pollution

Clouds, condensation and coffee

Clouds in my coffee. There is, perhaps unsurprisingly, plenty of atmospheric physics you can encounter in your cup.

As we approach the end of the year, it is a good time to notice the changes in the weather. If you are in the northern hemisphere, the nights grow longer as the days grow colder. If you are in the southern hemisphere it is the opposite. And yet around the world, we have things in common. There may be days when it is more cloudy and days when there is a heavy dew (or even in some places a frost) on the grass. But what has this to do with coffee?

It’s to do with some experiments that you can do at home or on your way to work. And, in particular, with two effects you can see in your coffee cup.

To start with the dew, perhaps you’ve noticed the condensation around the rim of the cup or the coffee pot when you brew the coffee and the hot steam condenses onto the cold mug around it. Condensation happens because the temperature of the mug is lower than the ‘dew point’ of water at that humidity and pressure. Below the temperature of the dew point, the water vapour will condense into the liquid droplets that we then see dotted around the mug.

coffee bowl pour over
You can see the condensation on the V60 brewer here. Looking at the dew formed in the mornings, what does it tell you about the temperature of space?

It is a similar effect on the grass: the temperature there is lower than the point at which the water vapour in the air starts to condense out of the air and so you get dew. William Charles Wells published his “Essay on Dew” in 1814. The result of more than two years of careful observation, Wells found that dew formed only under certain weather conditions and only on certain space (sky) facing surfaces. Wells’ results can be used to show that the space around the earth is much colder than the surface of our planet. His results (together with some back of the envelope calculations) can therefore also be used to show that the Earth is in a delicate balance and has a natural greenhouse effect. As the weather changes this year and you notice the dew, can you see how Well’s could come to this conclusion?

The second coffee experiment we could do at this time of year is to see whether pollution affects our steaming take-away coffee. While generally it’s always a better idea to sit in a cafe and take the time to enjoy your coffee, there are occasions when a take-away is necessary. Just as with the dew, clouds start to form when the air temperature drops below the dew point. However, water droplets in the air are unstable to evaporation and so as soon as a pure water droplet is formed, it will evaporate unless it has a diameter larger than about 0.1 µmª. This may seem small and yet to spontaneously form a droplet with this diameter would take the accumulation of several million water molecules (I will leave it to you to do the estimate!). This represents a very improbable occurrence and yet we can see that clouds are everywhere, how can this be?

contrail, sunset
Contrails are caused by condensing water droplets behind aeroplanes. But why are they white and what does that tell you about the water droplets within them?

The answer comes from the dust. Fortunately we are a dusty planet and these bits of dust in the atmosphere act as ‘nucleation’ points for water to condense onto. This makes the condensation of water into droplets much more likely and so clouds – which are an accumulation of droplets – can form.

Which brings us back to the coffee. If clouds require dust in order to form droplets, and the steam above your coffee is a grouping of water droplets, does it not make sense that your coffee should be steamier next to a polluted road than in the middle of a park (for the same temperature coffee)?

It’s an idea that I’ve never been able to test but the shift to colder weather here offers a(nother) perfect opportunity.

Does your coffee steam more when you take it away from a city cafe?

I look forward to hearing about the results of your experiments, in the comments here, on Twitter or on Facebook.

ª Introduction to Atmospheric Physics, Andrews, Cambridge University Press, 2008

Environmentalism inside and out at Farmstand, Covent Garden

Farmstand Drury Lane

Farmstand on Drury Lane

How can we live sustainably, buying locally, being mindful of our ecological footprint and still drink coffee? A recent trip to Farmstand on Drury Lane revealed a café conscious of its environmental responsibilities, somewhere that is trying to help us to make a difference while still enjoying good food and great coffee. Is it possible for us to have our coffee and drink it? The people behind Farmstand certainly seem to think so.

