiddle-farting with their PDAs. Sometimes you have all the time in the world to get your shit together and take your shot.... but sometimes you don't

I have a PDA, and I always carry it, but I rarely pull it out, because the cards are faster and simpler. More often than not, I pull it out to help someone who's "lost", doesn't have dope for their rifle, or whatever.

And no- it's not a dumb question

Marc

 

[Pete E]

Re: Density Altitude

Marc,

I can understand being leery about batteries and electronics, but surely you're relying on these to get your climate related input data if you want them "real time" so to speak anyway???

BTW I am not critizing here, but beeing new to all this, I'm trying to get, my head around how all this fits together and what, if any, electonics I need..

My Garmin 60CSX gives me pressure and altitude, but not temperature and obviously not wind speed but that can be estimated I suppose.

Regards,

Peter

 

[Lindy]

Re: Density Altitude

One thing a printed DA table will not do as quickly as a PDA is to calculate the effect of temperature variations on muzzle velocity. That variation may be significant at longer distances, depending on the powder in use.

I carry a PDA, and use the DA table as a backup.

 

[marc357]

Re: Density Altitude

Pete:

Actually, I'm not leery of electronic gadgets, I usually embrace them. Just been scrod by them enough times to know they f' up when you need them most. Yes, I have a Kestrel that I use to get the D/A, but I also have a chart to compute D/A, all I have to know is physical altitude, and temp. It's not perfect, but it's plenty close for the kind of shooting most of us are doing.....

And to be honest, when you pay attention to D/A for awhile, you start to get a feel for it. I always guess what it is before I pull out the Kestrel, and I am usually pretty close.


Lindy:

You're right about that. I knew I would need to find a way to get around that, if I was going to go "battery-less", so I am using Varget in the .308, and H-1000 in the .300WinMag, they are (as you know) both part of Hodgdon's "Xtreme" Line, which are supposed to be very temp. stable, and they seem to be. My elevation numbers are pretty spot on, regardless of temperature. It's that darn wind that's a bear, LOL

Marc

 

[Victor N TN]

Re: Density Altitude

Before the days when weather electronics made the Density Altitude calculations for you, I kept a notebook in the top of my shooting box. Just rechecked the DA between relays as time allowed and recorded it in the composition book.

The Kestrel 4000 or 4500 is on my wish list for next year. My old SkyMaster is several generations (and functions) behind.

Yes, I'm a little behind the state of the art.