Archive for March, 2019

Cloud in a bottle!

March 22, 2019

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.

Delta T versus Filling Fraction

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.




Learning about weather

March 17, 2019

I have just completed a FREE! ‘Learn About Weather‘ course, and slightly to my surprise I think I have learned some things about the weather!


Being an autodidact in the fields of Weather and Climate, I have been taught by an idiot. So ‘attending’ online courses is a genuine pleasure.

All I have to do is to listen – and re-listen – and then answer the questionsSomeone else has selected the topics they feel are most important and determined the order of presentation.

Taking a course on-line allows me to expose my ignorance to no-one but myself and the course-bot. And in this low-stress environment it is possible to remember the sheer pleasure of just learning stuff.

Previously I have used the FutureLearn platform, for courses on Global WarmingSoil, and Programming in Python. These courses have been relatively non-technical and excellent introductions to subjects of which I have little knowledge. I have also used the Coursera platform for a much more thorough course on Global Warming.

So what did I learn? Well several things about about why Global Circulation Cells are the size they are, the names of the clouds, and how tornadoes start to spin. But perhaps the best bit was finally getting my head around ‘weather fronts’.

Fronts: Warm and Cold

I had never understood the terms ‘warm front’ and ‘cold front’ on weather forecasts. I had looked at the charts with the isobars and thought that somehow the presence or absence of ‘a front’ could be deduced by the shapes of the lines. I was wrong. Allow me to try to explain my new insight.

Air Mixing

Air in the atmosphere doesn’t mix like air in a room. Air in a room generally mixes quite thoroughly and quite quickly. If someone sprays perfume in one corner of the room, the perfume spreads through the air quickly.

But on a global scale, air doesn’t mix quickly. Air moves around as ‘big blobs’ and mixing takes place only where the blobs meet. These areas of mixing between air in different blobs are called ‘fronts’


In the ‘mixing region’ between the two blobs, the warm – generally wet – air meets the cold air and the water vapour condenses to make clouds and rain. So fronts are rain-forming regions.

Type of front

However it is unusual for two blobs of air to sit still. In general one ‘blob’ of air is ‘advancing’ and the other is ‘retreating’.

This insight was achieved just after the First World War and so the interfaces between the blobs were referred to as ‘fronts’ after the name for the interface between fighting armies. 

  • If the warm air is advancing, then the front is called a warm front, and
  • if the cold air is advancing, then the front is called a cold front.

Surprisingly cold fronts and warm fronts are quite different in character.

Warm Fronts 

When a blob of warm air advances, because it tends to be less dense than the cold air, it rises above the cold air.

Thus the mixing region extends ahead of the location on the ground where the temperature of the air will change.

The course told me the slope of the mixing region was shallow, as low as 1 in 150. So as the warm air advances, there is a region of low, rain-forming cloud that can extend for hundreds of kilometres ahead of it.


So on the ground, what we experience is hours of steady rain, and then the rain stops as the temperature rises.

Cold Fronts 

When a blob of cold air advances, because it tends to be more dense than the warm air, it slides below it. But sliding under an air mass is harder than gliding above it – I think this is because of friction with the ground.

As a result there is a steep mixing region which extends a little bit ahead, and a short distance behind the location on the ground where the temperature of the air changes.


So as the cold air advances, there is a region of intense rain just before and for a short time after.

So on the ground what we experience are stronger, but much shorter, rain events at just about the same time as the temperature falls. There generally follows some clearer air – at least for a short while.


I had assumed that because of the messy nature of reality compared to theory, real weather data would look nothing like what the simple models above might lead me to expect. I was wrong!

As I was learning about warm and cold fronts last weekend (10 March 2019) by chance I looked at my weather station data and there – in a single day – was evidence for what I was learning – a warm front passing over at about 6:00 a.m. and then a cold front passing over at about 7:00 p.m.

  • You can look at the data from March 10th and zoom in using this link to Weather Underground.

This is the general overview of the air temperature, humidity, wind speed, rainfall and air pressure data. The left-hand side represents midnight on Saturday/Sunday and the right-hand side represents midnight on Sunday/Monday.


The warm front approaches overnight and reaches Teddington at around 6:00 a.m.:

  • Notice the steady rainfall from midnight onwards, and then as the rain eases off, the temperature rises by about 3 °C within half an hour.

The cold front reaches Teddington at around 7:00 p.m.:

  • There is no rain in advance of the front, but just as the rain falls – the temperature falls by an astonishing 5 °C!


Of course there is a lot of other stuff going on. I don’t understand how these frontal changes relate to the pressure changes and the sudden rise and fall of the winds as the fronts pass.

But I do feel I have managed to link what I learned on the course to something I have seen in the real world. And that is always a good feeling.

P.S. Here’s what the Met Office have to say about fronts…

What can we do to stop Climate Change?

March 16, 2019

Note: Reflecting on what matters to me most, I feel increasingly conscious that the only issue I care about deeply is Climate Change. In my mind, all other issues pale in comparison to the devastation to which we – you, reader and me – are condemning future generations because of our indifference and wilful ignorance.

