Cloud Studies

The nephoscope was invented in the nineteenth-century to measure the altitude, direction and velocity of clouds. Seventeenth- and eighteenth-century methods of determining cloud height required triangulation from the ends of two baselines. Other techniques involved using a map to trace the passage of cloud-shadows over the countryside, or a camera obscura to measure a cloud's angular height and bearing from 0º north, (known in astronomy as 'azimuth').

Early nineteenth-century mirror nephoscopes used a shallow tank of inky water to reflect the sky and employed the same triangulating technique as the camera obscura, in which the angle of reflection was used to calculate the height of a cloud. Unfortunately, the image produced using the reflecting pool was rather restricted and wind easily disrupted its surface.

In 1846, the French meteorologist G. Aimé incorporated a horizontal mirror into nephoscope design, which significantly improved the instrument. Fineman's nephoscope, first manufactured in 1886, employed darkened glass and was fitted with a compass needle (Image 1). A vertical pointer rotated around the glass edge to be aligned with a cloud observed in the glass reflection. Engraved on the surface of the glass, concentric circles functioned as a measurement guide.

This example of a Fineman's Mirror Nephoscope was produced by the French firm Pellin, which Philibért Pellin (1847-1923) acquired from Jules Duboscq (1817-1886) after several years of collaboration and partnership. Maintaining Duboscq's excellent reputation, Pellin and his son Félix produced optical and meteorological instruments of the highest calibre.

In addition to studying clouds in the field, late nineteenth-century scientists created artificial clouds for research in the laboratory. In 1895, C. T. R. Wilson (1869-1959) constructed a cloud chamber that produced a supersaturated water-vapour environment from which he studied the physical and optical properties of clouds (Image 3).

Early experiments with the cloud chamber enabled Wilson to determine the process of droplet formation. In his experiments, water vapour condensed around ions, which were produced from exposing the chamber to high-energy particles, such as x-rays.

In 1911, Wilson successfully photographed condensation from ionized radiation. High-energy particles, interacting with the water vapours, ionize and leave a visible and distinctive cloud trail. For instance, an electron produces thin and straight lines.

Wilson's cloud chamber experiments and the subsequent ray-track photography significantly contributed to physicists' understanding of particle behaviour and enabled scientists to establish different categories of particle 'type'. While physicists adopted the cloud chamber for their own research pursuits, Wilson remained dedicated to investigations of meteorological optics until his death.