Meteorology

Overview

About a hundred and ten pages across four books, written by Aristotle around 350 BCE as part of his larger project of natural philosophy, positioned deliberately at a particular point in his cosmological scheme. Having established, in the Physics, the general principles of change and the four terrestrial elements, earth, water, air and fire, and having described, in On the Heavens, the eternal, unchanging circular motion of the celestial bodies above the moon, Aristotle turns in the Meteorology to the region between: the sublunary atmosphere and the surface of the earth itself, where the four terrestrial elements mix, combine and interact in ways that are, unlike the perfectly regular heavens above, genuinely irregular, changeable and often violent.

The Greek meteora, from which the treatise’s title derives, meant roughly things high up or things in the sky, a considerably broader category than the modern English meteorology, which is why Aristotle’s treatise covers not only weather phenomena in the ordinary modern sense, but comets, the Milky Way, earthquakes, the saltiness of the sea, and, in its final book, the action of heat and cold in producing the various qualities found in compound terrestrial bodies more generally.

Book four is widely considered by modern scholars to be a somewhat later addition, possibly by another hand working within the Aristotelian school rather than by Aristotle himself, its content and method differing noticeably from the first three books.

What happens

The two exhalations. Aristotle’s foundational explanatory mechanism, established early and deployed throughout the treatise to account for an enormous range of otherwise disparate phenomena, is the theory of two exhalations drawn up from the earth by the sun’s heat. One exhalation is moist and vaporous, rising principally from bodies of water and producing, through various processes of cooling and condensation, clouds, rain, snow, hail, dew and frost. The other exhalation is dry and smoky in character, rising principally from the earth itself, and Aristotle uses it to explain wind, and, when this dry exhalation becomes trapped and compressed beneath the earth’s surface rather than escaping freely into the atmosphere, the violent shaking of earthquakes.

Comets and the Milky Way. Because Aristotle’s cosmology places the unchanging, eternal celestial bodies entirely above the sphere of the moon, and comets and the Milky Way visibly change, appear and disappear in ways the fixed stars and planets do not, he classifies both as atmospheric rather than genuinely celestial phenomena, explaining them as ignited exhalations burning in the upper atmosphere rather than as objects located, as later astronomy would establish, at vastly greater distances beyond the moon. This is among the treatise’s most significant errors, though it follows with real internal consistency from Aristotle’s broader cosmological commitment to a sharp division between an unchanging celestial realm and a changeable sublunary one.

Rain, wind, rivers and the sea. Books one and two work through the standard cycle of atmospheric moisture in considerable descriptive detail, and turn to an extended discussion of rivers, their sources and behavior, and to a notably extended treatment of the sea, its origin, its behavior, and above all the puzzle of its saltiness, which Aristotle attempts to explain through a theory involving the sun’s action drawing off the sea’s lighter, fresher portions through a process resembling distillation, leaving behind a residue of concentrated, earthy, saline matter, an explanation that does not survive scrutiny but that reflects real sustained attention to a genuine natural puzzle.

Thunder and lightning. Aristotle explains these as effects arising from the violent expulsion of the dry exhalation from within clouds, generating both the visible flash and the audible sound through a mechanism he compares, with real ingenuity if not accuracy, to the sudden collision or friction of the exhalation forcing its way out through the surrounding moist cloud.

The rainbow, halos and mock suns. Book three represents the treatise’s most enduringly impressive achievement, and the section modern historians of science generally single out for genuine praise. Aristotle’s analysis of the rainbow correctly identifies the underlying mechanism as reflection of sunlight from countless individual water droplets back to the observer’s eye at a specific, fixed angle, and he works out, with real geometric rigor, several consequences that follow from this correct identification, including why the rainbow always appears as an arc, why its apparent size and position depend systematically on the sun’s altitude in the sky, and why a rainbow is never seen as a complete circle from ground level. This section stands apart from much of the rest of the treatise in the degree to which its underlying physical mechanism, though not developed with anything like the mathematical optics that would later be available, is fundamentally correct rather than mistaken. He extends similar geometric analysis to halos around the sun and moon and to the rarer phenomenon of mock suns, parhelia.

Book four: the elemental qualities. The treatise’s final book, likely a later addition to the Aristotelian corpus rather than Aristotle’s own direct work, shifts register somewhat, addressing how the basic qualities of hot, cold, wet and dry act upon compound bodies to produce further qualities, hardness, softness, brittleness, malleability and the like, extending the elemental framework established in the Physics toward an account of the properties of ordinary terrestrial materials.

How to read it

Read book three, on the rainbow and related optical phenomena, with particular attention; it is the section of the treatise that has aged best, and working through Aristotle’s geometric argument for why the rainbow takes the shape and position it does is genuinely rewarding on its own terms, independent of the treatise’s many other now-discredited mechanisms.

Read the rest more as intellectual history than as reliable science. The theory of two exhalations, however ingeniously and consistently applied across comets, earthquakes, wind and the sea’s salinity, is simply not correct as an account of any of these phenomena, and the value of reading these sections lies chiefly in observing how a systematic ancient mind attempted to bring a bewildering range of disparate natural events under a single unifying explanatory principle.

Notice the cosmological logic behind classifying comets as atmospheric rather than celestial. This error follows directly and consistently from Aristotle’s broader division between an unchanging heavens and a changeable sublunary world, established in On the Heavens, and seeing exactly how one commitment in his larger system generates this particular mistaken conclusion is instructive about how theoretical frameworks can produce specific errors even while functioning with real internal consistency.

Read this treatise alongside the Physics and On the Heavens if you want the fuller cosmological picture it presupposes. The Meteorology occupies a specific, deliberately chosen place in Aristotle’s overall scheme of natural philosophy, between the eternal order above the moon and the settled elemental theory below the atmosphere, and understanding that placement clarifies why the treatise covers exactly the range of phenomena it does.

Approach book four as likely the work of a later hand within the school rather than assuming uniform authorship throughout, and notice how its approach and content differ somewhat from the first three books, a useful reminder that ancient philosophical corpora attributed to a single named author often include material of somewhat mixed and later origin.

Questions it raises

• What does the treatise’s mixed record, genuine geometric insight into the rainbow alongside serious error about comets and earthquakes, suggest about how to evaluate ancient scientific achievement fairly?
• Why did the specific error of classifying comets as atmospheric rather than celestial phenomena follow so directly from Aristotle’s broader cosmological commitments, and what does that suggest about how theoretical frameworks can generate predictable mistakes?
• Is a single, universal explanatory mechanism like the two exhalations a strength or a weakness in a scientific theory, when it can be stretched to account for phenomena as different as earthquakes, wind and the salinity of the sea?
• What allowed Aristotle to get the geometry of the rainbow substantially right without the mathematical optics that later science would use to complete the explanation?
• How should a modern reader weigh a scientific text whose specific mechanisms are almost entirely superseded, but whose systematic ambition to unify a disparate range of phenomena remains historically significant?
• Does the apparent difference in method and quality of book four suggest anything useful about how to read ancient philosophical texts that may combine an original author’s work with later additions from within the same school?