Aristotle devoted a book to the Categories. The overall goal of this work is devising the fundamental categories of things. Thus, according to Aristotle, some things seem not to be predicated of others, while other things can be predicated of them: those are the substances, the most fundamental entities. For instance, we say of Pablo Picasso that he was Spanish, but we cannot say of Spain – or any other thing – that it is Pablo Picasso; better still, we cannot say of anything that it is Pablo Picasso, without making an identity statement rather than a predication.
Over the past century or so, a key role in the categorization of things has been played by scientists. They are the ones to suggest what are the fundamental natural kinds. Natural kinds are a key topic in metaphysics and philosophy of science. They are supposed to pick out the deep structure of reality: quarks, leptons, electrons, protons, vitamin D, water, stem cells, carbs, saturated fats … these and alike are all kinds of things that inhabit our world, according to the scientific picture.
How to devise a natural kind? Typically this is done via the specification of two components: (i) some properties, alongside (ii) some typical behaviors explainable on the basis of some law of nature related to the properties of the kind in question. Water, for instance, has a typical chemical structure – H2O –, a property, which is used in conjunction with laws of attraction and repulsion between molecules to explain the behavior of water samples, such as the transition from a liquid to a solid state when temperature drops.
Storms as Natural Kinds
Storms seem to qualify as natural kinds. We tend to classify them based on some typical properties, which are then paired with laws of nature to explain the behavior of extremely large systems of clouds. The website of the National Hurricane Center provides some neat examples. What is a cyclone? "An atmospheric closed circulation rotating counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere." That the circulation is closed and has a specific direction are properties of the kinds of entities in question; the behavior of a cyclone will then be explained on the basis of the laws of gravitation and other, more local, laws valid for our planet, such as Coriolis effect.
The problem is that seem just not to fit our categories. The NHC glossary, for instance, does not feature an entry on "The Perfect Storm." And yet, when in 1991 a nor’easter swept the Northeast of the United States to then turn over warm waters and transform itself as what we would usually classify as a hurricane, meteorologists had a moment of classificatory puzzlement. It was not a hurricane, since it did not form in tropical climate and its core was cold; yet, its core became warm as the one of hurricane once it reached warm waters. The meteorologists aptly introduced the term "The Perfect Storm," perfect in that the system seemed to exploit at best local conditions to strengthen itself. Yet in meteorology there is no such natural kind known as perfect storm.
The Halloween Nor’easter of 1991 reminds us of the platypus, that taxonomists have struggled and still struggle to classify. And thus the old vexed question can be posed also with respect to storms: are storms natural kinds? Perhaps they are, but only if we concede that natural kinds have some degree of conventionality.