What do they mean, these guys and gals on television, when they tell us all too frequently about an "unstable" airflow over Ireland? Is the atmosphere about to topple on our heads? - or are they merely attempting to confuse by obfuscation?
Like most words used by the forecaster, "unstable" has a very specific meaning when used in the context of the atmosphere. In its simplest sense, it means the thermal structure of the air is such that it is likely to produce showers - probably heavy showers.
But let us try to assemble the subtleties of the concept.
Temperature, as we know, usually decreases with height. This is what might be called the "normal" state of the atmosphere, and within a certain range of temperatures the system is more or less in equilibrium; there is no marked tendency for the air at any level to move upwards or downwards.
But if something happens to accentuate the normal decrease in temperature with height, great movements of air can take place in the vertical.
It is then we say the atmosphere is "unstable".
Instability can occur, for example, when air in contact with the ground heats up on a sunny day; or it can be caused by the displacement of existing air near the ground by warmer air moving in from somewhere else.
Conversely, instability can occur if there is an influx of cold air at very high levels in the atmosphere, thus making it colder aloft and augmenting the decrease in temperature with height.
Or, indeed, an unstable atmosphere can be brought about by any combination of these events.
The end result, however, is always a greater fall than usual of temperature with increasing altitude.
So what happens when these unstable conditions come about?
If a "bubble" or "parcel" of air finds itself warmer than its environment, it will float upwards because of its lower density.
In unstable conditions, even though such a bubble cools as it expands with falling atmospheric pressure, it still finds itself warmer than its surroundings at every level it reaches in the atmosphere, because the temperature of the atmosphere drops so rapidly with height.
When the atmosphere is very unstable, a rising body of air like this develops into a convection current, a vertical jet of air gushing upwards like a fountain in slow motion.
The upward momentum of the rising column is accentuated by the condensation of any moisture which may be present - and the end result is one of those towering cumulonimbus clouds which give us showers - and sometimes thunderstorms.