A looping plume occurs when the atmosphere is:

Strongly unstable (superadiabatic), sunny with light wind

Strong inversion from the ground upward

Inversion above and below stack

Fumigation occurs when there is:

An inversion above the stack with an unstable layer below it

An inversion below the stack and unstable air above

A fanning plume is typical of:

Clear calm nights with a strong inversion from the ground upward

Sunny afternoons with strong convection

Which pair is correctly matched?

Lofting - inversion below stack and unstable/neutral above; Trapping - inversion above and below

Lofting - inversion above and below the stack

Trapping - inversion only below the stack

Practice · 5 marks

Types of plume behaviour from a stack

Short answer

Looping occurs in strongly unstable air, coning in neutral conditions, fanning in a strong inversion, lofting when inversion is below stack, fumigation when inversion is above with unstable layer below, and trapping between two inversions. Fumigation and looping give highest ground-level concentrations.

Explain with sketches the different plume shapes (looping, coning, fanning, lofting, fumigation and trapping) observed from a chimney, stating the atmospheric condition in which each occurs.

The shape of a plume depends on the lapse rate (stability) of the atmosphere around and above the stack.

Plume	Atmospheric condition	Description
Looping	Strongly unstable (superadiabatic), sunny day, light wind	Large eddies carry the plume up and down in loops; high but intermittent ground concentration near the stack
Coning	Neutral or slightly stable (ELR $\approx$ DALR), windy or cloudy	Cone-shaped, spreads about equally in the vertical and horizontal; reaches the ground at a longer distance
Fanning	Strong inversion (stable) from the ground upward, clear calm night	Plume spreads sideways only, like a flat ribbon, and travels far at stack height; low ground-level concentration unless the stack is short
Lofting	Inversion below stack height, unstable/neutral above	Plume spreads upward and away; good condition, no ground-level effect
Fumigation	Inversion above the stack, unstable layer below (after sunrise breaks the night inversion)	Plume is trapped below the inversion lid and mixed down to the ground; worst case, high ground concentration
Trapping	Inversion above and below the stack height	Plume is confined between two stable layers and cannot disperse up or down

 Looping:     stack ~\_/~\_/~\_/~>   (large up-and-down loops)
 Coning:      stack <=========>      (cone opening downwind)
 Fanning:     stack ------------->   (thin, flat ribbon)

 Lofting:     stack  ^^^^^^^^^^^>    (spreads upward only)
              ==== inversion below stack top ====

 Fumigation:  ==== inversion above stack ====
              stack vvvvvvvvvvv>     (mixed down to ground)

 Trapping:    ==== inversion above ====
              stack <=========>      (confined)
              ==== inversion below ====

Fumigation and looping are of greatest concern for ground-level pollution; the chimney height is normally chosen for the worst of these cases.

✓ Answer checked by Claude Opus 5.5 (high effort)

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