The bare brick walls inside the spacious Farmstand have a certain rustic charm that serves to emphasise the environmental concerns of the café. A focus on local, free range meat and GM free vegetables means that this is definitely a place to be considered when looking for a lunch spot (though on this occasion, we only tried the coffee). Coffee is obviously not locally grown but is roasted by Workshop which is, relatively speaking, just down the road. Tea meanwhile comes from Postcard teas, just up the street. Water is complementary and is provided on tap so as to reduce plastic waste. The service was friendly and with such a bright and airy feel it is a very pleasant space to enjoy an Americano (though I imagine it is fairly crowded at lunchtimes). However, the Americano was served in a take-away cup (when I specified I was staying in). After a bit of digging on their website, I discovered that they use compostable and/or recyclable packaging sourced from London Bio Packaging. However, as it is not easy to either recycle nor to compost cups in regular waste collection (including recycling collections), it would be interesting to know details of how they dispose of their cups so as to know how they reconcile this with the otherwise careful environmental policy.

Interior vertical gardening

Green wall inside Farmstand

As you enter the café, there is a staircase on the left hand side. Potted plants are fixed to the railings making what seems to be almost a miniature green wall. A great way to get houseplants into a small space, this seemed a small scale example of the green walls that are starting to pop up around our cities. Green walls are vertical gardens. They can be grown either with climbing plants or with a second structure on the wall that supports the hundreds of plants. Along with an aesthetic appeal (certainly true of the structure at Farmstand), these green walls have environmental benefits too.

A big environmental problem in cities is particulate pollution from exhausts. Specifically, particulate matter that is less than 10 μm diameter (think Turkish coffee grind) can irritate the lungs and cause health problems for the city’s inhabitants. Particulates less than 2.5 μm diameter are even more dangerous to health. Worldwide, in 2012, 3.7 million early deaths were associated with poor air quality. In London, a 2010 study showed that approximately 4000 deaths per year were the result of exhaust fumes. Which brings us to the first reason that green walls in cities may be such a good thing: Plants adsorb the pollutants.

Green wall Singapore

A green wall at the Ocean Financial Centre in Singapore, Image shared under cc license (attrib. share alike) by smuconlaw.

Over a three month period, a study by Imperial College showed that a single green wall on Edgware Road tube station had removed 515 g of particulate matter from the atmosphere. Using a mix of plants on the wall was found to increase the air turbulence around the wall and so increase the adsorption of the pollutants. Of course, different plants performed differently (in terms of their ability to remove particulate matter from the air). One of the plants on the wall (Convolvulus cneorum) could take out up to 2.73±0.16 g/m² of particulate matter*. On the other hand, another plant on the wall (Hedera helix) took out much less, removing only 0.28±0.02 g/m². However, we know Hedera helix by another name: Ivy. And ivy plants can produce a lot of foliage per plant very quickly. Convolvulus cneorum on the other hand, is a small plant with small leaves. While its efficiency could be very high, the amount of pollution it can remove may not be as great as an ivy plant, purely as a consequence of its leaf size.

Which brings us to questions of aesthetics and practicality. The wall at Edgware Road is planted with many different types of plant in order to produce an effect that reduces pollution while also being good to look at. Similar walls have sprouted up all over the world. However, for short term projects that require a large amount of foliage quickly, planting ivy can be a good option as a pollutant remover. Some of the temporary structures built along Park Lane for the Crossrail project are now covered with ivy. Although I had initially thought that this was due to a lack of weeding, it turns out that this is part of a step towards pollution reduction in our cities (modelling data has indicated that these green walls can reduce the local particulate pollution by 10-20% depending on the geometry of the wall and the plant species growing).

A small step perhaps, but one that is definitely in the right direction. The green wall at Farmstand could therefore be said to illustrate the idea that if we are to make a difference to our external world, we must start by reforming our own interior one. We need to make green walls not green wash and we can start by paying attention to what we plant inside and out.

Farmstand is at 42 Drury Lane, WC2B 5AJ

*The study looked at particulate matter between 2.5 µm and 10µm diameter (i.e. PM(2.5)-PM(10)).

 

 

Clouds in my coffee

clouds over Lindisfarne

How do clouds form?

Does your coffee appear to steam more next to a polluted road than in the countryside?

