How to Calculate the Density Altitude

don_w

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First, let's define what density altitude is. The density altitude (DA) is the altitude at which the density of the International Standard Atmosphere (ISA) is the same as the density of the air being evaluated. (The Standard Atmosphere is simply a mathematical model of the atmosphere which is standardized so that predictable calculations can be made.) So, the basic idea of calculating density altitude is to calculate the actual density of the air, and then find the altitude at which that same air density occurs in the Standard Atmosphere.

There are many ways to calculate the DA; the easiest of course is to buy a handheld weather station that does all the work for you. But, most of us don't have one of those. So, I'll show you in this post how I calculate DA.

Basically, you need to know four pieces of information. The actual track elevation, the ambient air temperature, the barometric pressure, and the humidity. Once you have that info, you run it through a calculator to arrive at the DA.

The DA calculator I use is this one: http://wahiduddin.net/calc/calc_hp_dp.htm. Note the units which are required: Temp is in degrees F, barometric pressure is in inches of mercury (Hg), the humidity is expressed as the dew point (in degrees F), and the track elevation (altitude) is feet (above sea level). If you don't know the track elevation, try locating it using Google Earth... it will give you the elevation.

I have been using this website, http://www.wunderground.com/, to get my weather data. I identified a weather station as close to the track as possible (Google Earth can also help you there), and then select the date that I was at the track. For example, here is what I would have used had I been at Barona drag strip today: http://www.wunderground.com/weatherstation/WXDailyHistory.asp?ID=KCARAMON8&month=4&day=16&year=2008

If you scroll down, you'll see weather data has been posted every 5 minutes or so. I use the time of day recorded on the ET timeslip, and pick the data point closest to the time of each run. The next three columns are the temp, dew point and pressure, and they are in the correct units for the calculator I linked above.

Input the four required data fields into the calculator. Let's say I made a run at 4:00 today (16:00). The temp was 73.9*, the dew point was 45.3*, and the pressure was 29.96 inHg. The elevation at Barona is 1,700 feet. When I click the "Calculate" button, I see down below that the DA was 3,140 feet.

That's it!! I hope this helps some of you.

And if any of you have other methods, by all means, post them in this thread, too.
 

andyman

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Thanks for all the information... i was always confused on a couple parts of the various calcs like the Hg stuff. but wunderground is a kickin site, i've been using that one for who know how long. it's nice to know the temp of the sea water before i go wading in it :)
 

Boss281

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Beautiful summary. I had trouble following all this in my thread about where to find minute by minute historical weather data, and this puts it all in one place. Bookmarked!

John
 

Rygen

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Thanks for this, very well done.
 

ArtQ

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Don, good stuff for DA calculations.

http://www.smokemup.com/auto_math/index.php#edrag

This link has a great selection of calculators.

My favorite is the Auto lookup calculator.

Input the date and time of your run, select the track from a comprhensive list of tracks accross the country, plug in your time and it will give you DA, temp, alt setting/baro pressure and track elevation. It will also automatically correct your time to standard day conditions (Sea level, 60 deg, 29.92, 15.7)
If your track is not listed just send a PM and it will be added to the list. It will cost you a few bucks a month....3.00 to 5.00, forgot to pay attention last time, but it is really worth the info for the minimal cost. I'm pretty sure if you go a month without using it , they don't charge you.

EXAMPLE RESULTS:

Time recorded 10:54 AM
Temperature °F 55.0
Dew Point °F 45.0
Altimeter Setting 30.08 Inches Mercury
Absolute Pressure: 29.5 Inches
Density Altitude: 370.7 feet
Track Elelvation: 548 feet​

UnCorrected ET:
12.01 (sec) @ 112.17 (MPH)​


Corrected ET to Sea Level:
11.963 (sec) @ 112.617 (MPH)​


dot_clear.gif
dot_clear.gif
To use this calculator enter the date, time, select dragstrip, ET, and trap speed. This calculator will look up the weather information for the date you entered from the closest airport to the dragstrip you select. Based on these weather conditions the calculator will calculate the density altitude and correct the timeslip to sea level.
Don't see your dragstrip listed? Send me PM with track name, state, and track elevation. Be patient we plan to add in all the dragstrips in the USA shortly.
 
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don_w

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That's pretty cool, Rench.

The only downside I see offhand is this, "This calculator will look up the weather information for the date you entered from the closest airport to the dragstrip you select". In some cases, that might be huge discrepancy in weather data.
 

ArtQ

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The important thing is it uses the track elevation for the calculations and not the airport elevation. The variables in the weather in say a 50 mile radius would be minimal. This would cover the majority of tracks. The other plus is if you always use the same data it makes it easy to track mod results and car performance all year long regardless of DA variance. You can also plug in a set of numbers before leaving for the track to get weather and DA info. When you consider a 150' change in DA is worth approx .01 in ET this puts you well in the ball park.
 

don_w

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The variables in the weather in say a 50 mile radius would be minimal.
Granted, that might be true in most places, but not here in Socal. It could be 65* at the coast, and 115* in the desert 50 miles inland. Hell, even 5 to 10 miles can be a 10* - 20* difference on most days.

But, for anywhere that has little to no weather variability compared to the airport, that looks like a kick ass method!!
 
