The Moon as a symbol of hope

July 19, 2019

Eclipse July 2019

I sat out by the Diana Fountain in Bushy Park on Tuesday night and took a picture of the eclipsed Moon.

As I sat in the peaceful darkness, I thought about the fact that when I was nine-years old, human beings had sent a rocket ship to the moon, and men had walked about and collected some rocks.

As technology has advanced since the 1960s, the engineering in the Apollo program has not been eclipsed. Indeed, it seems ever more remarkable.

And amongst the moths and the bats, I reflected that “…if human beings can do that, then we can do anything that can be done…”. 

That qualification “…that can be done…” is there because although the aim of the Apollo programme was built on a whimsical folly, the engineers who made it happen could only use practical steps to make it real.

Some the steps they took seem astonishing, but there was – obviously – nothing ‘impossible’. No steps relied on wishful thinking.

The excellent bookHow Apollo Flew to the Moon” , (my review is here) highlighted some of most astonishing facts:

  • The total mechanical output power of five first stage rockets was 60 GW. This is equivalent to peak electrical supply of the entire United Kingdom.
  • On its return from the moon, its speed just before entry into the Earth’s atmosphere was more than 11 kilometres per second.
  • Since Apollo 17 returned in 1972. no human being has been more than 700 kilometres from Earth’s surface.

And sitting in the dark I reflected that if we could achieve all these things then, surely we can – and eventually will – get our act together on Climate Change.

It may seem impossible now, but even the most politically deaf regimes will eventually dance to the theme of climate change – they have no choice.

And if the US were to devote to this problem even a small fraction of the energy and enterprise that it devoted to Apollo, they could yet inspire us all again, and leave a legacy to be proud of for all our children.

Travelling too much

July 5, 2019

Its been a while since I wrote here and the main reason for this silence is that I have been travelling too much.

And by ‘travelling’ I specifically mean flying. So far this year I travelled far enough to fly clear around the Earth – more than 40,000 km. And in every sense, it is just too much.

There is always a good reason to travel – work and ‘business’ always provide sound reasons for travel. But the undeniable fact of the matter is that flying is bad for the environment.

So while I greatly enjoyed a secondment in New Zealand the trip caused the emission of 7.6 tonnes of carbon dioxide – an amount which overshadows the steps I take to minimise emissions at home.

And the carbon dioxide emissions are not even the worst part. I was reminded by a recent Physics World news item that:

… contrail cirrus clouds are the single largest source of the aviation industry’s contribution to climate change, far outpacing the impact of aircraft carbon dioxide emissions. 

It is hard to just say ‘No’

I have asked my colleagues in many fields how they feel about flying. Everyone I have asked thinks it is a really important issue.

Some have, bravely in my opinion, shunned air travel, refusing offers to travel because of the emissions they would cause.

While not underestimating how hard that choice is, it is easier for those who live in large cultural centres such as London. London offers a great many local opportunities for work and pastimes, and also has ground-based National and International travel networks, that are not available to those who, for example, live in New Zealand.

And the freedom to not travel is not open to everyone. Some are obliged to travel internationally for work.

For colleagues working in the field of climate change, many of whom forswear personal air travel, international meetings are an essential part of seeking international solutions to this international problem.

But even these colleagues who don’t fly for personal reasons normally have funerals to attend. And perhaps when their children grow they may ache to see their grandchildren who may be living far away.

The bridge to the future

If we imagine a future more sustainable world, then flying around the world must surely become less common.

In market economies the only ways to achieve this are to either

  • (a) raise the price of air travel, or
  • (b) ration the amount of air travel or
  • (c) some combination of (a) and (b).

None of these look like options that people will vote for without a great deal more understanding of the problem at hand.

To misquote LP Hartley,

The future is a foreign country, they do things differently there.

But how do we get there?

World Metrology Day 2019

May 19, 2019

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Monday 20 May – World Metrology Day 2019 – is a day towards which I have been working for the last 14 years.

