Some examples of low-level static instability

E. Linacre


1. The shape of a chimney plume is an indication of the ambient stability: looping (sinuous), coning, fanning (horizontal dispersion under an inversion), fumigating (dispersing down to the ground), lofting (steadily rising), thermalling (separate little vertical clouds, due to plume's buoyancy in a dry neutral environment), flagging (spread over whole layer from ground to chimney top), downdraft (occasional flagging in lee of building), puffing (pulsing at about 1 - 5 puffs per second), bifurcating (breaking up into a chain of isolated puffs, each rising in the centre and descending on the sides).

2. Water spouts form as the result of a cold downflow of air from cumulus cloud during rainfall, creating very intense local instability near the warm ocean surface, and hence updraught (2).

3. Soaring birds tend to rise in thermals to about 800 metres. Any higher makes it hard to see their prey. Vultures soar in thermals but there are no vultures in the Sahara nor in Australia (where thermals are deep and plentiful) because it is too arid for their prey (3).

4. Thermals consist of mushroom-shaped volumes which ise most rapidly in the centre. So they turn themselves inside out in rising 1.5 times their diameter. Mixing occurs at the top of the mushroom, and its diamter increases in to proportion to the elevation (4).



(1) Scorer, R.S. 1997. Weather & Climate (John Wiley) 686pp. (p. 408)

(2) ibid p. 528

(3) ibid p.543

(4) ibid p.314