The candle’s flame heats the surrounding air, creating a warm pocket within the opening of the jar. For this activity to work, the air needs to be absolutely still. Any breeze will blow away the layer of heated air.
Smoke from the incense rises due to a convection current, where the heated gases float above the surrounding dense, cooler air at the bottom of the jar. Yet the warm pocket at the top interrupts this convection current, pushing the rising smoke out towards the sides of the jar and allowing it to cool. The warm layer therefore acts like a ceiling, trapping the incense smoke in the jar.
If the warm layer is blown away, the smoke will spill out and escape.
In meteorology, we call a pocket of heated air such as this an ‘inversion layer’. Normally, the air is warmest when it is close to the ground. This is because the Sun’s radiation heats the Earth’s surface faster than it heats the atmosphere. The hot air rises until it cools in the upper atmosphere, before dropping back to the surface in a cycle.
Inversion layers can occur when warm air blows over the top of a cooler layer. If it interrupts the normal convection current, it is called a ‘capping inversion’. If the area is surrounded by hills, pollutants can build up within the trapped air, creating severe smog problems. Cities such as Los Angeles in California regularly have an inversion layer preventing the spread of smoke and exhaust.
Because warmer air refracts (bends) light differently than cooler air, an inversion layer can create some strange optical illusions. It is thought to be the cause of some mirages where objects beyond the horizon are seen, or when a green flash of light appears at sunset because the light is split into different colours.
Warning: This activity involves using matches. Younger scientists should get an adult to help.
You will need
- Glass jar (at least 300ml in size)
- 20cm copper wire (or two large paper clips)
- Rubber band
- Tea light candle
- Lighter or matches
- Incense cone
What to do
- Bend the length of copper wire into a squared ‘U’ shape. The tea light candle will need to sit in the bottom of the U, with the arms pointing up along the sides.
- Use a rubber band to hold the copper wire onto the candle.
- Bend the ends of the wire into small hooks, so the entire tea light can hang completely inside the opening of the jar. (Note: if you have no copper wire, two paper clips can be used instead. Bend each of them into a squared ‘S’ shape).
- Light the incense cone and place it on the bottom of the jar, roughly in the centre.
- Light the tea light candle and hang it in the opening of the jar, near the incense.
- Find a quiet spot where the air is still.
- Watch the smoke rise from the incense cone. What happens when it reaches the warm air around the candle?
Attach hooks to your candle. Hang your candle in the opening of the jar. Light the incense and place it in the jar. Watch the ‘smog’ swirl under the inversion layer.