Back in 2005, my NPL colleagues Richard Rusby,  Jonathan Williams and I compiled a report on possible methods for measuring the Boltzmann constant. The aim of the measurement would be to obtain an estimate of the Boltzmann constant with sufficiently small uncertainty that the International Bureau of Weights and Measures (BIPM) would feel able to redefine what we mean by ‘one kelvin’ and ‘one degree Celsius’ in terms of this new estimate.

To cut a very very long story short, we succeeded. And tomorrow, that project comes to fruition.

Of course it wasn’t just me. Or even me and my immediate colleagues in the thermal team at NPL. We were helped by colleagues from across the laboratory, and from other institutions. Notably:

  • Cranfield University who manufactured the key component in the experiment,
  • The Korean National Laboratory KRISS and the Scottish Universities Environmental Research Council who helped with isotopic analysis of argon gas.
  • Colleagues helped us from:
    • LNE-CNAM in France,
    • INRIM in Italy,
    • NIST in the USA,
    • PTB in Germany,
    • CEM in Spain.

And I have probably missed an important institution or partner from this list because – frankly – it has been a long haul!

But even this list doesn’t include all the other teams involved in the wider kelvin re-definition project.

Several other institutions also sought to independently measure the Boltzmann constant using a range of different techniques and the value chosen by the International Bureau of Weights and Measures (BIPM) was the weighted average of estimates from this international effort.

In all, hundreds of scientists, engineers and technical staff around the world have supported this effort and I feel humbled to have had the opportunity to take part in a project of this scale.

And it is not just the kelvin, today three other units will also be be redefined – the mole, the ampere and the kilogram.

In this troubled world, it is a real comfort to me to feel the friendships built and professional relationships created during these last 14 years.

I think it shows that the International System of Units is a living international institution which really works; which brings people together from around the globe to make measurements better. The SI is an institution of which the whole world can feel proud.

Happy World Metrology Day 🙂

Should I still be using gas?

May 6, 2019

TL/DR The carbon emissions associated with electrical generation in the UK have fallen so much it is has become greener – but not necessarily cheaper – to cook and heat using electricity.

Electricity Generation

The carbon intensity of a source of electricity is a measure of how much carbon dioxide (measured in kg) was emitted to make one unit of electrical energy: an electrical kilowatt hour (kWe).

The chart below shows the carbon intensity of electricity generated by various techniques. The average carbon intensity depends on generating mix – and how that varies with time.

Carbon Intensity of different generating sources

When I began talking about Climate Change back in 2004, Coal, Gas and Nuclear were the main generating sources for UK electricity. And the average carbon intensity – if I remember correctly – was around 0.50 kg CO2 per kWe.

The generating mix in 2019 is radically different. The carbon intensity varies daily and seasonally between about 0.1 kg CO2 per kWe (at times of low demand and high renewable generation) and 0.4 kg CO2 per kW(at times of high demand and low renewable generation). The average value is less than 0.3 kg CO2 per kWand falling.

The chart below shows how the carbon intensity of electricity varied through the month of December 2018. The average value was 0.243 kg CO2 per kW.and the maximum and minimum values were 0.390 kg CO2 per kWand 0.094 kg CO2 per kWrespectively.

Carbon Intensity in December 2018

In my home

I can choose to heat by using electricity or gas.

  • If I heat using electricity then I can convert electrical energy into heat with 100% efficiency, so for every kW of electrical power I use, I generate 1 kW of thermal power (kWth). And hence release roughly 0.3 kg of carbon dioxide.
  • If I heat using gas then I can convert chemical energy in the gas into heat with high efficiency – but not generally 100%. So (looking at the chart at the start of the article) for every 1 kWth, I emit at least 0.47 kg of carbon dioxide.

Now the price per kWh (one kilowatt hour) that I am charged by EDF, the French government-backed company that supplies my electricity and gas is:

  • 26.6 pence for 1 kWh of electrical/thermal energy during the day.
    • Generally higher carbon intensity ~ 0.4 kg CO2 per kWe.
  • 5.0 pence for 1 kWh of electrical/thermal energy during the night.
    • Generally lower carbon intensity ~ 0.15 kg CO2 per kWe
  • 4.2 pence for 1 kWh of thermal energy (via gas) at any time.
    • Always at least ~ 0.46 kg CO2 per kW

So the reduction in the carbon intensity of the UK’s generating mix means that switching to electricity now makes ‘green sense’. i.e. If I generate 1 kW heat in my house using electricity then less carbon dioxide is emitted than if I just burned the gas directly

But in order to make financial sense I would need to make sure that I didn’t use any ‘daytime’ electricity.

