Stability and Instability

Meteorology · Chapter 9 · DGCA CPL and ATPL

Why some air shoots up into towering storms while other air lies flat as fog: the three lapse rates, and the one comparison that decides it.

Remember

Rising air expands and cools; sinking air is squashed and warms. No heat goes in or out: that is an adiabatic process.
DALR 9.8 °C/km (3 °C/1000 ft) for dry air. SALR about 5 °C/km for saturated air, nearing DALR where it is colder than −40 °C. ELR is whatever the real air is doing: ISA uses 6.5.
Compare the lifted air with the air around it. Warmer: it keeps rising (unstable). Colder: it sinks back (stable). Same: neutral.
ELR DALR: absolutely unstable. DALR > ELR > SALR: conditionally unstable (stable if dry, unstable once saturated).
An inversion is a negative lapse rate (warmer above). It is very stable and traps haze, smoke and fog under it. Common on clear winter nights and under highs.

Why does one day give towering storms and another a flat grey sheet?

Rise or sink?

Six real photographs. Swipe or tap Next.

Think of air as hot-air balloons Meteorologists imagine a small bubble of air, called a parcel, and follow it as it moves up or down. Like a hot-air balloon, a parcel that is warmer than the air around it is lighter, so it floats up. One that is colder is heavier and sinks.
Rising air cools itself Air is a poor conductor of heat, so a rising parcel keeps its heat to itself. But higher up the pressure is lower, so the parcel expands, and expanding uses up its energy: it cools. Sinking air is squeezed and warms up. Cooling or warming with no heat going in or out is called an adiabatic process. It largely shapes how temperature changes with height. Adiabatic = no heat enters or leaves. Isothermal = temperature stays the same. The adiabatic process largely decides the vertical temperature profile.
Unstable: once pushed, it keeps going Give unstable air a small push upwards and it carries on by itself, like a ball rolling off the top of a dome. That is how these towering storm clouds grow: warm, moist air keeps rising for kilometres. Unstable air means bumpy flight, showers and good visibility between them.
Stable: pushed up, it sinks back Push stable air up and it sinks back to where it started, like a ball at the bottom of a bowl that rolls back after you nudge it. Stable air gives flat, layered cloud like this sheet of stratocumulus, smooth flying, and often poor visibility because haze cannot mix away.
An inversion holds everything down Sometimes the air gets warmer as you go up. That is an inversion, and it is very stable: rising air hits the warm layer and stops. On calm, clear nights the ground cools fast and so does the air touching it. By dawn, fog lies trapped in the valleys under the inversion, as in this view from orbit. Inversion = negative lapse rate = stable. Common in winter at night, and under highs (anticyclones). It traps smoke, haze, dust, mist and fog.
Which kind of day? A pilot reports smooth air, a flat layer of cloud and thick haze below it.

Takeaway: Air rises when it is warmer than the air around it and sinks when it is colder. Stability is just that comparison, made again and again as the air climbs.

Why does rising air get colder without losing any heat?

Adiabatic cooling

Air is a poor conductor, so a rising bubble keeps its own heat.

Isothermal or adiabatic? In an isothermal process heat flows in or out so the temperature stays the same. In an adiabatic process (Greek: "not passing through") no heat enters or leaves. Air is a bad conductor, so a moving bubble of air is effectively insulated from its surroundings: adiabatic.
Going up A parcel lifted into lower pressure expands. Expanding is work, paid for from its internal energy, so its temperature falls: about 9.8 °C per km (3 °C per 1000 ft) for dry, unsaturated air. This is the dry adiabatic lapse rate (DALR).
Coming down A sinking parcel is squeezed by rising pressure; the work done on it warms it at the same rate. Adiabatic processes largely decide how temperature is spread up and down the atmosphere.

Memory hook: Up: expand, cool. Down: squeeze, warm.

Takeaway: In an adiabatic process no heat enters or leaves. A rising parcel moves into lower pressure, expands, and uses its own energy to do so: it cools at 9.8 °C per km if dry. A sinking parcel is compressed and warms at the same rate.