During these strange times of working from home, perhaps you, like me, have been preparing a lot more coffee. For me this has included, not just my regular V60s, but a type of cafe-au-lait for someone who used to regularly drink lattes outside. My previous-latte-drinker turns out to be a little bit discerning (the polite way of phrasing it) and so prefers the coffee made in a similar way each day. Which is why I’ve been weighing the (oat) milk I’ve been using.
So, each morning to prepare a coffee, I’ve been using a V60 recipe from The Barn and then, separately, weighing out 220g of refrigerated oat milk into a pan that I then heat on the stove. Generally I heat the milk for just over 5 minutes until it is almost simmering whereupon I pour it into a mug (with 110 – 130g of coffee inside – depending on the coffee). Being naturally lazy, I keep the cup on the scales so that it is easier to pour the milk in and then, completely emptying the pan into the coffee, the scales register an increase of mass (of milk) in the cup of 205-210g. Which means about 10-15g of milk goes missing each morning.
Now clearly it is not missing as such, it has just evaporated, but it does prompt a question: can this tell us anything about the physics of our world? And to pre-empt the answer, it actually tells us a great deal. But to see how, we need to go on an historical diversion to just over three hundred years ago, when Edmond Halley was presenting an experiment to the Royal Society in London. The experiment shares a number of similarities with my heated oat milk pan. It was later written into a paper which you can read online: “An estimate of the quantity of vapour raised out of the sea by the warmth of the Sun; derived from an experiment shown before the Royal Society at one of their late meetings: by E Halley“.
Halley heated a pan of water to the temperature of “the Air in our hottest summers” and then, keeping the temperature constant, placed the pan on a set of scales to see how much water was lost over 2 hours. The temperature of the air in “our hottest summers” cannot have been very high, perhaps 25-30C and there was no evaporation actually seen in the form of steam coming from the pan (unlike with my milk pan). Nonetheless, Halley’s pan lost a total of 13.4g (in today’s units) of water over those two hours.
Halley used this amount to estimate, by extrapolation, how much water evaporated from the Mediterranean Sea each day. Arguing that the temperature of the water heated that evening at the Royal Society was similar to that of the Mediterranean Sea and that you could just treat the sea as one huge pan of water, Halley calculated that enough water evaporated to explain the rains that fell. This is a key part of the water cycle that drives the weather patterns in our world. But Halley took one further step. If the sea could produce the water for the rain, and the rain fed the rivers, was the flow of the rivers enough to account for the water in the Mediterranean Sea and, specifically, how much water was supplied to the sea compared to that lost through the evaporation? Halley estimated this by calculating the flow of water underneath Kingston Bridge over the Thames. As he knew how many (large) rivers flowed into the Mediterranean, Halley could calculate a very rough estimate of the total flow from the rivers into the Mediterranean.
The estimates may seem very rough, but they were necessary in order to know if it was feasible that there could be a great water cycle of rain, rivers, evaporation, rain. And although Halley was not the first to discuss this idea (it had been considered by Bernard Palissy and Pierre Perrault before him), this idea of a quantitative “back of the envelope” calculation to prompt more thorough research into an idea, is one that is still used in science today: we have an idea, can we work out, very roughly, on the back of an envelope (or more often on a serviette over a coffee) if the idea is plausible before we write the research grant proposal to study it properly.
So, to return to my pan of oat milk simmering on the stove. 15g over 5 minutes at approaching 100C is a reasonable amount to expect to lose. Only, we can go further than this now because we can take the extra data (from the thermostats we have in our house and the Met Office observations for the weather) of the temperature of your kitchen and the relative humidity that day and use this to discover how these factors affect the evaporative loss. Just as for Halley, it may be an extremely rough estimate. However, just as for Halley, these estimates may help to give us an understanding that is “one of the most necessary ingredients of a real and Philosophical Meteorology” as Halley may have said before he enjoyed a coffee at one of the Coffee Houses that he, Newton and others would retire to after a busy evening watching water evaporate at the Royal Society.