Weatherwatch with Irv Lee

Atmosphere Stability

Stability of the atmosphere plays a major part in the enjoyment of any flight. It not only affects the type of clouds and precipitation you might encounter, but also visibility, wind strength and turbulence.

The stability of the atmosphere is governed by temperature changes with altitude (the 'lapse rate). Temperatures at certain altitudes can be found on Metform 214, alongside the wind vectors used for navigation calculations. Few pilots look at temperature change with altitude, but it's a direct indication of stability.

The four types of atmosphere can be explained with the help of a saucer and a marble:

1 Stable Visualise the marble in the saucer, and give it a gentle push. The marble gradually settles back in its original position.

2 Unstable Visualise the marble on an upside-down saucer. Any disturbance makes the marble accelerate away, never to return.

3 Conditionally stable Use a saucer which has a small flat base and a small ridge around it. Turn it upside down and sit the marble on it. The marble is stable to small pushes, but a sufficient force will move it over the ridge.

4 Neutrally stable Remove the saucer, place the marble on the floor. A small disturbance moves the marble a little distance, but it does not depart completely or return to the original position.

Consider a 'parcel' of air near the ground which could be disturbed by uneven heating of the ground, or by meeting hills as it travels in the wind. The parcel of air will contain dissolved water vapour, with the maximum amount determined by its temperature. If the parcel of air rises, expansion occurs in the lower pressures aloft, causing cooling. If our parcel of air is disturbed upwards in stable conditions, it will calmly settle down again.

If stability conditions are neutral, there will be no obvious consequences. However, if atmospheric conditions are unstable, it will accelerate skywards, releasing cloud and precipitation as it cools.

Effects for the pilot

Very Stable Atmosphere
Cloud Either none or layered stratus
Precipitation None in clear skies, potential widespread drizzle or light rain under stratus
Visibility Deteriorating in haze under clear skies, or poor in any drizzle under stratus
Winds Light if high pressure close, stronger if low pressure nearby
Turbulence Probable in stronger winds on the lee of hills, or under any inversion boundary present

Very Unstable Atmosphere
Cloud Large vertical cumulus or towering cumulus
Visibility Excellent. Pollution is carried into higher atmosphere
Precipitation Sudden heavy showers, often seen and avoided
Winds Stronger, with dangerous gusts near towering cumulus
Turbulence Likely under any cloud, dangerous near towering cumulus

These differences are of great significance to the pilot, and it is the temperature change with altitude ('lapse rate') which determines stability. We all know the theoretical lapse rate is 2 degrees C per 1000ft, based on a ground level temperature of 15 degrees C cooling to -55 degrees C at the stratosphere boundary of 35,000ft. Great in theory, but when the real air above us arrives in the UK from different sources -polar seas, the arctic, sub tropical Atlantic - reality rarely matches the textbook.

However, theory and reality do agree on the rate our 'packet' of air will cool as it 'expands with altitude', if disturbed upwards.

Initially, it cools at 3 degrees C per 1000ft of ascent. Cooling, it cannot now hold as much water vapour, and latent heat produced by release of the water vapour slows this 'ascent cooling' to half its previous value. A parcel of air warmer than the surrounding atmosphere will rise under natural buoyancy, while cooler air would sink. The real atmosphere itself will have its own temperature lapse rate, but our 'disturbed' parcel of air can only cool at two values for every 1000ft: 3 degrees initially, or 1.5 degrees when it is too cool to hold all the original water vapour.

If the real atmosphere's lapse rate is more than 3 degrees per 1000ft, instability must be present, as rising air cannot cool quickly enough to match. Therefore the air will always be more buoyant than surroundings, and continue to rise. If the atmosphere reduces by under 1.5 degrees per 1000ft, stability is present, as the disturbed air must always be cooler than surroundings and will sink again.

If the atmosphere temperature reduces by a value between the 1.5 and 3 degrees per 1000 feet, it is conditionally stable. If disturbed air has already started to release cloud, it will cool slower and therefore continue to rise. If no water vapour release has started (ie cloud), the opposite is true.

This article first appeared in FLYER magazine's June 2000 edition