Essay: The Meteors

Overview
Rene Descartes' The Meteors (1637), published alongside the Discourse on Method, recasts meteorology and atmospheric optics in mechanical and geometrical terms. Rejecting scholastic qualities, it treats winds, clouds, rain, halos, and the rainbow as outcomes of particles moving, colliding, and refracting light according to definite rules. The essay is organized as a sequence of discourses that move from general physical principles to particular phenomena, with experiments and geometric constructions anchoring the explanations.

Physical Principles
Descartes frames heat as a kind of motion in a subtle matter that agitates and separates corpuscles of water and earth. Differences in particle size, shape, and motion explain why vapors rise, why they condense, and why mixtures stratify in the air. Rarefaction and condensation are mechanical processes, not transformations in substance. The same conceptual toolkit, pressure, motion, impact, underlies his accounts of winds, storms, and luminous appearances.

Vapors, Clouds, and Precipitation
Evaporation lifts minute particles of water into the air, where they aggregate around heterogeneous admixtures of salts and oily matter. Clouds form when the vibrating motion imparted by heat diminishes and particles begin to cling; fog is a near-surface variant under cooler conditions. The type of precipitation depends on altitude, temperature, and the speed of descent. Rain occurs when droplets coalesce and fall before freezing; snow arises from slower aggregation and freezing aloft, producing faceted crystals; hail results from rapid alternations of freezing and thawing in turbulent updrafts that layer ice around nuclei. Dew and hoarfrost are attributed to nocturnal cooling that saps the motion of near-surface vapors, promoting condensation on colder bodies.

Winds, Lightning, and Thunder
Winds are large-scale motions driven by spatial imbalances of heat and pressure, channeled by topography and temperature differentials between land and water. Lightning is a sudden incandescence produced when agitated, combustible exhalations discharge and ignite, rapidly heating the air along their path. Thunder follows as the surrounding air collapses and rebounds against this heated channel; echoes and varying distances account for rumbling and delays between flash and sound. Haze and dry fogs come from suspended fine particles that scatter light without precipitating.

Halos and Sundogs
Descartes interprets rings around the sun and moon and mock suns as optical effects caused by refraction through myriad small ice crystals in high clouds. The geometry of prismatic crystals deflects rays by characteristic angles, producing bright arcs and lateral images when the crystals share preferred orientations. He links the appearance, size, and color faintness of lunar versus solar halos to light intensity and the degree of crystal alignment, anticipating later accounts in terms of hexagonal ice prisms and plates.

The Rainbow
The centerpiece of the essay is the quantitative theory of the rainbow. Using the sine law of refraction and experiments with a water-filled glass sphere, Descartes traces rays entering raindrops, refracting, reflecting internally, and emerging toward the observer. He shows that brightness concentrates near a particular deviation angle, about 42 degrees for the primary bow, where exiting rays cluster due to a minimum in deviation. A second bow, formed by two internal reflections, appears higher and fainter, with reversed color order and a larger characteristic angle. He connects the ordering of colors to systematic variation in refraction with ray paths and uses the geometry to explain why each observer sees a personal bow centered on the antisolar point, why the sky inside the primary is brighter, and why supernumerary features do not ordinarily appear.

Method and Legacy
Throughout, Descartes weds mechanical causes to measurement and construction, offering not just narratives but calculable predictions, especially in optics. The Meteors helped displace qualitative meteorology with physical explanation and established a lasting template for atmospheric science: start from simple particles and precise laws, use models and controlled experiments, and let geometry decide what in the sky must be so.