README

Density Altitude calculator class for PHP based on formulas published by the U.S. National Weather Service.

You know that warm air is less dense than colder air - this is the principle that makes hot air balloons fly. Humidity, air pressure and temperature can all change the air in a way that makes altitude seem higher or lower than it actually is. When the air is thinner because the temperature is warmer, we say that the "density altitude" is higher. It is although you were actually at a higher altitude on a standard day.

Density altitude can affect the oxygen intake to engines and athletes, the performance of aircraft, and even beer brewing and cake baking.

DANGER!

Do not use this class for safety-sensitive applications:

Do not use for flight planning, operation or navigation
Do not use for the diagnosis, prevention, or treatment of any medical condition
Do not use in nuclear facilities
Do not use in any situation where failure or inaccurate information could lead to injury, damage to property, loss of life, or catastrophe

You must indicate your understanding of this disclaimer when calling the class

Use

To calculate density altitude, we need to know the actual altitude, the temperature, the dewpoint, and the air pressure.

The class can accept inputs in several common units.

The class will output the density altitude in either feet or meters depending on the units used to set the altitude initially.

Inputs may be in any order.

$calculator = new DensityAltitude();
$calculator->altitudeInFeet(998.4);
$calculator->temperatureInC(27);
$calculator->dewpointInC(14);
$calculator->altimeterPressureInInchesHg(30.28);
$calculator->DANGERthisClassNotIntendedForSafetyCriticalUses('understood');
echo round($calculator->calculateDensityAltitude()); // 2225

You must include the method acknowledging the safety considerations of this class. This is required so that future maintainers of your code are aware of the class's limitations and do not blindly copy the code to more critical applications.

Notes

This class is not certified. This class has no engineering or scientific review.

This class was not developed by a physicist, meteorologist or engineer - nor has it been tested or reviewed by one.

Notes on the formulas used

The developer is a private pilot, so has some clue about density altitide. The formulas are based on https://www.weather.gov/epz/wxcalc_densityaltitude from the National Weather Service, apparently by Tim Brice (tim.brice@noaa.gov) and Todd Hall (todd.hall@noaa.gov). The formulas are referenced in PDF from https://www.weather.gov/media/epz/wxcalc/densityAltitude.pdf and https://www.weather.gov/media/epz/wxcalc/pressureConversion.pdf These formulas have a lot of steps, multiple mixed units, no references or explanations.

Wikipedia's article on density altitude, https://en.wikipedia.org/wiki/Density_altitude , references three different formulas, none of which have citations.

Wikipedia notes that the NWS standard should round to the nearest 100 feet. The documents from the NWS web site do not say this, but also give no indication of significant figures.

When comparing the formula and class to FAA AWOS reports, the 100 foot rounding seems to give correct results.

Other formulas and calculators appear to give much different results. I don't have a clear enough understanding of density altitude to make judgements on why other reports and calculators produce significantly different results.

https://airdensityonline.com/free-calcs/ is an example of another calculator that seems more knowledgable than me, but gives pretty different answers.

Notes on testing

Even though I am a pilot, I will not rely on this class for navigation - I will instead rely on the Density Altitude charts, official reports, or the E6B flight calculator.

To test, I compared values computed to ASOS reports from airports and the NWS JavaScript calculator referenced above.

I changed input values to make sure the calculated values moved in the correct direction (nothing wired backwards).

I tested examples from flight training materials I own.

Contributions

I welcome contributions - especially bug reports and criticism.

I would like for this class to someday be trustworthy enough for actual flight operations, but I do not believe this is something that one person can do on their own - it's too easy to overlook mistakes.

rabollig / density-altitude

Maintainers

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