• Karoliina Pulkkinen


Updated: May 11

Consider the clouds. Then consider those who consider clouds. Cloud-spotters, meteorologists, and atmospheric scientists come to mind. Looking into how the labour of studying clouds is carved up reveals information about the phenomena under study, and also the relationship between disciplines that study it – including atmospheric physics and chemistry.

When philosophers bring up the relationship between chemistry and physics, it is often examined in terms of reducibility. One prominent view is that chemistry is reducible to physics, although philosophers of chemistry are often skeptical of such claims (Weisberg and Needham 2010). In his excellent 2017 book on the subject, Hinne Hettema takes as his starting point that "the unity of chemistry and physics is based on intertheoretical explanation and that it makes sense to look at reduction as a formal paraphrase of this explanatory unity"(2017, vi). For Hettema, the appeal of reductionism arises from the fact that

"[R]eduction of chemistry to physics is still paradigmatic for, or at least fundamental to, the notion of reduction per se: as reduction relations go, the domains of chemistry and physics are close and significantly overlapping, and there is a lot of theoretical and ontological ‘borrowing’ and a significant amount of continuity between the two sciences."(Hettema 2017, vi)

I agree that it makes sense to explore the relationship between the two sciences in terms of reducibility, but what other options are there on the table? It serves as some consolation that many varieties of reduction have been proposed in the literature, but different types of reduction are, well, types of reduction. But just as tracking the Windsor family tree does not tell why uncle David’s nickname for Queen Elizabeth was “Shirley Temple,” reducing the chemistry-physics relation to reducibility conceals that there might be other ways to examine how the two disciplines relate when providing inter- and intratheoretical explanations.

The variation in the ways that chemistry and physics connect applies especially to atmospheric sciences – an interdisciplinary field that studies the structure and dynamics of Earth’s atmosphere (Pielke 2018). Take clouds again. They constitute a central topic because clouds reflect incoming radiation back to space and form one of the most pervasive uncertainties in estimating the effects of climate change.

In explaining the relationship of chemistry and physics in the study of clouds, Dennis Lamb and Johannes Verlinde (2011, 480) clarify that

"The term microchemistry in cloud physics parallels that of microphysics. Both disciplines deal with the particles making up clouds, but the emphasis in cloud microchemistry is on the chemicals contained in the particles, not on the particles themselves. Atmospheric trace chemicals influence cloud properties in important ways, and the cloud microphysics also determine the fates of atmospheric chemicals."

Same particles, different emphases; both needed to understand how clouds are composed and structured. The particles in question are the tiny atmospheric aerosol particles that serve as the nuclei upon which liquid cloud droplets form (Bellouin 2015). Examples of aerosols include sea salt from natural sources and sulphate and soot from anthropogenic sources. Once in the atmosphere, aerosol particles interact in different ways with each other, with water vapour, with existing cloud droplets, and with ice (if present). Such processes then determine cloud properties. This gives us one answer to the question concerning the relationship between physics and chemistry: the two come together to understand different aspects of the same entity of interest – in this case, the atmospheric aerosol.

Coordination of this kind of explanatory collaboration does not always appear straightforward. Consider the figure below (adapted after Ahrens 1994), which shows a warming front with the capacity to cleanse precipitating clouds.

For explaining the above process of cleansing the cloud, both microphysics and -chemistry is needed. But it is not entirely clear how this explanatory collaboration is conducted. Lamb and Verlinde explain as follows:

"The warm, moist air that rides over a cooler air mass forms cloud that is often convective near the surface front and stratiform farther downwind (...) The microphysical processes responsible for precipitation formation allow some of the chemicals to be removed from the most polluted air and end up in the rain closest to the surface front. As verified by field measurements, this first rain is relatively dirty compared with the rains that fall farther from the surface front. Precipitation, with its chemical burden, simultaneously closes the hydrologic cycle and the atmospheric cycles of many trace chemicals. Determining how chemicals end up in precipitation is an important goal of cloud microchemistry. (Lamb and Verlinde 2011, 482).

According to the authors, it is a microphysical process that chemicals are removed from air and end up in the rain closest to the surface front. But how do those chemicals end up in precipitation? That is a goal of cloud microchemistry. Determining what part of the process falls to the realm of chemistry or physics appears as hard as defining the whether someone lingering on the doorway is exiting or entering.

Although I would not deny that looking into the relationship of physics and chemistry in atmospheric sciences would offer good candidates for reduction, I suggest it is at least equally worthwhile to understand different forms of explanatory collaboration. In either task, we might take solace in the words of Garbage,

I’m only happy when it rains

I’m only happy when it’s complicated

(W/ special thanks to Sabine Undorf.)


Ahrens, C.D. (1994). Meteorology Today. St. Paul, MN: West, 591 pp.

Bellouin, N. (2015). AEROSOLS | Role in Climate Change (G. R. North, J. Pyle, & F. B. T.-E. of A. S. (Second E. Zhang, eds.). https://doi.org/https://doi.org/10.1016/B978-0-12-382225-3.00054-2

Hettema, H. (2017) The Union of Chemistry and Physics: Linkages, Reduction, Theory Nets and Ontology. Springer.

Lamb, D., & Verlinde, J. (2011). Physics and Chemistry of Clouds. https://doi.org/10.1017/CBO9780511976377

Pielke, R. (2018). Atmospheric science, Encyclopædia Britannica, https://www.britannica.com/science/atmospheric-science

Weisberg, M., & Needham, P. (2010). Matter, Structure, and Change: Aspects of the Philosophy of Chemistry. Philosophy Compass, 5(10), 927–937. https://doi.org/10.1111/j.1747-9991.2010.00335.x

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