One of the best parts of the FREE! ‘Learn About Weather‘ course, was the chance to make a cloud in a bottle. Here’s my video!
The demonstration involves squeezing a bottle partly filled with water and then letting go. One can see a cloud form as one lets go, and then disappear again when one squeezes. Wow!
But there is a trick! You need to drop a burning match into the bottle first!
Heterogeneous versus homogeneous nucleation
How does the smoke make the trick work? It’s to do with the way droplets form – a process called nucleation.
There are two ways for droplets to nucleate. An easy way and a hard way. But those words are too short for scientists. Instead we call them heterogeneous and homogeneous nucleation!
- ‘Heterogeneous nucleation‘ means that the water droplets in a cloud form around dust or smoke particles. The ‘hetero-” prefix means ‘different’, because there is more than one type of entity involved in forming droplets – dust and water.
- ‘Homogeneous nucleation‘ means that the water droplets in a cloud form spontaneously without any other type of particle being present. The ‘homo-” prefix means ‘the same’, because there is just one substance present – water.
The experiment shows that hetero-gen-e-ous nucleation is dramatically easier than than homo-gen-e-ous nucleation. And in reality – in real clouds – practically all droplet formation is heterogeneous – involving dust particles.
The reason is easy to appreciate.
- To form a tiny droplet by homogeneous nucleation requires a few water molecules to meet and stick together. It’s easy to imagine three or four molecules might do this, but as new molecules collide, some will have higher than average energy and tend to break the proto-droplet apart.
- But a dust or smoke particle, though small by human standards (about 0.001 mm in diameter), is roughly 10,000 times larger than individual molecules. So its surface provides billions of locations for water molecules to stick. So when the average energy of the water molecules is at the appropriate level to form a liquid, the water molecules can quickly stick to the surface and cause a droplet to grow.
How big is the temperature change?
Squeezing the bottle compresses the air quickly (in much less than 1 second) and so (because the air is a poor conductor of heat), there is no time for the heat of compression to flow from the gas into the walls and the water (this takes a few seconds) and the air warms transiently.
I was curious about the size of the temperature change that brought about this cloud formation.
I calculated that if the air in the bottle changed volume by 5%, there should be a temperature change of around 6 °C – really quite large!
Squeezing the bottle warms the air rapidly – and then over a few seconds the temperature slowly returns to the temperature of the walls of the bottle and the water.
If one lets go at this point the volume increases by an equivalent amount and the temperature returns to ambient. It is this fall which is expected to precipitate the water droplets.
To get the biggest temperature change one needs a large fractional change in volume. I couldn’t do the calculation of the optimum filling fraction so I did an experiment instead.
I poked a thin thermocouple through a bottle top and made it air tight using lots of epoxy resin.
I then squeezed the bottle and measured the maximum temperature rise. The results are shown below.
The results indicate that for a bottle filled to around three quarters with water, the temperature change is about 6 °C.
But as you can see in the video – it takes a few seconds to reach this maximum temperature, so I suspect the instantaneous change in air temperature is much larger, but that even this small thermocouple takes a couple of seconds to warm up.
Happy Experimenting
The Met office have more cloud forming tricks here.
Tags: Weather
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