Temperature gradients (lapse rates)
Lapse rate = temperature decrease per unit altitude. Three main types: environmental lapse rate (ELR), dry adiabatic lapse rate (DALR), saturated adiabatic lapse rate (SALR).
Source: AMS Glossary; WMO; FAA-H-8083-25B PHAK Ch. 12.
Environmental Lapse Rate (ELR)
- Actual temperature decrease of ambient air at a place and time.
- Variable, influenced by weather and time of day.
- ISA value: 0.65 °C / 100 m (standard).
Dry Adiabatic Lapse Rate (DALR)
- Temperature decrease of a dry air parcel rising adiabatically.
- Constant: 1.0 °C / 100 m (= 9.8 °C/km, = 3 °C/1000 ft).
- "Adiabatic" = no heat exchange with environment.
Saturated Adiabatic Lapse Rate (SALR)
- Temperature decrease of a saturated parcel (with condensation) rising.
- Less than DALR because condensation heat is released.
- Typical: 0.5–0.6 °C / 100 m (varies with T and P).
ISA lapse rate
The assumed temperature decrease with altitude in the ICAO standard atmosphere is 0.65 °C / 100 m — binding for ISA calculations.
Stability via ELR vs DALR/SALR
- ELR < SALR (e.g. ELR = 0.3 °C/100 m): absolutely stable — no vertical convection.
- SALR < ELR < DALR (e.g. ELR = 0.7 °C/100 m): conditionally unstable.
- ELR > DALR (e.g. ELR = 1.2 °C/100 m): absolutely unstable — strong convection, Cb possible.
(Stability details in the next lesson.)
Isothermal layers and inversions
Layers in the troposphere where air temperature remains constant are isothermal layers, while those where it increases with altitude are inversions, categorized as surface inversions or free/altitude inversions:
| Type | Description |
|---|---|
| Isothermal | Temperature constant with altitude |
| Surface inversion | Temperature rises in the first metres/hundreds of feet above ground |
| Free / altitude inversion | Temperature rises in higher layers — e.g. subsidence inversion in a high |
| Frontal inversion | At a front (warm air over cold) |
Cloud-base height rule
The cloud base altitude in thermal lift can be approximated as 400 feet per degree Celsius of temperature-dew-point spread at the surface, yielding about 8000 ft AGL for a 20 K spread:
- Spread 5 °C → cloud base ~2000 ft AGL.
- Spread 10 °C → ~4000 ft.
- Spread 20 °C → ~8000 ft.
Physical basis: dry air cools ~1 °C/100 m = ~1 °C/300 ft rising; dew point falls ~0.2 °C/100 m → spread closes at ~0.8 °C/100 m ≈ 1 °C/120 m ≈ 1 °C/400 ft.