Atmospheric stability explains whether air parcels rise, sink, or stay near their starting level. This topic helps students understand cloud formation, thunderstorms, temperature inversions, and daily weather patterns. A lapse rate cheat sheet is useful because stability depends on comparing several rates that are easy to confuse.
It also connects temperature changes with humidity, condensation, and vertical motion in the atmosphere.
The main idea is to compare the environmental lapse rate with the dry adiabatic lapse rate and the moist adiabatic lapse rate. Unsaturated air cools at about 10°C per 1000 m as it rises, while saturated air cools more slowly, usually about 4°C to 7°C per 1000 m. If a rising parcel remains warmer than the surrounding air, it is unstable and continues rising.
If it becomes cooler than the surrounding air, it is stable and resists vertical motion.
Key Facts
- The environmental lapse rate, ELR, is the actual temperature change with height in the atmosphere, often written as ELR = change in temperature / change in altitude.
- The dry adiabatic lapse rate, DALR, is about 10°C per 1000 m for rising or sinking unsaturated air.
- The moist adiabatic lapse rate, MALR, is usually about 4°C to 7°C per 1000 m because condensation releases latent heat.
- A rising air parcel expands and cools because air pressure decreases with altitude.
- Absolutely stable air occurs when ELR is less than MALR, so both dry and saturated rising parcels become cooler than the environment.
- Absolutely unstable air occurs when ELR is greater than DALR, so rising parcels stay warmer than the environment and continue rising.
- Conditionally unstable air occurs when MALR is less than ELR and ELR is less than DALR, meaning saturated air can rise but unsaturated air tends to sink.
- A temperature inversion occurs when temperature increases with height, creating very stable conditions that can trap pollution near the ground.
Vocabulary
- Lapse Rate
- A lapse rate is the rate at which air temperature changes with altitude.
- Environmental Lapse Rate
- The environmental lapse rate is the actual measured temperature change with height in the surrounding atmosphere.
- Dry Adiabatic Lapse Rate
- The dry adiabatic lapse rate is the cooling or warming rate of unsaturated air, about 10°C per 1000 m.
- Moist Adiabatic Lapse Rate
- The moist adiabatic lapse rate is the cooling rate of saturated rising air, usually about 4°C to 7°C per 1000 m.
- Atmospheric Stability
- Atmospheric stability describes whether air resists vertical movement or continues rising or sinking after being displaced.
- Temperature Inversion
- A temperature inversion is a layer where air temperature increases with altitude instead of decreasing.
Common Mistakes to Avoid
- Confusing ELR with DALR is wrong because ELR describes the real atmosphere, while DALR describes a moving unsaturated air parcel.
- Using the dry adiabatic lapse rate after condensation begins is wrong because saturated air cools more slowly due to latent heat release.
- Assuming all rising air forms clouds is wrong because clouds form only when air cools to its dew point and becomes saturated.
- Calling an inversion unstable is wrong because temperature increasing with height strongly resists vertical mixing.
- Ignoring units in lapse rate calculations is wrong because rates are usually compared in °C per 1000 m, so altitude units must be consistent.
Practice Questions
- 1 An unsaturated air parcel at 20°C rises 2000 m. Using the dry adiabatic lapse rate, what is its final temperature?
- 2 The temperature at the ground is 18°C and the temperature at 3000 m is 0°C. What is the environmental lapse rate in °C per 1000 m?
- 3 If the ELR is 6°C per 1000 m, the DALR is 10°C per 1000 m, and the MALR is 5°C per 1000 m, is the atmosphere absolutely stable, absolutely unstable, or conditionally unstable?
- 4 Explain why saturated rising air cools more slowly than unsaturated rising air.