Mmmm. Well at least I have a choice!

Resources

 

Cultural Vertigo

April 28, 2019

I wrote this back in December 2012, and I can still remember the visceral shock of seeing our addiction to energy manifested in the glowing arteries of London.

I still feel the same way

Protons for Breakfast Blog

London at night from the air London at night from the air. The roads look the veins and arteries of a living being.

ver·ti·go (Noun): A sensation of whirling and loss of balance, associated particularly with looking down from a great height, or caused by disease…

I have known for some time that I suffer from two forms of vertigo. The first is the normal form, induced by looking down over the edges of cliffs or tall buildings: I have to believe that this perfectly normal.

The second is age vertigo which involves similar dizziness, nausea and panic, but is induced by meeting adults who are much younger than me. My head spins as I focus on the vastness of the gap separating me from them – a gap across which we can converse, but not traverse. I cannot travel back to meet them, and by the time they reach my place on the cliff-face of…

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Is a UK grid-scale battery feasible?

April 26, 2019

This is quite a technical article, so here is the TL/DR: It would make excellent sense for the UK to build a distributed battery facility to enable renewable power to be used more effectively.

=========================================

Energy generated from renewable sources – primarily solar and wind – varies from moment-to-moment and day-to-day.

The charts below are compiled from data available at Templar Gridwatch. It shows the hourly, daily and seasonal fluctuations in solar and wind generation plotted every 5 minutes for (a) 30 days and (b) for a whole year from April 21st 2018. Yes, that is more than 100,000 data points!

Wind (Green), Solar (Yellow) and Total (Red) renewable energy generation for the days since April 21st 2018

Wind (Green), Solar (Yellow) and Total (Red) renewable energy generation for 30 days following April 21st 2018. The annual average (~6 GW) is shown as black dotted line.

Slide7

Wind (Green), Solar (Yellow) and Total (Red) renewable energy generation for the 365 days since April 21st 2018. The annual average (~6 GW) is shown as black dotted line.

An average of 6 GW is a lot of power. But suppose we could store some of this energy and use it when we wanted to rather than when nature supplied it. In other words:

Why don’t we just build a big battery?

It turns out we need quite a big battery!

How big a battery would be need?

The graphs below shows a nominal ‘demand’ for electrical energy (blue) and the electrical energy made available by the vagaries of nature (red) over periods of 30 days and 100 days respectively. I didn’t draw the whole year graph because one cannot see anything clearly on it!

The demand curve is a continuous demand for 3 GW of electrical power with a daily peak demand of 9 GW. This choice of demand curve is arbitrary, but it represents the kind of contribution we would like to be able to get from any energy source – its availability would ideally follow typical demand.

Slide8

Slide9

We can see that the renewable supply already has daily peaks in spring and summer due to the solar energy contribution.

The role of a big battery would be cope to with the difference between demand and supply. The figures below show the difference between my putative demand curve and supply, over periods of 30 days and a whole year.

Slide10

Slide11

I have drawn black dotted lines showing when the difference between demand and supply exceeds 5 GW one way or another. In spring and summer this catches most of the variations. So let’s imagine a battery that could store or release energy at a rate of 5 GW.

What storage capacity would the battery need to have? As a guess, I have done calculations for a battery that could store or release 5 GW of generated power for 5 hours i.e. a battery with a capacity of 5 GW x 5 hours = 25 GWh. We’ll look later to see if this is too much or too little.

How would such a battery perform?

So, how would such a battery affect the ability of wind and solar to deliver a specified demand?

To assess this I used the nominal ‘demand‘ I sketched at the top of this article – a demand for  3 GW continuously, but with a daily peak in demand to 9 GW – quite a severe challenge.

