Ideas of stability and instability are linked to the notion offeedback (Note 7.A). Stability arises from negative feedback, where an initial impulse results in an opposite effect, offsetting the original perturbation. In contrast, instability is the result of positive feedback, where the effect aggravates the impulse. In general, instability leads to change, which persists until some negative-feedback process becomes dominant.

Feedback processes occur throughout the natural world. Notably, the enhanced greenhouse effect (Note 2.L) becomes more serious if it is indeed dominated by positive feedbacks in the atmosphere, exaggerating any fluctuation of global temperature. Also, the interaction of positive and negative feedbacks produces the ceaseless changes of atmospheric conditions we call weather, notwithstanding the steady input of solar radiation. Our incomplete understanding of all the feedbacks involved, and their interactions, partly accounts for the uncertainties in weather forecasting and climate-change predictions (Chapter 15).

The feedback processes most relevant to weather forecasting are those acting rapidly, especially processes connected with two particular kinds of instability (i) (vertical) static instability and (ii) baroclinic instability. We will discuss the first of these in the present chapter, and the second in Chapter 13. In general, static instability depends on the temperature and humidity profiles, and is important at low latitudes. It governs the occurrence of thunderstorms. On the other hand, baroclinic (or dynamic) instability depends on the wind profile, and concerns the development of frontal disturbances which determine the weather in mid-latitudes (Chapter 13).

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