Essay on dew

On dew, greenhouses & IR thermometers: Coffee & Science at Amoret

starting with a coffee
Always good to start with a coffee. The evening started with two coffees (from Ethiopia and El Salvador). What will be the effects of climate change on the coffee industry?

January 2020 was the 6th warmest on record in the UK, with a mean temperature 2C higher than the 1981-2010 average. Early in February it was announced that Antartica had recorded the highest temperature ever recorded there of 18.3C, beating the previous record of 17.5C in March 2015. The atmospheric concentration of CO2 in January 2020 was measured to be 413 ppm following the trend that has seen the atmospheric CO2 concentration increase more than 10% from just the year 2000. That the polar regions would warm faster than other parts of the planet had long been a prediction of global warming based on increased CO2 emissions. Nonetheless, to see the figures reported quite so starkly was startling.

Each month brings new headlines and more concerns about whether we are responding fast enough to limit global warming to 1.5 or 2C. And yet, the greenhouse effect was proposed back in 1824; the idea that carbon dioxide (and water vapour) were greenhouse gases suggested during the 1850s (1,2) and it was back in 1895 that Arrhenius predicted that doubling the atmospheric levels of CO2 (relative to 1890s levels) would result in a global temperature increase of 5-8C.

So given that it is such an established theory, why are we still arguing about it? And, more importantly perhaps, what has this to do with coffee?

It is, in many ways, an ideal connection for the theme for one of the Coffee & Science evenings that we’ve been holding at Amoret Coffee in Notting Hill. And so it was that a group of us got together over coffee to discuss the greenhouse effect and its links to coffee.

coffee bowl pour over
The first connections can be seen with the condensation. How does dew form, and why does it suggest that space is cold?

The first coffee-greenhouse connection is in the condensation. When you make a pour over, or even if you pour your coffee into a cold mug, you will notice the condensation forming on the colder glass (or ceramic) surfaces as the steam evaporates. We know that the droplets form because the temperature of the surface is below that at which water vapour will re-condense into liquid. Technically, this temperature is known as the dew point. And it is partly to dew that we owe our understanding of the greenhouse effect.

Back in 1814, William Charles Wells made a series of detailed observations about how, where and when dew formed. He was able to show that more dew formed on clear (or not terribly cloudy) nights and on surfaces that were exposed to the sky; they were space facing. Which brings us to a second coffee connection: just as your coffee cup warms you by radiating its heat (in the infra red) to your hands, so all objects with heat radiate their energy out. Wells realised that this meant that space was cold because, just as a coffee cup if it is not being heated and not surrounded by reflecting material (think about the inside of a thermos flask) will radiate its heat and get cold* so the surfaces of the earth, if there is no energy coming in from space and no surfaces above them to reflect their heat back at them, will also get cold.

If space is cold, you can calculate what the temperature of the Earth should be if the energy it is losing is balanced by the energy it gains from the Sun and when you do this, it turns out that the mean temperature of the Earth should be -18C or about 30C lower than that observed**.

Earth from space, South America, coffee
One common home.
The Blue Marble, Credit, NASA: Image created by Reto Stockli with the help of Alan Nelson, under the leadership of Fritz Hasler

This leads to the idea that there is a natural greenhouse effect whereby gases in the Earth’s atmosphere form a layer which lets through a large amount of the energy from the Sun but lets a lot less energy escape back through it from the Earth (owing to the lower frequency of the radiation being emitted by the Earth compared with that coming in from the Sun). This ‘natural’ greenhouse effect results in a warming of the Earth to a delicate balance and to the temperatures that we experience on Earth***. Fairly clearly, if this delicate balance is disturbed by adding extra greenhouse gases to the atmosphere it will lead to a warming effect (as Arrhenius predicted back in 1895), the question is how much and how fast?

We were very fortunate to be joined for the evening by Dr Robin Lamboll of the Grantham Institute of Imperial College London. Robin explained the latest science and understanding of the effects of climate change and of adding increased CO2 into the atmosphere. Particularly highlighting how an increase in CO2 leads to an increase in water vapour (owing to the initial temperature increase produced by the CO2) which is itself a greenhouse gas, and so the warming effects of a small amount of CO2 can be amplified by this mechanism.

At this point the conversation diverged away from coffee, not just because Robin is a tea drinker (!) but we moved onto the effects of sulphur dioxide in the atmosphere, local vs global temperature effects and the science of Eunice Newton Foote. We discussed what we know, and what we are just starting to understand, such as how what happens in one part of the world may lead to consequences in other parts of the world (weather wise). We also got to a discussion of albedo and the reflection of heat by ice via playing with a couple of infra red thermometers that we had to hand and the different ways that human eyes and shrimp eyes detect colour. How is this connected to climate change and coffee? I’m afraid that there is a connection but the path to it is a little circuitous for a write up. It’s the sort of thing that pops up when you have a number of people of different backgrounds all contributing to the discussion. This is what, from my point of view, makes these evenings so interesting (and on a personal level induces such pre-event nerves): the fact that the conversation can go in so many directions, with such different contributions from the attendees, that each evening takes on a different character, with a different set of connections and a new set of things to think about. I hope that others feel the same way!

