don_w
Dyno Numbers - Who Cares?
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.
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.