Natural Navigator Tristan Gooley asks you to consider a conundrum

THIS is a good time of year to ask yourself:

How can a smell make a train appear?

One small clue can change the way you think about your surroundings quite dramatically. I would like you to imagine that you are on a walk on a cold morning and you pick up the faint, constant scent of musty smoke. However, you can see no sign of a fire. I want you to ask yourself what deductions and predictions you might be able to draw. Now think for a minute before reading on.

Smelling Smoke on a Cold Morning

If you can smell smoke in the air on a cold morning, it is likely that there is a temperature inversion, which occurs when a layer of warmer air traps a cooler layer near the surface. The smoke from factories and home fires gets trapped near the ground and spreads along under the warmer layer, giving the air a musty whiff of smoke.

When there is a temperature inversion it creates a ‘sandwich effect’ and sound, light and radio waves get bounced between the top of the cool lower layer of air and the ground.

Sounds travel further in these conditions and can be heard more loudly, so you will hear airports, roads or trains that you cannot normally. The effects can be more dramatic if there are any very loud noises nearby and a violent demonstration of this effect took place in the middle of the last century.

An explosion creates an extreme form of sound, called a shock wave. In 1955 one of the earliest nuclear weapon tests in Russia created a shock wave that bounced back down off the inversion layer, knocked over a building in Semipalatinsk and killed the three occupants.

Light is refracted in an inversion, and this unusual bending of the rays leads to optical illusions. In normal atmospheric conditions, very distant objects appear shortened and squashed; this is why we often see a squashed fat sun at sunset. In an inversion the opposite happens and objects appear to be stretched vertically. This can lead to an optical illusion called a ‘Fata Morgana’, from the Italian nickname for this strange type of mirage. A Fata Morgana is best known for making objects appear to levitate in the distance: bridges and boats are seen to hover above the water. This type of inversion refraction effect will also improve your chances of seeing the rare phenomenon known as the ‘green flash’, when a momentary burst of green light is seen at the exact moment of sunset.

Excerpt from The Walker’s Guide to Outdoor Clues & Signs. © Tristan Gooley 2014. More info about the book on this page.

Tristan Gooley is the author of The Natural Navigator and The Walker’s Guide to Outdoor Clues and Signs. You may visit his website at www.naturalnavigator.com and follow him on Twitter at @naturalnav

We have organised a walk with Tristan himself in late January.

Book now for Tristan’s Natural Navigation walk on the morning of Saturday 24 January