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TGJ

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First, let's define what density altitude is. The density altitude (DA) is the altitude at which the density of the International Standard Atmosphere (ISA) is the same as the density of the air being evaluated. (The Standard Atmosphere is simply a mathematical model of the atmosphere which is standardized so that predictable calculations can be made.) So, the basic idea of calculating density altitude is to calculate the actual density of the air, and then find the altitude at which that same air density occurs in the Standard Atmosphere.

There are many ways to calculate the DA; the easiest of course is to buy a handheld weather station that does all the work for you. But, most of us don't have one of those. So, I'll show you in this post how I calculate DA.

Basically, you need to know four pieces of information. The actual track elevation, the ambient air temperature, the barometric pressure, and the humidity. Once you have that info, you run it through a calculator to arrive at the DA.

The DA calculator I use is this one: http://wahiduddin.net/calc/calc_hp_dp.htm. Note the units which are required: Temp is in degrees F, barometric pressure is in inches of mercury (Hg), the humidity is expressed as the dew point (in degrees F), and the track elevation (altitude) is feet (above sea level). If you don't know the track elevation, try locating it using Google Earth... it will give you the elevation.

I have been using this website, http://www.wunderground.com/, to get my weather data. I identified a weather station as close to the track as possible (Google Earth can also help you there), and then select the date that I was at the track. For example, here is what I would have used had I been at Barona drag strip today: http://www.wunderground.com/weatherstation/WXDailyHistory.asp?ID=KCARAMON8&month=4&day=16&year=2008

If you scroll down, you'll see weather data has been posted every 5 minutes or so. I use the time of day recorded on the ET timeslip, and pick the data point closest to the time of each run. The next three columns are the temp, dew point and pressure, and they are in the correct units for the calculator I linked above.

Input the four required data fields into the calculator. Let's say I made a run at 4:00 today (16:00). The temp was 73.9*, the dew point was 45.3*, and the pressure was 29.96 inHg. The elevation at Barona is 1,700 feet. When I click the "Calculate" button, I see down below that the DA was 3,140 feet.

That's it!! I hope this helps some of you.

And if any of you have other methods, by all means, post them in this thread, too.

I use this info to calculate the D.A. as it is more accurate than what I had used in the past.

Thanks Don...

We have one and I think it's very accurate. We also have a horsepower calculator based on MPH and weight which I believe is very accurate.


http://www.modulardepot.com/?show=resources


I use DA Correction time calculator on ModDepot. I used it for quite some time though.

Thanks Ken...
 

don_w

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I found a site that has the weather and calc all on one page,
Very easy to use...

Click here --->: Dragtimes.com Density Altitude Calculator
Nice find!!

The biggest drawback I see for this though, is that it says "Weather data is obtained from an airport near the track." Like I said above, that might be OK in most places, but not here in Socal. It could be 65* at the coast, and 115* in the desert 50 miles inland. Hell, even 5 to 10 miles can be a 10* to 20* difference on most days. I tested it for Barona, and it assumed the elevation was 13' (like the airport), but Barona is at 1700'. And the weather is totally different at the track than it is at the airport.

But, for anywhere that has little to no weather variability compared to the airport, that looks like an easy way to go!! I also think it would be better if the weather data was more frequent than 1-hr intervals.
 

Cdvision

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Nice find!!

The biggest drawback I see for this though, is that it says "Weather data is obtained from an airport near the track."
That's awesome for our track as it literally backs on to the Edmonton International Airport.
 

james432

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is Track Elevation 632 feet above sea level good or bad....because i can only get 12.90s with my car i see people running 12.50s with the same stuff but i think my 20 inch dr are hurting me..this winter im getting holeshots.....
 

stkjock

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gotta love the NJ tracks.... all elevations are below 100', and DAs of -1000 to -1500 in the late fall are common!!!
 

s8v4o

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Does anybody have more info or experience with these little devices. They can measure DA and a variety of other data. Seems very cool nonetheless http://www.kestrelmeters.com/Kestrel-4500-Weather-Meter.pro


Features

  • Digital compass provides direction and stores data with wind speed
  • Automatic Crosswind calculator
  • Headwind/Tailwind speeds
  • Current, Maximum and Average Wind Speeds
  • Air, Water & Snow Temperature
  • Wind Chill
  • Relative Humidity
  • Dewpoint
  • Heat Stress Index
  • Barometric Pressure
  • Altitude Please go to barometer and altitude help for more information.
  • Density Altitude
  • Wet Bulb Temperature
  • Graph and recall trends
  • Easy to read, backlit display
  • Time and date
  • Customize screens to display user-selected measurements
  • Flip-top impeller cover allows use of other functions while protecting the impeller (hard cases available.)
  • Automatically store measurements, even when the unit is turned off
  • Manually store measurements with the press of a button
  • Chart up to 2,000 measurements
  • Impeller can be replaced without tools
  • Exterior temperature, humidity, and pressure sensors for fast and accurate readings
  • Humidity sensor can be recalibrated in the field with our Relative Humidity Calibration Kit.
  • Includes neck and wrist lanyards, protective pouch and 2 AAA batteries
  • Waterproof and floats.
  • 5-year warranty
  • Assembled in the USA
 
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