The two graphs below show the energy that would be stored in the battery for 30 days after 21 April 2018, and then for the whole following year.

  • When the battery is full then supply is exceeding demand and the excess is available for immediate use.
  • When the battery is empty then supply is simply whatever the elements have given us.
  • When the battery is in-between fully-charged and empty, then it is actively storing or supplying energy.

Slide12

Over 30 days (above) the battery spends most of its time empty, but over a full year (below), the battery is put to extensive use.

Slide13

How to measure performance?

To assess the performance of the battery I looked at how the renewable energy available last year would meet a levels of constant demand from 1 GW up to 10 GW with different sizes of battery. I consider battery sizes from zero (no storage) in 5 GWh steps up to our 25 GWh battery. The results are shown below:

Slide15It is clear that the first 5 GWh of storage makes the biggest difference.

Then I tried modelling several levels of variable demand: a combination of 3 GW of continuous demand with an increasingly large daily variation – up to a peak of 9 GW. This is a much more realistic demand curve.Slide17

Once again the first 5 GWh of storage makes a big difference for all the demand curves and the incremental benefit of bigger batteries is progressively smaller.

So based on the above analysis, I am going to consider a battery with 5 GWh of storage – but able to charge or discharge at a rate of 5 GW. But here is the big question:

Is such a battery even feasible?

Hornsdale Power Reserve

The Hornsdale Power Reserve Facility occupies an area bout the size of a football pitch. Picture from the ABC site

The Hornsdale Power Reserve Facility occupies an area about the size of a football pitch. Picture from the ABC site

The biggest battery grid storage facility on Earth was built a couple of years ago in Hornsdale, Australia (Wiki Link, Company Site). It seems to have been a success (link).

Here are its key parameters:

  • It can store or supply power at a rate of 100 MW or 0.1 GW
    • This is 50 times smaller than our planned battery
  • It can store 129 MWh of energy.
    • This is just under 40 times smaller than our planned battery
  • Tesla were reportedly paid 50 million US dollars
  • It was supplied in 100 days.
  • It occupies the size of a football pitch.

So why don’t we just build lots of similar things in the UK?

UK Requirements

So building 50 Hornsdale-size facilities, the cost would be roughly 2.5 billion dollars: i.e. about £2 billion.

If we could build 5 a year our 5 GWh battery would be built in 10 years at a cost of around £200 million per year. This is a lot of money. But it is not a ridiculous amount of money when considering the National Grid Infrastructure.

Why this might actually make sense

The key benefits of this kind of investment are:

  • It makes the most of all the renewable energy we generate.
    • By time-shifting the energy from when it is generated to when we need it, it allows renewable energy to be sold at a higher price and improves the economics of all renewable generation
  • The capital costs are predictable and, though large, are not extreme.
  • The capital generates an income within a year of commitment.
    • In contrast, the 3.2 GW nuclear power station like Hinkley Point C is currently estimated to cost about £20 billion but does not generate any return on investment for perhaps 10 years and carries a very high technical and political risk.
  • The plant lifetime appears to be reasonable and many elements of the plant would be recyclable.
  • If distributed into 50 separate Hornsdale-size facilities, the battery would be resilient against a single catastrophic failure.
  • Battery costs still appear to be falling year on year.
  • Spread across 30 million UK households, the cost is about £6 per year.

Conclusion

I performed these calculations for my own satisfaction. I am aware that I may have missed things, and that electrical grids are complicated, and that contracts to supply electricity are of labyrinthine complexity. But broadly speaking – more storage makes the grid more stable.

I can also think of some better modelling techniques. But I don’t think that they will affect my conclusion that a grid scale battery is feasible.

  • It would occupy about 50 football pitches worth of land spread around the country.
  • It would cost about £2 billion, about £6 per household per year for 10 years.
    • This is one tenth of the current projected cost of the Hinkley Point C nuclear power station.
  • It would deliver benefits immediately construction began, and the benefits would improve as the facility grew.

But I cannot comment on whether this makes economic sense. My guess is that when it does, it will be done!

Resources

Data came from Templar Gridwatch

 

Here and there. Now and then

April 21, 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.