“An Essay on Dew”, Wells book of 1815 summarising his observations on dew. An excellent piece of observational science.

Our next Coffee & Science evening is scheduled for March 2020. Please do sign up to the events list or keep an eye on the Facebook events page to learn details as they are announced. Thanks again to Dr Robin Lamboll for coming along in January. I look forward to seeing both familiar faces and some new people in March.

Bean Thinking’s Evenings of Coffee & Science @ Amoret Coffee are held approximately every 2 months from 5.30 until about 8pm at Amoret Coffee in Notting Hill. More details can be found here.

*Two caveats here: firstly the coffee will also get cold through convection and conduction, the connection is illustrative rather than precise – though were you to put your coffee into a vacuum it would cool via radiative cooling only. Secondly, Wells himself never made the coffee connection but instead considered the latest physics theories about heat.

**In “Introduction to Atmospheric Physics”, David Andrews, (2000)

***For details about how we can know what the temperatures have been over such a time period and the effects of other cyclical temperature variations on the climate, it’s worth reading “The Ice Chronicles” P Mayewski & F White, (2002)

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

In the Greenhouse at CoffeeGeek

Coffee Geek and Friends, Coffee Victoria

Coffee Geek and Friends

Earlier this year, a new café opened up in Victoria. Coffee Geek and Friends is located at the far end of Cardinal Place as you enter from Victoria Street. Cardinal Place is an odd sort of shopping centre, a small collection of shops with a glass roof. The building site near Coffee Geek as well as the constant stream of people rushing to and fro make Coffee Geek an ideal place to spend some time watching the world go by. Coffee is by Allpress espresso and is served in very individual mugs. Apparently there is a range of geek-ery in the cafe including a ‘centre piece’ water filter but I admit I missed that as I was too focussed on my coffee. Coffee Geek and Friends is definitely a cafe to keep in mind (along with Irish & June’s) if you need a good place to meet near Victoria Station.

It was a very humid day when I enjoyed my coffee at Coffee Geek and, because the mug had not been pre-warmed before my Americano/long black (my notes don’t specify which) was poured into it, condensation quickly formed around the rim of the mug. The condensation forms for the same reason that dew forms after a cool night: the vapour pressure of the water above the coffee (or the ground) has reached the dew point at the temperature of the mug. The lower the temperature, the lower the vapour pressure has to be for the water in the atmosphere to start condensing into liquid droplets. Hence you will often find that your coffee is more ‘steamy’ on a winter’s, rather than a summer’s day.

Condensation on mug in CGaF

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

Just over two hundred years ago, William Charles Wells made a study of dew. He observed the weather conditions under which dew formed. He observed on which surfaces dew collected. He noted whether the dew formed on space facing surfaces or ground facing surfaces. After several years of careful study he published his “Essay on Dew” in 1814. His work, showed that the earth radiated heat at night (when it was not being kept warm by the Sun) and therefore that space was cold. Cloud cover reduced the amount by which the ground cooled which implied that cloud cover was acting as a type of blanket for the Earth, keeping the heat trapped inside. Later calculations of the balance between the heat radiated by the Earth and the heat received by the Sun confirmed that, without some heat getting trapped by clouds and ‘greenhouse’ gases in the atmosphere, the earth would be a good 30 C cooler than it is observed to be. Although these calculations are just rough, “back of the envelope” figures, detailed calculations confirm that the Earth is in a delicate balance, heated by the Sun, cooled by radiation and kept warm (and live-able) by a layer of natural greenhouse gases. This “natural greenhouse effect” has been necessary for our development, the problem is that now we are adding yet more greenhouse gases to the atmosphere which threatens to tip the established delicate balance by a few degrees.

Cardinal Place roof, greenhouse

The roof of Cardinal Place shopping centre. A very appropriate place for a meditation on the greenhouse effect

What we now call the greenhouse effect are these extra gases, which are more efficient at trapping heat within our atmosphere. If you can imagine what has been happening over the past three hundred years or so as we have been pumping yet more of these gases into the atmosphere at an accelerated rate, we are in danger of tipping this delicate balance towards further heating of the earth. The 2015 Paris Climate Conference is being held with the aim of requiring all nations to agree to a legally binding commitment to reduce the amount of extra greenhouse gases that we emit to a level that will only result in a temperature increase of 2C. To achieve this requires all of us to work together to reduce our own ‘carbon footprint’. Each of us will have to find our own, individual ways to reduce our emissions but perhaps when we look at the condensation on the rim of our coffee cup, we could remember William Charles Wells and his essay on dew and just think, what can I do, at this moment, to reduce my carbon footprint? Maybe it could be something as simple as turning off that phone (to conserve the battery) and watching what is going on in a café instead. A small gesture but one that would be good for us as well as the earth.

Coffee Geek and Friends is at the northern end of Cardinal Place shopping centre (opposite Westminster Cathedral).

As a Coffee Geek note, I would like to just comment that my notes on Coffee Geek and Friends were written using a “linux-sure” ball point pen. Not particularly environmentally friendly but definitely quite geeky.