Weather Forecasting

'Red sky at night, shepherd's delight. Red sky in morning, shepherd's warning.' By the end of the eighteenth century, weather lore remained a dominant form of weather prediction. Based on the reading of natural signs, such as the behaviour of animals or the appearance of the sky, weather-wise expressions commonly circulated as a type of forecasting. Nineteenth-century meteorology saw the development of weather forecasting based on measurable phenomena, such as wind, the intensity of the Sun and, in many cases, the character and appearance of clouds.

Herschel's 'Weather Table'

Herschel's weather table charted the effect of the position of the Sun and phases of the Moon on terrestrial weather as an aide to the farmer's almanac, a popular form of weather-wise literature (Image 1). Though the weather table took the name 'Herschel', neither William, nor his son John, devised the chart. In fact, in 1809 William publically denounced its wrongful claim to the Herschel name in the Philosophical Magazine.

The influence of the Moon on the atmosphere, however, was a common subject of investigation in eighteenth-century meteorology with Sir William Herschel (1738-1822) being a supporter of lunar theories of meteorology. The gravitational pull of the Moon on large bodies of water to produce tide was widely known. This lunar influence was linked to changes in atmospheric electricity then understood as a sort-of 'fluid' that permeated the air. Like water, electrical 'fluids' in the atmosphere were susceptible to the relative position of the Moon.

John Herschel (1792-1871) continued his father's work on lunar theories of meteorology, and was an instrumental figure in re-shaping traditional modes of visualizing weather. Working within the framework of terrestrial physics, an area of study that included investigation of tides, magnetic forces and ocean waves, Herschel studied weather as a global phenomenon.

Herschel was heavily influenced by the ideas of friend Charles Lyell (1797-1875), who suggested a relationship between the seasonal position of the Sun and the exchange of heat between the southern and northern hemispheres. In A Treatise on Astronomy (1833) published as part of the 'Cabinet Cyclopædia' series, Herschel devoted a chapter to his own atmospheric theories.

He described the circulation of hot and cold airs and their role in creating trade winds, and the deflection of airs according to the Earth's rotation. The belts of Saturn and Jupiter as observed through the telescope presented a visible analogy to Herschel's theory that understood atmospheric conditions as informed by elasticity of air, heat and gravity.

Further methods of forecasting

In the wake of John Herschel and others' work, weather prediction increasingly became linked with the measurement of specific physical variables using scientific instruments. Throughout the late nineteenth and early twentieth centuries, a variety of pocket weather forecasters appeared to aid this work.

Pocket weather forecasters

The scientific instrument firm Negretti and Zambra produced a weather forecaster intended to be used with a barometer and weather vein. Two dials were rotated according to barometric reading of air pressure and the direction of the wind (Image 2). The corresponding chart on the reverse indicated likely changes in weather for the next 12 hours.

In 1938, the firm D. & K. Bartlett produced a weather forecaster based on the type of cloud and the direction of the wind (Image 3). The central dial rotated to reveal a weather prediction.

Sunshine recorder

Theories of solar heat affecting the global atmosphere persisted throughout the nineteenth and twentieth centuries. The Campbell-Stokes sunshine recorder was invented in 1853 by John Francis Campbell (1821-1885) and later improved by George Gabriel Stokes (1819-1903) in order to measure the hours and intensity of sunshine (Image 4).

Campbell's original sunshine recorder consisted of a glass sphere positioned in a wooden bowl. The glass focused sunlight to a point that burned a pattern into the wooden bowl depending on the brightness of the Sun. The path of the Sun and the intensity of the light would be charred as a line. For instance, if the day was overcast, the burnt line would be very faint.

Stokes improved the instrument by devising a metal cradle for the glass, which held a strip of card behind the sphere that could be changed daily. The card was printed with a grid to better record the Sun's height above the horizon.