An equation for the ‘weather glass’

Abstract

The ‘weather glass’, containing air and (usually) water, is a compact barometer, less accurate but more convenient than the Torricellian barometer of 1644. The weather glass pre-dates the Torricellian barometer, and coexisted with it until it was out-competed from about 1860 by the more accurate, comparably compact aneroid barometer. Today, the weather glass is sold as an elegant interior decoration and is also constructible in a student laboratory. Elementary physics allows one to model its response to changing pressure at constant temperature and also the adjustments that need to be made if the temperature varies. In an air- and water-containing weather glass, the principal factor affecting movements of the water level is the expansion and contraction of the atmospheric nitrogen, oxygen, and argon in a headspace above the water, well represented by the ideal gas law. The water acts primarily as a piston allowing the expansion and contraction to be observed. In contrast, Torricellian and aneroid barometers respond directly to the pressure difference between the atmosphere and a vacuum.