Lapse rates & stability

A. 3 °C per 1000 ft (9.8 °C per km)

Unsaturated air cools at 9.8 °C per km (about 3 °C per 1000 ft) as it rises. Saturated air cools more slowly (about 1.5 °C per 1000 ft low down) because condensation releases latent heat. 2 °C per 1000 ft is the ISA lapse rate.

A. Conditionally unstable

Such air is stable while it stays unsaturated but becomes unstable once it is lifted to saturation. The condition is saturation - hence "conditional" instability, the usual set-up for towering cumulus.

A. Stability

In an inversion a rising parcel immediately finds itself colder than the warmer air above, so it sinks back. An inversion is the most stable layer there is - a lid on convection.

A. Assume any value

The real lapse rate at any place and time can take any value: positive, zero (isothermal) or negative (an inversion). Only the ISA fixes it at 2 °C per 1000 ft.

B. Instability

Turrets growing out of a medium-level layer show the air at that level is unstable - often a warning of thunderstorms later in the day.

B. Subsidence

Subsidence is the slow sinking of air over a wide area, as in a high. Sinking air warms adiabatically and dries, which dissolves cloud.

B. ELR > DALR

If the environment cools faster with height than a rising dry parcel does, the parcel is always warmer than its surroundings and keeps going up - instability.

B. Winter nights

Long, clear, calm winter nights let the ground radiate its heat away, chilling the air next to it below the air above - a radiation inversion. That is also why winter mornings bring fog.

B. Smooth air, poor visibility and stratiform cloud

Stable air resists vertical movement: smooth flying, layer (stratiform) cloud, steady light precipitation, and haze or fog trapped low down.

A. Subsidence

The subtropical high-pressure belts sit near 30 degrees. Air sinks there, warming and drying as it descends, so clouds rarely form - hence the Sahara, Thar and Australian deserts.

B. 25°C

Dry air cools at the dry adiabatic lapse rate of about 9.8 °C per km, so 35 °C at the surface becomes about 25 °C after a 1 km climb.

A. Stable

Stability of dry air is judged against the DALR. An ELR of 6.8 °C/km is less than the DALR of 9.8 °C/km, so a displaced dry parcel is always colder than its surroundings and sinks back - the air is stable.

B. 10 °C

DALR is about 10 °C per km. 2 km x 10 = 20 °C cooler: 30 - 20 = 10 °C.

C. Absolutely unstable

A rising parcel, dry or saturated, cools more slowly than its surroundings, so it stays warmer and keeps rising. When the ELR exceeds both adiabatic rates the air is absolutely unstable.

C. Conditionally unstable - unstable when saturated and stable when unsaturated

Stability depends on where the ELR lies against the two adiabatic rates. With the ELR between SALR and DALR, the same layer is stable for dry air and unstable for saturated air.

A. Warmer than in a dry ascent

Condensation gives back the latent heat, which offsets part of the expansion cooling. Saturated air therefore cools more slowly (SALR) and stays warmer than dry air lifted the same distance.

C. A temperature inversion

Temperature increasing with height makes any lifted parcel colder than its surroundings, so it sinks back - stable. A super-adiabatic lapse rate is the most unstable case.

A. Steady (continuous) precipitation

Stable air lifted gently makes layer cloud and steady, continuous rain or drizzle with poor visibility. Showers and heaped cloud with good visibility are the marks of unstable air.

A. Ascent of air

Air piling in at the surface has to go somewhere, and the ground stops it going down - so it rises. Surface convergence means rising air, cloud and weather (lows, troughs, the ITCZ).

A. Remains constant with height

Isothermal means equal temperature: through the layer the temperature neither falls nor rises with height. Such a layer is stable, like an inversion, only less strongly.

C. Stable

Temperature in the lower stratosphere stays constant and then rises with height. That inversion makes the stratosphere very stable, which is why convective cloud stops at the tropopause.

B. At -40 °C

Saturated air cools more slowly than dry air because condensation releases latent heat. At very low temperatures the air holds almost no water vapour, so there is little heat to release and the SALR approaches the DALR - at about -40 °C.

A. Absolutely stable

The temperature falls only 2 °C in 1000 m, an ELR of 2 °C per km. That is less than even the SALR (about 6 °C per km), so a rising parcel, dry or saturated, is always colder than its surroundings - absolutely stable.

A. ELR

The DALR and SALR are the rates at which a rising parcel cools, and they barely change. What changes from day to day is the actual temperature structure - the ELR. Comparing it with the parcel rates tells you whether the air is stable or unstable.

C. ELR compared with the adiabatic lapse rates

DALR and SALR are fixed properties of a rising parcel. The Environmental Lapse Rate is what the atmosphere is actually doing at that moment, so stability is judged by comparing the ELR against them: ELR less than SALR is absolutely stable, greater than DALR is absolutely unstable, and in between is conditionally unstable.

B. Negative

A positive lapse rate is the normal case, temperature falling with height; zero is isothermal. When temperature RISES with height the lapse rate is negative, and that is an inversion. Inversions are very stable - they cap convection and trap haze, smoke and moisture below them.