This is a question that has been bothering me for some time. Perhaps it seems an odd question and maybe it is, but it is all about how clouds form. Maybe as you read this you can glance out the window where you will see blue skies and fluffy white clouds. Each cloud consists of millions, billions, of water droplets. Indeed, according to the Met Office, just one cubic metre of a cloud contains 1 hundred million water droplets. We know something about the size of these droplets because the clouds appear white which is due to the way that particles, including water droplets, scatter sunlight. Clouds appear white because the water droplets scatter the sunlight in all directions. In contrast, the particles in a cloudless sky scatter blue light (from the Sun) more than they scatter red. Consequently, from our viewpoint, the scattered light from the clouds appears white while the sky appears blue. The sort of directionless light scattering that comes from the clouds happens when the scattering sites (ie. the water droplets) are of a size that is comparable to, or larger than, the wavelength of light. This means that the water droplets in a cloud have to be larger than about 700 nm in diameter (or approximately just less than a tenth of the size of the smallest particle in an espresso grind). The particles in the atmosphere on the other hand scatter blue light more than they scatter red light because they are smaller than the wavelength of the blue light. You can find out more about light scattering, blue skies and cloudy days, with a simple experiment involving a glass of milk, more details can be found here.

glass of milk, sky, Mie scattering

A glass of (diluted) milk can provide clues as to the colours of the clouds in the sky as well as the sky itself

So each of the one hundred million water droplets in a cubic metre of cloud is at least about a micron in diameter. We can then estimate how many water molecules make up one droplet by dividing the mass of a droplet of this size by the mass of one water molecule. This turns out to be more than 1000 million water molecules that are needed to make up one droplet of cloud. So, 1000 million water molecules are needed for each of the 100 million drops that make up one, just one, cubic metre of cloud. These numbers are truly huge.

But can so many molecules just spontaneously form into so many water droplets? Unlike a snowball, the water droplet in a cloud cannot start very small and accumulate more water, getting larger and larger until it forms a droplet of about a micron in size. Water droplets that are much smaller than about a micron are unstable because the water molecules in the drop evaporate out of it before they get a chance to form into a cloud (precise details depend on the exact atmospheric conditions). Water droplets need to come ‘ready formed’ to make the clouds which seems unlikely. So how is it that clouds can form?

Condensation on mug in CGaF

Look carefully at the rim of the mug. Do you see the condensation?

It turns out that the water droplets form by the water condensing onto something in the atmosphere. That something could be dust, or salt or one of the many other sorts of aerosol that are floating around in our skies. Just as with a cold mug filled with hot coffee, the dust in the air gives the water molecules a cold surface onto which they can condense. This sort of water droplet can ‘snowball’ into the bigger droplets that form clouds because the water is now condensing onto something and so does not evaporate off again so easily. At the heart of each water droplet in a cloud is a bit of dust or a tiny crystal of salt. Which brings me back to my question. It is much more dusty along a polluted road  than it is in the clean air of the countryside. Is this going to be enough of an effect to affect the probability of cloud formation? Does your coffee steam more as you cross the road than when you walk through the park?

It is a question that demands an experiment (and associated video). Last year, the Met Office suggested this simple experiment for observing clouds in a bottle. Unfortunately however, I have yet to make this experiment work in a way that would allow me to test whether polluted air produces thicker clouds than cleaner air. If you have any suggestions as to a good experiment (that will work on camera!) please let me know either in the comments section, by emailing me, or on Facebook. In the meanwhile, I’d be interested to know what you think, so if you think this post is about you, please let me know.

