I am watching a local race that is being streamed, and was thinking, I have never really figured what this is. On 1320go they of course have the weather data on the timeslip page.

As an example, the track is at 2306 feet altitude. On the one recent pass, Standard Relative Air Density is 90.90% and the effective altitude is 3248 feet. But Dividing 2306 by .909 only gives 2329. What does the 90.90% actually mean in regards to the correct altitude? I am sure it is a good number to use in predicting ETs but doesn't make sense to me, as to how it is calculated.

This is way oversimplified, but it's sort of like Density Altitude, where 0 feet represents 'perfect' air and a Density Altitude of 3500 feet says that the current conditions are the same as otherwise perfect conditions but at an altitude of 3500 feet. It's a way to make a standard reference.

A Relative Air Density of 100% is similar to a Density Altitude of 0 feet. That's ideal. The 90.90% you see is saying that the air is about 9.1% less dense than optimal.

In my data, Density Altitude and Relative Air Density are 99.5% the same thing. They just move in opposite directions. When DA goes up, the RelAD goes down.

The #1 prediction factors for my stuff are moisture based: Dew Point, Grains, Vapor Pressure.

The 'all-in-one' items such as Standard HP Correction Factor and even DA are pretty good. Relative Air Density is not as good as those but the car follows it OK.

that is what I figured, 90% of base, but the math didn't make sense unless I calculated it wrong.

if 2306 is 100% then shouldn't 90% be a little over 2500?

since I am on gas I will stick with DA then since I understand that one

is the relative air a more useful number when entered into prediction software maybe?

It's not just expressing DA as a percentage. There is more to it than that. It's a different measure than DA, but similar concept.

As I stated, my car would be better off using DA than Relative Air Density if I was only using one single metric. I am on alcohol and your mileage may vary.

maybe this will help you in the future....

https://airdensityonline.com/free-calcs/

quote:

Originally posted by TD6297:
that is what I figured, 90% of base, but the math didn't make sense unless I calculated it wrong.

if 2306 is 100% then shouldn't 90% be a little over 2500?

since I am on gas I will stick with DA then since I understand that one

is the relative air a more useful number when entered into prediction software maybe?

There is also another problem with your theory/formula. The Air Density number doesn't know that you are at 2306 feet. It is comparing the current air conditions to dry air at sea level, not to perfect air at your current altitude.

quote:

Originally posted by J178RED:
maybe this will help you in the future....

https://airdensityonline.com/free-calcs/

Looks decent but I think racers will always want to leave the Elevation at 0. The calculation on the site is using Elevation to convert a 'corrected' barometer reading to uncorrected, but uncorrected is what we are measuring with our weather stations.

quote:

Originally posted by Lenny5160:

quote:

Originally posted by J178RED:
maybe this will help you in the future....

https://airdensityonline.com/free-calcs/

Looks decent but I think racers will always want to leave the Elevation at 0. The calculation on the site is using Elevation to convert a 'corrected' barometer reading to uncorrected, but uncorrected is what we are measuring with our weather stations.

Correct. Many calculators assume you know physical altitude because they are derived from aviation formulas, where the elevation of an airport is a known fact.

quote:

Originally posted by Lenny5160:


since I am on gas I will stick with DA then since I understand that one


There is also another problem with your theory/formula. The Air Density number doesn't know that you are at 2306 feet. It is comparing the current air conditions to dry air at sea level, not to perfect air at your current altitude.

ahhh ok. So it is not a comparison of the conditions relative to the location, but relative to sea level. That makes sense now. Thanks!