Density Altitude: Hot, High and Humid

Updated: 2026-09-27 · First published: 2026-09-27 · About this site

Two aircraft, the same weight, the same runway, the same pilot. On a January morning the take-off roll is 350 m; on an August afternoon it is 600 m and the climb rate after lift-off is barely half. Nothing about the aeroplane changed. The air did. Density altitude is the number that captures how much thinner the air has become than the aeroplane's performance charts assume, and it is the most reliable predictor of the summer take-off accident.

Three altitudes

True altitude — height above sea level. What the map shows.
Pressure altitude — the altitude in the standard atmosphere at which the current pressure would be found. What the altimeter shows with 1013 set.
Density altitude — the altitude in the standard atmosphere at which the current air density would be found. What the engine, propeller and wing actually experience.

The standard atmosphere (ISA) assumes 15 °C and 1013.25 hPa at sea level, with temperature falling 2 °C per 1,000 ft. A real day is almost never standard, and density altitude is the correction for that.

From a METAR to a number

Take a METAR at a field 2,500 ft above sea level: … 33/08 Q1008 ….

Pressure altitude. The QNH is 1008, which is 5 hPa below standard. Each hPa is about 27 ft, so pressure altitude ≈ 2,500 + 5 × 27 = 2,635 ft.
ISA temperature at that altitude. 15 − 2 × 2.635 ≈ 9.7 °C.
Temperature deviation. The OAT is 33 °C, so ISA + 23.3.
Density altitude. 2,635 + 120 × 23.3 ≈ 5,430 ft.

The aircraft is sitting on a runway at 2,500 ft, but it will perform as though it were taking off from 5,400 ft. Every airport page on this site does this calculation from the live METAR and the field elevation and shows it with the other decoded values, so the number is there before you open the performance chart.

What it does to the aeroplane

Effect	Why	Rough size
Less engine power	Fewer oxygen molecules per intake stroke (normally aspirated)	−3% per 1,000 ft
Less propeller thrust	The prop is a wing; thin air gives less lift per revolution	included above
Higher true airspeed for the same lift	Indicated airspeed is what the wing feels, but the ground speed at rotation is higher	+2% TAS per 1,000 ft
Longer take-off roll	Less acceleration and a higher rotation ground speed, compounded	+10% per 1,000 ft (piston)
Lower climb rate	Excess power shrinks from both ends	can halve by 6,000 ft DA
Longer landing roll	Higher ground speed at touchdown	+4–5% per 1,000 ft

The take-off case is the dangerous one because the effects multiply. At 6,000 ft density altitude a light single that needs 400 m at sea level may need 700 m, and after it lifts off the climb gradient may be too shallow to clear terrain that was never a factor in winter. Aircraft rarely fail to get airborne on a hot day; they get airborne and then fail to climb.

Humidity — the part the formula forgets

Water vapour molecules weigh less than the nitrogen and oxygen they displace, so humid air is less dense than dry air at the same temperature. The effect is modest — a few percent at 35 °C and high humidity, worth a few hundred feet of density altitude — but it is always in the wrong direction and the simple formula ignores it. On a tropical coast at 32/27 (a spread of 5 °C at 32 °C) add 300–400 ft to whatever the rule of thumb gives.

Practical rules

Compute density altitude whenever the OAT is above about 25 °C or the field is above 3,000 ft. It takes thirty seconds.
Use the AFM take-off chart with the pressure altitude and temperature, not the field elevation. The chart already turns them into density.
Apply a personal margin. The chart figures were produced by a test pilot with a new aircraft on a hard runway.
Depart early. Between 07:00 and 14:00 the temperature at a Mediterranean or Anatolian field can rise 15 °C, which is 1,800 ft of density altitude — see the diurnal curve on the meteogram for LTAI Antalya or LTAC Ankara.
Reduce weight before you reduce margin. Fuel is the easiest weight to leave behind and the only one you can put back at the next stop.

Frequently asked questions

What is the formula for density altitude?

Density altitude ≈ pressure altitude + 120 × (OAT − ISA temperature at that pressure altitude), all in feet and °C. ISA temperature is 15 °C at sea level falling 2 °C per 1,000 ft. It is an approximation; flight computers and the AFM charts do it exactly.

How do I get pressure altitude from a METAR?

Pressure altitude = field elevation + (1013 − QNH) × 27 ft, approximately. With QNH 1003 at a 2,000 ft field the pressure altitude is about 2,270 ft. In the US with an altimeter setting in inHg use (29.92 − setting) × 1,000.

Does humidity affect density altitude?

Yes, but less than temperature. Water vapour is lighter than dry air, so humid air is less dense; at 35 °C and 90% humidity the effect is equivalent to a few hundred feet of extra density altitude. Most simple formulas ignore it, which is why they under-estimate on humid days.

By how much does take-off distance increase with density altitude?

A widely used approximation is 10% per 1,000 ft of density altitude above sea level for the take-off ground roll of a normally aspirated piston aircraft. At 5,000 ft density altitude that is roughly 50% more runway than the sea-level, standard-day figure. Use the AFM chart for the real number.

Does a turbocharged engine solve the problem?

It restores engine power at altitude, but the propeller still produces less thrust in thin air and the wing still needs a higher true airspeed to generate the same lift. Take-off distance and climb gradient still degrade — less than for a normally aspirated aircraft, but not to zero.