How is air density calculated?
From the equation of state for dry air.
- Density falls with height and with higher temperature
- Humid air is less dense than dry air (water vapour is lighter than N₂ and O₂)
DGCA Aviation Meteorology · formula sheet
DGCA Aviation Meteorology Formulas and Key Concepts
17 formulas and the ideas behind them, grouped by topic. Understand each one, then practise it on the question bank.
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Atmosphere & ISA
What are the ISA sea level values and lapse rate?
15 °C, 1013.25 hPa, 1.225 kg/m³. Temperature falls 1.98 °C / 1 000 ft (6.5 °C/km) up to 11 km (36 090 ft).
- Above 11 km the temperature is constant at −56.5 °C to 20 km
- Tropopause height ≈ 8 km at poles, 16–18 km at the equator
- ISA deviation = actual − ISA temperature
Pressure & Altimetry
How do you find pressure altitude?
Correct the elevation for the difference between 1013 hPa and the QNH.
- QNH 1003 hPa at elevation 500 ft gives PA = 500 + 300 = 800 ft
What temperature correction applies to an altimeter?
True altitude differs from indicated by about 4 ft per 1000 ft per °C of ISA deviation.
- Colder than ISA: true altitude is lower than indicated (dangerous over high ground)
- Warmer than ISA: true altitude is higher
How do you convert QNH to QFE?
Subtract the elevation expressed in hPa.
- QNH 1013 at elevation 900 ft gives QFE ≈ 983 hPa
How does pressure altitude relate to true altitude?
True altitude = pressure altitude corrected for the actual (non-ISA) temperature.
- Cold air: true altitude below indicated
- Correction ≈ 4 ft per 1 000 ft per °C of ISA deviation
- Density altitude = pressure altitude + 120 × (OAT − ISA)
Temperature, Humidity & Cloud Base
How do you estimate cloud base?
Use the dew-point spread. The temperature falls 3 °C per 1000 ft and the dew point 0.5 °C, so they close 2.5 °C per 1000 ft.
- T = 24 °C, Td = 14 °C: spread 10, base ≈ 4000 ft above the surface
Stability & Adiabatics
How do you find the cloud base from temperature and dew point?
Cloud base height (ft) ≈ 400 × (T − Td), or 125 m per °C of spread.
- T 25 °C, Td 13 °C: 12 × 400 = 4 800 ft above the station
- Convection cloud forms at the condensation level
- Dew point falls about 0.5 °C/1 000 ft in rising air
Winds & Upper Winds
What is the geostrophic wind and when does it apply?
The wind balanced between the pressure gradient force and the Coriolis force, blowing parallel to straight isobars.
- Closer isobars: stronger wind
- Speed is inversely proportional to sin latitude and density
- Not valid near the equator (within about 15°)
What is the geostrophic wind?
The wind that blows parallel to the isobars when the pressure gradient force balances the Coriolis force.
- Geostrophic speed ∝ gradient / (ρ sin latitude)
- No geostrophic wind at the equator
- Thermal wind: wind shear due to horizontal temperature differences, blows with cold air to the left (N)
Meteorological Calculations
How do you calculate density altitude?
Start from pressure altitude and add 120 ft for every °C above ISA.
- PA 4000 ft, OAT 25 °C: ISA 7 °C, deviation +18, DA = 4000 + 2160 = 6160 ft
How do you find temperature at altitude in ISA?
Take off 2 °C per 1000 ft from the surface value.
- ISA deviation = actual temperature − ISA temperature
How do you estimate the freezing level?
Divide the surface temperature by the lapse rate.
- Surface 12 °C: freezing level ≈ 6000 ft
True altitude with a temperature correction: how?
Work out the ISA deviation at the level, then multiply.
- Indicated 10 000 ft, ISA −15 °C: error −600 ft, true altitude 9400 ft
How is the temperature at altitude estimated from the surface?
T = T₀ − 2 °C per 1 000 ft (ISA lapse rate 1.98).
- If the surface is 25 °C, at 10 000 ft the temperature is ≈ 5 °C (ISA)
- Freezing level: height where T = 0 °C
- Height of 0 °C isotherm = surface temp (°C) / 2 × 1 000 ft
Make the Aviation Meteorology formulas stick
In class every formula is built from a diagram first, then practised until it is automatic.