How do we stop Climate Change?

We can’t.

We can’t stop Climate Change because the process is already well underway. We are already experiencing human-induced Climate Change and we are committed to many decades more global warming, even in the most optimistic of scenarios.

Given this dismal reality, reducing the extent of the Climate Change to which we are committing ourselves and our children is, in my opinion, the greatest challenge facing humanity.

But I don’t know what to do other than two things: (a)Try to emit less carbon dioxide personally – a real challenge while living a ‘normal’ life. And (b) tell everyone I know that I think this is the greatest challenge facing humanity.

Who needs to act? 

Our schoolchildren are striking to try to force us ‘grown ups’ to do something. I support them. But perhaps we are ‘grown olds’ rather than ‘grown ups’.

‘Grown ups’ often look to serious-minded economists for guidance. Economics is seen – by the establishment at least – as a more sober and practical activity than science – less prone to doom-laden negativity. The 2006 Stern Review was a clear-headed and practical economic plan to address climate change. As far as I can tell, it has been ignored in practice.

The extent of the failure of Economics to address the challenge of Climate Change was made clear to me the other week reading The Economist’s Free Exchange column. Paraphrasing:

The implicit criticism of the economic approach to climate change is not that it is flawed or politically unrealistic, but that it is a category error, like trying to defeat Hitler with a fascism tax.

Might a fascism tax have worked? No, I don’t think so. Stopping Hitler required general mobilisation of the entire population. It involved commitment from all to a goal that made sense despite the hardship and sacrifice

But in fact economics is not just failing to provide a solution, or a mechanism for a solution, it is at the heart of the problem. It currently doesn’t make economic sense to do things which will minimise the global calamity. This is so at the grand scale – where Oil Companies are valued based on reserves which it would be insane to exploit – and on the micro-scale – where insulating a house requires investments that don’t pay back.

Economists would argue that what we need to make the price of emitting carbon dioxide reflect its true long-term cost via a carbon tax. Then the action of the market will efficiently find solutions. But I don’t think this will be any more effective than a fascism tax would have been against Hitler.

Rather, limiting Climate Change is  likely to require something akin to general mobilisation. The effort will require near universal commitment despite the detriment to almost  every facet of almost every activity in almost everybody’s life.

But as have seen in both the UK and France, at the moment people are not prepared to undergo even modest hardships.

  • The gilets jaunes in France were initially protesting about increases in the price of fuel aimed at reducing fuel use – a carbon tax.
  • In the UK, the Fuel Price Escalator was abandoned because increasing fuel prices, even modestly, is unpopular.

Weaning ourselves off carbon consumption is unpopular. People suffer. And as always, poor people suffer the most.

Why is it so hard?

Weaning ourselves off carbon is hard because burning fossil fuels is easy, cheap and brings immediately appreciable benefits to humanity. That is humanity in general, and also humanity’s individuals – you and me.

  • People love the freedom to move around cheaply. It allows them to earn money in new ways. It allows for the efficient concentration of manufacturing and distribution of goods that makes consumer goods cheap. We have no consciousness at all that for every 8000 km (5000 miles) we drive in a typical car – we emit (roughly) one tonne of carbon dioxide into the atmosphere. Yes. A TONNE! This carbon dioxide will continue to warm the atmosphere for roughly one hundred years – long after the people who emitted it are dead.
  • People love cheap energy. Cheap electricity brings light and heat and gadgets to our homes and allows for low manufacturing costs. We have no consciousness at all that for every 4 units (kWh) of electricity we use (cost ~ £1) – we emit (roughly) one kilogram of carbon dioxide into the atmosphere. In 2017 I used around 6000 units of electricity at home resulting in the emission of roughly 1.5 tonnes of carbon dioxide which will continue to warm the atmosphere for roughly one hundred years – long after my death.

Is there any cause for hope?

… Hope means hoping when things are hopeless, or it is no virtue at all…

GK Chesterton

Personally, I don’t see any. I acknowledge that:

  • progress has been made already – we now emit less carbon dioxide for each unit of electricity delivered than we did even 10 years ago.
  • there are positive political developments. There is talk of Green Deals, political actions that will transform the economic landscape and drive a transition to a renewable energy economy.
  • the practical engineering solutions exist now which can take us from where we are to a post-carbon world a decade or two. No new technology is required.

But these solutions will generally involve people like me, in places like the UK, using less energy, travelling less, and consuming less. I’d vote for these this things in a heartbeat, and pay for these things, but I think I would be in a minority.

Unless people are convinced of the rationale for the changes – changes which will make life materially worse and bring them no benefit in their lifetime – they simply won’t entertain the inconvenience of any practical solution.

So hopeless as I feel, the best I can do is to explain as clearly as I can to everyone I know – and other people I don’t know – why I think that Climate Change is the greatest challenge facing humanity.

And as GK Chesterton also said,

If a thing’s worth doing, it’s worth doing badly.”.

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