The tendency towards thermal convection depends on the vertical profile of temperatures and the consequent vertical static instability of the layer, To understand this, it is helpful first to consider the concept of 'instability' in general.
A situation is said to be 'unstable' if a small disturbance automatically becomes amplified. The traditional example is a marble on an upturned saucer: a slight displacement of the marble to one side leads to an accelerating movement in the same direction. Conversely, a 'stable' situation resembles a marble on a saucer placed normally, where a slight temporary displacement, or even a large one, is followed by the marble rolling back to its initial position, the status quo is restored. So the saucer's shape aggravates any disturbance in the unstable case, whereas the shape hinders change in the stable case. Between the stable case and the unstable, there is the situation of neutral stability. An example is a marble on a level table, where a disturbance leads to displacement but then there is neither acceleration onwards nor restoration to the original position.
The atmosphere is stable in the first five types of uplift mentioned above and therefore resists uplift. But it is statically unstable in the case of uplift due to thermal convection.
Other examples of stable, unstable or neutral conditions are found in nature, in human society, in the oceans (Chapter 11)—in fact, everywhere. Some cases of instability have been mentioned already. For instance, there is the effect of any extra melting at the edge of Arctic ice, which reduces the albedo there (Table 2.3), causing greater absorption of solar energy and hence an increase of local heating and therefore even more melting. Hence, melting leads to more melting. Or there is the case of global warming, leading to increased oceanic evaporation (Section 4.3), hence more rainfall, more vegetation and therefore a reduction of the albedo of land surfaces, greater absorption of sunshine and consequently more warming. Other cases are considered in Note 7.A and in Section 7.4. In every instance, there is a circular chain of events, with each cycle augmenting the earlier change.
Stable situations have been mentioned previously too, where the tendency is towards restoring the original state. The Urey effect (Section 1.2) concerns the way in which ultraviolet light forms oxygen from water, but then the oxygen that is created itself obscures the UV and thus inhibits further formation of the gas. Secondly, any global cooling would increase the wavelength of the ground's longwave radiation (Note 2.B), which would therefore pass less readily through the atmospheric window (Note 2.H), so that the world would warm up again. Stable situations lead to curtailment of the initial fluctuation, not reinforcement.
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