Essay on dew

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.