But even so, I find it hard to know how to act…

On the one hand… 

It has been a beautiful April day.

On the other hand… 

Today, Sea Ice Extent in the Arctic is lower than it has ever been on this date since satellite measurements began in 1979. (Link)

Arctic Sea Ice Extent for March to May from every year since 1979.

Arctic Sea Ice Extent for March to May from every year since 1979.

On the one hand… 

I strongly support the aims of Climate protesters in London. I share their profound frustration.

On the other hand… 

I feel the protesters are not being honest about the impact of the actions they advocate.

For example, I think if their wishes were granted, we would all be obliged to use much less energy and I only know two ways to do that.

  • The first method is to increase the price of energy – famously not a route to popularity.
  • The second method is to ration energy which has not been attempted in the UK (that I can recall) since the 1974 Oil Crisis.

One could use some combination of these two methods, but I don’t know of any fundamentally different ways.

We are all in favour of ‘Saving the Planet’, but higher energy costs or rationing would be wildly unpopular. This would increase the cost of almost all products and services.

I would vote for climate action and an impoverishment of my life and my future in a heartbeat. But I am well off.

Unless other people are convinced, and until we find a way to address this problem which is acceptable to those who will be most hurt in the short term – poorer people –  it will never actually happen. And all I care about is that it actually happens.

On the one hand… 

I strongly support the goal of a zero-carbon economy.

On the other hand… 

If the existing carbon-intensive economy reduces in scope too fast, then we will lack the resources to create the new economy.

On the one hand… 

David Attenborough spoke movingly on television this week about ‘Climate Change: the facts.

David Attenborough

David Attenborough

I watched his programme and while it’s not the story I would have told, it seemed to me to be a pretty straightforward and a fair presentation.

On the other hand… 

Not every one thought it fair. Here are specific comments (1, 2) or follow these links for torrents more similar stuff (Link#1, Link#2, Link#3). I disagree with these people, and their specific points are broadly irrelevant. But their votes are worth just as much as mine.

On the one hand… 

I am trying hard to lower the amount of energy I personally use.

I am measuring the energy use of appliances, reading my meters once a week and switching things off.

My aim is to reduce the electrical power being used by an average of more than 200 watts.

Over one year this will reduce my carbon dioxide emissions by around 0.35 tonnes. (Link).

On the other hand… 

Last year I was invited to give a keynote talk at a conference in New Zealand. I was honoured and said ‘Yes’.

This will cause an additional 7.4 tonnes of carbon dioxide to be emitted. (Link)

CO2 flight to New Zealand

Andrea Sella has written about this issue and perhaps we are at the end of the era of hypermobility.

On the one hand… 

I felt sad when I saw Notre Dame in flames.

On the other hand… 

I feel sad about droughts and floods and wild fires and destroyed livelihoods and brothers and sisters in poverty around the world.

If billions of euros can be found ‘in an instant’ for Notre Dame, why can’t we address these much more serious and urgent problems as dynamically?

And on this Easter day, I think:

What would Jesus do? 

 

I’m gonna sit right down and write myself a paper

April 9, 2019

 

I’m gonna sit right down and write myself a paper

(with apologies to Fats Waller)

I’m gonna sit right down and write myself a paper

And make believe that it’s all true

My Method’s technically sweet

with results to knock me off my feet.

Refer-en-ces at the bottom

I’ll be glad I got ’em

 

I’m gonna pick the best of my ‘typ-i-cal’ data

Make a table and a graph or two

I’ll do the theory section later

With my collaborator

We’ll add some x’s, y’s, and zee’s

To impress the referees.

 

I need more citations for that job at CalTech.

They won’t take me with a h-index of two

But if I can get this paper

Into ‘Science’, or to ‘Nature’

I’ll be on my way…

…to Cal-i-forn-i-a…

 

I’ll conclude and say that more work is required

And further funding should accrue…

I’m gonna sit right down and write myself a paper

And maybe I’ll acknowledge you.

 

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.

Bottle

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!

Learning

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’

Slide1

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.

Slide2

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.

Slide3

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.

Data

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.

Slide4

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!

Slide5

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…


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