 

 

Seeing the trees for the wood at OJO Coffee, Bangsar, KL

coffees on display at OJO

OJO Coffee, Bangsar, Kuala Lumpur

It is very easy to sit for a long time watching the people and the surroundings at OJO Coffee in Bangsar, Kuala Lumpur. Initially I had thought that this medium-sized café with an impressive number of power points dotted around it was an independent. However similarities with CoffeaCoffee around the corner and a couple of other clues (CoffeaCoffee t-shirts) suggest that it is actually part of the CoffeaCoffee chain, something that was confirmed when I asked the barista. However, the standard of coffee in this chain should prompt some of the smaller independents to up their game a bit (and certainly all of the UK based chains). Not content with just serving the typical coffees of ‘latte’, ‘cappuccino’ etc. (which are made using their own blend), OJO’s additionally serves about 15 types of single origin coffee made with your choice of method (Hario V60, Aeropress or French press). For a while this summer I became a bit of a regular at OJO and so I would particularly recommend the Indonesian Sumatran prepared by V60, but with so many coffees to choose from (from the relatively local Indonesians to South American coffees from much further afield) there is plenty to try at this café.

wooden mosaic

The wall made of wood at OJO

The interior of OJOs is decorated with many types of wood. Different cuts of wood are made into a sort of wood mosaic on the wall while the tables are made using several types of wood so as to give a symbolism about the Sun that is a type of motif of the café. Much of the floor is wood too and so this got me thinking about the rainforests in this country. Malaysia has a rich variety of wildlife and forest, it is home to the Orangutan as well as many other species. Teak trees that can be used for more expensive furniture grow along the roadside. Much of this timber can be obtained sustainably and in a way that respects the rainforest and I am certainly not suggesting that the wood in OJOs was anything but sustainable. However, perhaps inevitably, there are many pressures on these invaluable forests. Some of these pressures have, in the recent past, resulted in significant deforestation. One such pressure is that of palm oil.

Palm oil is a massively useful commodity. It is now used in food products from margarine to biscuits to raisins (surprising but true, check the ingredients list of a packet of raisins) and non-food products such as soaps. It is literally everywhere. Both Malaysia and its neighbour, Indonesia, have profited enormously from growing and exporting palm oil. Unfortunately, at times the rainforest is cleared to make way for the palm oil plantations. As it is easier to burn felled trees to clear the land rather than to painstakingly pull the roots up by hand, the cleared forest is burned. But the ground is not any ordinary soil, the ground is often peat based which means that the fires on the surface penetrate deep below the ground and produce phenomenal amounts of smoke.

If at this point you were wondering where the ‘physics’ bit of this café-physics review is, I assure you it is coming. It is indeed linked to this environmental story and to OJOs, please keep with me.

Each year, parts of Malaysia, Singapore and Indonesia are enveloped by a haze produced by this burning peat land (It made the BBC in 2013 when it was particularly bad, but some haze is present for a few weeks every year). Haze has the appearance of thick fog but smells of smoke. At times, visibility can be reduced such that the tops of nearby tall buildings are obscured. Each time land needs clearing for new palm oil plantations, this smoke is produced. The haze can be reduced by local weather patterns but on many days, the haze is cleared by the torrential rains that can occur in this part of the world.

the haze is coming in

L-R: The haze comes in over part of KL in 2013 (series of 3 pictures)

It is commonly said that ‘rain clears the air’ but this is not completely true. It is not the raindrops themselves that somehow wash the air free of the dust of the haze, it is the vortices that form behind them*. Just as a spoon dragged through coffee produces vortices behind it, so a raindrop falling through the air forms vortices in its wake. The size of these vortices will depend on the size of the drop and the speed at which it falls through the air; a tea spoon and a dessert spoon pulled at different speeds through the coffee similarly produce different forms of vortex. So the amount of dust that is ‘sucked in’ and falls to the ground will depend on the type of rain that falls. Perhaps if you are in Malaysia, Singapore or Indonesia when this haze is present, you could make a study of which sort of rain clears the air most effectively. I have an idea but not the evidence to see if the idea is correct, it would be interesting to know what you think.

As I left OJO one afternoon, the rain had started to come down. The rain, or at least the vortices behind the raindrops, cleared some of the haze that had been around earlier. It is a temporary solution to a longstanding problem. A more long lasting solution may be to start (or continue) asking manufacturers of those biscuits you are eating: just how sustainable is the palm oil they are using?

OJO is at No 23, Jalan Telawi 3, KL

* JR Saylor and BK Jones, Physics of Fluids, 17, 031706 (2005)