Function calculate_density_altitude 

pub fn calculate_density_altitude(
    _altitude_ft: f64,
    pressure_inhg: f64,
    temp_f: f64,
) -> f64

Calculate density altitude from environmental conditions

MBA-643: Fixed to interpret pressure as STATION PRESSURE (actual local pressure), not altimeter setting (sea-level corrected). This matches how weather stations and most ballistic tools report pressure.

Pressure altitude follows the published NWS station-pressure equation: PA = 145366.45 * (1 - (P_hPa/1013.25)^0.190284) https://www.weather.gov/media/epz/wxcalc/pressureAltitude.pdf

DA = PA + 66.7 * (OAT_F - ISA_temp_F)

_altitude_ft is ignored, deliberately

The first parameter is unused and has been since MBA-643 made pressure_inhg a station pressure: station pressure already encodes the altitude, so taking one again would be taking it twice. The parameter survives only because removing it is source-breaking for callers. Passing a different altitude changes nothing, which is worth knowing before spending time on a discrepancy it cannot explain.

Humidity is not an input, for compatibility

⚠️ NOT AN OVERSIGHT. This is the FAA-rule figure the engine has always printed, and crate::atmosphere::resolve_atmosphere_for_density_altitude (MBA-1366) is its exact algebraic inverse — the entry mode that lets a shooter type a single density altitude instead of altitude + pressure + temperature. That entry mode takes a temperature but no humidity (the run’s humidity is applied separately), so a humidity term here would change what every previously entered density altitude resolves to, as well as every printed card. It is not a mathematical necessity: a humidity-aware forward could be inverted given the run’s humidity, the way temperature already is. main.rs’s density_altitude_round_trips_through_the_dope_card_formula* tests are what pin the pair together; a change to the maths here must keep them passing.

The TRAJECTORY is unaffected by this: the solver’s air density comes from crate::atmosphere::calculate_air_density_cimp (CIPM-2007) and its speed of sound from crate::atmosphere::moist_speed_of_sound, both of which do take humidity. This function feeds a READOUT, never the integrator.

Why it disagrees with some other tools

Tools that fold humidity into the reported figure — the National Weather Service’s own density-altitude calculator among them — answer a different question: “which ISA altitude has the air density I actually have” rather than “what does the FAA rule say”. On a humid day they read HIGHER. At 15 °C and 1013.25 hPa, 50% RH makes the air about 0.32% less dense than dry air, which is 33.1 m (109 ft) of ISA altitude; this function reports 0 there, by construction. Both quantities get called “density altitude” in the field. Reported externally 2026-09.

The engine computes that second quantity too, as crate::atmosphere::density_matched_altitude_m, and the atmosphere.density_altitude bridge command reports the two side by side under separate names.