Coffee and atmospheric physics

by: Prof Maarten Ambaum

Every morning I trundle down to the office kitchen and I make myself a whole thermos flask of coffee which keeps me going for the rest of the day. In fact, most people in our Department have a similar daily ritual. During coffee breaks, science is discussed as well as more mundane things (a lot of politics, these days). Coffee is the fuel of science!

There are deeper links between science and coffee as well: recently our hot water boiler in the office kitchen was replaced by a fancy new hot water boiler. This new boiler has a so-called “eco-mode” which claims to save energy, essentially by using the boiler at half capacity. This claim could not go untested; we are a science department after all! Some basic thermodynamics (the science of heat and energy) and some experiments showed that the eco-mode is nothing like it: it does not save energy and we haven’t used the eco-mode since. A blog with the fun details can be found here.

In fact, this autumn I will again be teaching our new cohort of master’s students the ins and outs of atmospheric thermodynamics. It is a profoundly interesting part of physics and it is at the fundament of our understanding of the climate and weather. And of our understanding of hot water boilers, of course.

A good understanding of fundamental physics is crucial in our field of science. For example, most climate sceptics use arguments that fall over at the level of fundamental physical understanding.

Many people still cannot accept the idea that adding carbon dioxide to the atmosphere could ever heat up the atmosphere in any substantial way. This kind of argument can be debunked comprehensively by basic thermodynamics. The key is that adding carbon dioxide to the atmosphere is similar to putting a thicker duvet on your bed: a thicker duvet will make you feel warmer, not because you produce more heat or somehow the duvet makes you warm. The key is that the heat energy you produce has a harder job of escaping to the environment through a thicker duvet and it can only do so by increasing the temperature in your bed, allowing the same amount of heat to escape through the thicker duvet.

The same is true for the earth’s climate: the atmosphere acts as a blanket on the earth’s surface. The earth’s surface is heated directly by the sun (which remains broadly constant in its energy output), so if the atmospheric blanket gets thicker (by adding carbon-dioxide), the earth’s surface needs to get warmer for the heat to escape at the same rate.

There are many fascinating additional details to this picture, way too many to address here. Many of those I will be teaching to our new group of students (for example, how and why does carbon dioxide change the effective thickness of the atmospheric blanket), and many are also still actively researched in our Department (for example how changing cloud properties might change the effective thickness of the atmospheric blanket, but also how they might change the amount of energy from the sun reaching the earth’s surface). But the underlying fundaments are rock solid physics.

Here’s a brainteaser to keep you busy: for my coffee to stay hot for longer, should I pour it in bigger or a smaller mug?


Ambaum, M. H. P., 2010: Thermal Physics of the Atmosphere, J. Wiley & Sons, Chichester, 256pp.

Ambaum, M. H. P., and M. Prosser, 2019: Is our “ECO mode” hot water boiler eco-friendly?

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