Why does the air go wrinkly when it gets hot?, asks seven-year-old

Holiday season is in full swing but there is no rest for the curious mind. This question was sent in by seven year-old Dominic, from the beach in France, while on holiday.

Dominic would like to know…

Why does the air go wrinkly when it gets hot?

To answer this we have to consider two things; the first is how light travels through things and the second is the difference between cold and hot air.


When light travels from one medium to another, it tends to bend and change direction slightly. This is called refraction. A good way to observe this is to place a straw into a glass that is half full with water.

When you look through the glass from the side, the straw appears to bent or break as it enters the water. What you see is actually the result of light bending as it moves from the air to the water.

Light will also bend when it travels from cold air to hot air, or visa-versa. The molecules of cold air are packed more tightly together than hot air, so we say cold air is more dense than hot air. It is the difference in density that makes light bend.


When the ground is hot, some of its heat transfers to the air directly above it. This creates a layer of hot air near the ground. The air a little higher up is cooler so there is air of two different densities, one above the other.

Hot air rises and cold air falls. Once the air near the ground heats up it starts to rise up and mix with the cold air that is travelling downwards. This creates some air turbulence, a mixture of air that is hot and cold. The light travelling through this patch of air bends in different directions. What we see is a kind of wavy or wrinkly air.


This refraction of light in hot and cold air can create some very interesting effects, more than just wrinkly air. Sometimes when a layer of cold air sits over a layer of hot air, the light is bent in a way that creates an illusion; we think we see something that isn’t actually there at all.

The classic example is when people see a pool of water in the dessert, but it is not there at all.

You may have experienced a similar illusion much closer to home. Have you ever thought you saw a pool of water on a hot road, only to discover the pool seems to move away as you get closer?

This is called the mirage effect; light is bent towards our eyes instead of bouncing off the road surface. The distortion is due to tight pockets of hot and cold air sitting above the road.

We think we are seeing a blue pool of water but what we are really seeing is the sky. It is like a reflection but is actually due to refraction.


The mirage effect has inspired some really interesting technology, like the cloaking devise made from sheets of carbon nanotubes.

These nanotubes are really good conductors, they can rapidly transfer heat to surrounding areas, creating hot and cold gradients that bend the light away from the object concealed behind the device.

Naomi is a science communicator and mother to three inquisitive children. She can be found at sciencewows.ie

Feel free to email your questions to drhowsciencewows@gmail.com


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