HP Forums

Full Version: Density Altitude Equation on HP 35s
You're currently viewing a stripped down version of our content. View the full version with proper formatting.
For fun I hammered out a density altitude app in Java and decided to change things up so it works on the HP 35s. I used the NOAA calculators and associated formulas from here: http://www.srh.noaa.gov/epz/?n=wxcalc

Here is the EQN formula for the HP 35s...

D = 145442.16 * (1 - ((17.326 * B * ((288 - 0.0065 * (A / 3.2808)) / 288) ^ 5.2561 / ((9 / 5 * ((5 / 9 * (T - 32)) + 273.15 / (1 - 0.379 * ((6.11 * 10 ^ ((7.5 * (5 / 9 * (W - 32))) / (237 + (5 / 9 * (W - 32))))) / (33.8638816 * B))) - 273.15) + 32) + 459.69)) ^ 0.235))

D = Density Altitude
B = Barometer(inHg)
A = Station Altitude
W = DewPoint(F)
T = Temp(F)

Solve for D (Density Altitude)...
B = 30.11
A = 5130
W = 39
T = 89
Result for D = 8040.64

In the formula I’m calculating Vapor Pressure, Virtual Temperature, and Station Pressure to get the Density Altitude.

I seem to get a very slight variance on the 35s vs the NOAA online Density Alt calculator but still its very accurate.
Very cool! Putting this equation on my 35S is now on the list of things to do.
Yep, must add that to my aviation suite. Strictly for interest as the takeoff and landing data on the b777 is computed on board.

But leaving Bogata, altitude of airport is 8000 feet on a hot day (40'C) with a low pressure 29.80 it would be interesting to see what the density altitude is. Also comparing these answers to my circular JEPP cr3 which has the log curve built in to the display.

Thanks, but what are those brackets things (((()))))?

I dropped this into the 50G equation solver and D=8040.64949872?
There's a much simpler version on Wikipedia: https://en.wikipedia.org/wiki/Density_altitude (though I haven't tested it as I know nothing about the subject)

It's also generally much simpler for people to stick to standard international units instead of laboriously converting to and from their local units that the rest of the world knows nothing about.

The calculation on Wikipedia is okay but it assumes dry air therefore your Density Altitude will indicate lower than actual. We are calculating humidity (water vapor) indirectly by using the dew point. Humidity displaces air which decreases lift which results in a higher density altitude. That said formulas which assume dry air are not specifically dangerous to use given temperature is the primary driver. If using a dry air Density Altitude formula results in an aircraft's performance to be at threshold then one might assume calculating humidity will be more definitive in finding that threshold, and it will. But on those hot humid days its better to leave the plane in the hanger and do something else until temps cool down.
Reference URL's