In perfectly still conditions, with a constant temperature gradient reducing with altitude, smoke from a chimney will rise straight up (photograph, far left). However, in an early morning, the ground may be cooler than the air above. In this case an inversion layer is created in the first few hundred metres of air (photograph, near left, and letter below) and smoke from a chimney will disperse horizontally, rather than vertically
Smoke stacks
What is smoke? Why does it sometimes rise straight up and why, on a cool morning, does wood smoke sink into the hollows?
鈥� Smoke rises because it is part of a stream of hot gases that are normally warmer than the surrounding air. The vertical and straight nature of the smoke plume depends very much on the wind conditions. Still air produces fairly straight plumes.
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On cold mornings, this warm stream cools until its density meets and matches that of the surrounding air. At this point it will spread out into a layer or it may even descend. This is a very common sight in our cold Canadian winters.
Wood smoke is often seen in a layer above houses where it is burnt and sometimes it subsequently descends to street level. This is why government air quality inspectors often ask the public to use other fuels such as natural gas when such conditions are occurring.
Richard Turle
Ottawa, Canada
鈥� Smoke is a mixture of gases and particles. It is usually the product of a combustion process and, in this case, its composition depends on the fuel and the technology used.
The main gas involved in smoke processes is carbon dioxide. But other gases such as carbon monoxide, nitrogen oxides, hydrocarbons and sulphur dioxide might also be present. In smoke from domestic sources, the particles are usually soot.
On a winter morning, especially after a cloudless night, the ground will be cooler than the air above it. So the first few hundred metres of air forms an inversion layer, where the temperature rises rather than falls with height.
In these conditions, smoke rising from a chimney does not disperse vertically but horizontally instead. That鈥檚 because, in the inversion layer, the warm air is higher than the cold air so there is little vertical turbulence to carry the smoke upwards.
But as the sun rises, it starts heating the surface of the ground, which in turn heats the layer of air above it, so the temperature inversion is reversed. This heating also causes considerable vertical turbulence in the lower part of the layer because warm air from below tends to rise above cold air which, in turn, tends to sink.
During the early morning when the height of the chimney is slightly less than that of the inversion layer, the top of the smoke plume will be stable, while further down it will be unstable. So the plume will follow the vertical turbulence of the unstable air below the inversion, although it will not be able to overcome the inversion and disperse above it.
Of course, much more complicated patterns can arise depending on other factors such as wind speed, the general lie of the land and any water bodies that are close by.
Kostas Kourtidis
Laboratory of Atmospheric Physics University of Thessaloniki Greece
This week鈥檚 questions
Dead end
I鈥檝e just had my appendix removed. My surgeon told me I wouldn鈥檛 miss it because it no longer serves a purpose in humans. But it does serve a purpose in some animals. What exactly ?
Paul Whitten
Edinburgh
Plant poser
Did all the oxygen in Earth鈥檚 atmosphere come from photosynthesising plants? If not, where did it come from?
Gabby Griffith
Cardiff