Water vapour adds close to 400 feet to the density altitude at Changi on an average April afternoon, by our calculation. The Meteorological Service Singapore (MSS) climate station at Changi records a mean daily maximum of 32.4 °C for April over 1991–2020, and its mean temperature and humidity give a dew point of about 24–25 °C on our conversion. At standard pressure that air comes to about 2,400 ft; with the water vapour left out, about 2,000 ft. Across all twelve months, with the dew point estimated two ways, humidity added between about 360 and 410 ft. A rule of thumb of 120 ft per degree Celsius above standard lands roughly 300 ft short.
Density altitude is the height in the standard atmosphere where the air would be as dense as the air present. The FAA's pamphlet P-8740-2 (2008) calls it "pressure altitude corrected for nonstandard temperature variations". It describes the air rather than any aircraft's performance.
Water vapour pushes Changi's density altitude up in every month of the year
We used Changi's 6.66 m elevation from Singapore's aeronautical information publication (AIP, the aerodrome data a civil aviation authority publishes) and 1013.25 hPa, since MSS publishes no pressure normal. Temperatures are mean daily maxima from the MSS climate page; dew points are our conversion of its 24-hour mean temperature and humidity. The standard values we assumed are 15 °C and 1013.25 hPa at sea level, cooling 6.5 °C per kilometre.
| Month | Max (MSS), °C | Dew point (ours), °C | Dry air, ft | With water vapour, ft | Humidity adds, ft |
|---|---|---|---|---|---|
| January | 30.6 | 23.8 | 1,817 | 2,190 | 373 |
| April | 32.4 | 25.0 | 2,017 | 2,417 | 400 |
| May | 32.3 | 25.3 | 2,006 | 2,413 | 407 |
| December | 30.5 | 24.2 | 1,806 | 2,189 | 383 |
The other eight months fall inside that range. Estimated from each month's mean daily minimum humidity instead, the term is 363 to 390 ft. These are monthly means, never one afternoon's readings. Humid air is less dense because water vapour is lighter than the dry air it displaces, and the factor of 0.622 in the US National Weather Service formula expresses that.
Dew point drives almost all of the humidity effect
At sea level, holding the dew point at 24 °C while the air ran from 26 to 36 °C changed the humidity term by 3 ft or less. Raising the dew point from 20 to 26 °C moved it from about 300 ft to 427 ft. Relative humidity is a poor handle, as our guide to humidity without temperature explains.
A 2016 paper by Guinn and Barry makes the same point in its abstract: at a given pressure altitude, the humidity correction depends on the dew point alone. Its shortcut is that the correction "is simply twenty times the dew-point temperature in Celsius". AOPA's 2019 article words it "double the dew point temperature and add a zero". Our tropical figure is about 16 ft per degree of dew point, so a 24 °C dew point in 32 °C air at sea level gives 377 ft against the shortcut's 480. At Changi's April normals the shortcut puts the whole density altitude 4.1% high, inside the 5% the abstract claims.
Heat and humidity tug the 120-foot shortcut in opposite directions
The 120 ft rule adds that amount to pressure altitude for each °C above standard, for dry air. No FAA document we opened states it; the pamphlet prints a chart instead. On our assumed values, the dry-air slope at standard temperature is 118.6 ft per °C at every altitude, and the average factor shrinks as air warms, to 111.5 ft per °C at 30 °C above standard at sea level. So in heat the rule reads high for dry air, by about 98 ft at Changi's April normals, while missing the 400 ft that water vapour adds. Against the pamphlet's own chart, read as pressure altitude at standard pressure because it states none, the rule came out higher in all 30 cells, by 50 to 834 ft.
Until 12 October 2026 our density altitude calculator headlined the 120 ft rule, had no humidity input and took the altimeter setting only in inches of mercury. At Changi's monthly normals it read 283 to 310 ft below the full figure. The revision made that day computes the full method from a dew point or relative humidity, accepts hectopascals and shows the rule as a labelled comparison line. For Changi's April normals it gives 2,417 ft, with the rule at 2,115 ft.
The NWS formula pulls water vapour into the sum through virtual temperature
The National Weather Service office at El Paso publishes a density altitude calculator and its formula, taking air temperature, station pressure and dew point. It says "the virtual temperature must be calculated first": the temperature dry air would need to be as light as the moist air. With its published constants, Changi's April normals come out 15 ft above our model.
The FAA pamphlet says humidity "is not generally considered a major factor in density altitude computations because the effect of humidity is related to engine power rather than aerodynamic efficiency", and adds: "If high humidity does exist, however, it is wise to add 10 percent to your computed takeoff distance and anticipate a reduced climb rate." Those are the FAA's words to pilots.
Each hectopascal below standard nudges the figure up about 34 feet
GEN 2.1 of the CAAS eAIP effective 3 September 2026 reads "Altimeter Setting Hectopascals (hPa)", and the CAAM eAIP of 11 August 2026 reads "Altimeter setting Hectopascal". CAAS GEN 2.2 defines QNH as the "Altimeter sub-scale setting to obtain elevation when on the ground"; QFE is the pressure at aerodrome elevation. The FAA pamphlet's standard is 29.92 inches of mercury, "1013 hPa in other parts of the world".
With Changi's April temperature and dew point, density altitude came to 2,325 ft at a QNH of 1016 hPa and 2,661 ft at 1006 hPa. That is about 34 ft per hectopascal, so a pressure 7 hPa below standard adds roughly what 2 °C of extra heat does.
A hotter reference temperature tips Seletar about 300 feet above Kota Kinabalu
We ran five aerodromes at standard pressure, each at the elevation and reference temperature its AIP publishes. Every row, Malaysian ones included, uses a dew-point sweep of our choosing that brackets Changi's monthly values; we found no Malaysian Meteorological Department normals for these airports.
| Aerodrome | Elevation (AIP) | Reference temperature (AIP), °C | Dry air, ft | Dew point 22 °C, ft | Dew point 24 °C, ft | Dew point 26 °C, ft |
|---|---|---|---|---|---|---|
| Changi (WSSS) | 6.66 m | 32.6 | 2,039 | 2,373 | 2,416 | 2,464 |
| Seletar (WSSL) | 14 m (46 ft) | 33.3 | 2,145 | 2,479 | 2,522 | 2,570 |
| Kuala Lumpur International (WMKK) | 21 m (69 ft) | 32.0 | 2,030 | 2,364 | 2,408 | 2,456 |
| Penang (WMKP) | 3 m (10 ft) | 32.57 | 2,021 | 2,355 | 2,398 | 2,446 |
| Kota Kinabalu (WBKK) | 4 m (14 ft) | 30.7 | 1,819 | 2,153 | 2,196 | 2,244 |
Penang's 32.57 °C replaced 31.95 °C under CAAM's AIP amendment 03/26, effective 11 August 2026. At a dew point of 24 °C, Seletar comes out 326 ft above Kota Kinabalu. A reference temperature 2.6 °C higher accounts for about 287 ft of that by our arithmetic, and a field 32 ft higher for about 39 ft more.
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