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Blue mist, Greenwich, 1866

The theory that infectious disease was the result of a 'miasma' or poisoned atmosphere was once general. Malaria derives its name from mala aria or 'bad air'. Medieval physicians thought that the Black Death was the result of corrupted air, which sometimes became visible in the form of mist or smoke. The Chronicler of Este said that the plague of 1348 was the result of a rain of fire between China and Persia, which created a vast cloud of smoke. Whoever saw this smoke died in a few hours, as did anyone who looked at persons who had seen the smoke. Other physicians thought the lethal miasma emanated from the corpses of innumerable victims of earthquake, flood and drought in China, or had issued from cracks in the ground caused by earthquakes in 1347.

In 1717, a Dr. Aubrey, aboard a slave ship off the coast of West Africa, experimented with the miasma theory. He believed that the fogs that appeared at dusk over the river estuaries caused the disease dengue, or 'break-bone fever'. He made a paste of oatmeal and left it on deck all night, then fed it to some chickens. The "evil, malevolent, contagious, destructive quality of those fogs that fall there in the night" was demonstrated by the fact that soon after eating it, the fowls, "in a kind of vertigo dropt down and expired." A fatal epidemic in West Africa in 1754 was ascribed to "a noxious, stinking fog."

The late summer of 1866 was cool, wet and windy in England, apart from eight days of fine, hot weather in the middle of July. There was a cholera epidemic in London, which began in June and reached its peak between July 29 and August 4, when 1,053 persons died.

On Monday, July 30, a fine day, with a westerly wind and a maximum temperature of 70° F. (rather cool for July), James Glaisher, F.R.S. and Superintendent of the Meteorological Department at Greenwich Observatory, observed "a dense blue mist". This was "apparent on all sides, and extended fully to the tops of the trees, though not there so easy to distinguish." Against a background of distant trees, it resembled "thin smoke from a wood fire."

This blue mist had been seen before. In the cholera epidemic of 1854, "a remarkable blue mist was observed which prevailed night and day."

James Glaisher, who was one of the most respected meteorologists of the 19th century, and had achieved a balloon altitude record in 1862, observed the blue mist nearly every day from the end of July until early October. On August 2, a dense blue mist obscured the River Thames and everything northward of it. (This was a cloudy, warm day with occasional showers). On August 5, with thin rain and cloud clearing, there was "a blue mist on all sides". Next day, August 6, overcast, with rain in the afternoon and evening, "the blue mist increased in density with the falling rain." On August 11, a cloudy, hazy day, it was noted, "The blue mist is very thick and is general. This blueness has been constant since first seen on July 30th."

The blue mist then faded. On August 21 (cloudy), the blue mist, "although not very dense, is very apparent in all directions." By August 26, (a very fine day of 78°F.), it had disappeared.

The blue mist reappeared thinly on August 29 (overcast with heavy showers). On September 3, a very fine day, it "was dense at times, after 3 hr. p.m. none was visible." On September 8, overcast, and wet in the afternoon, there was a "blue mist early in the morning, succeeded by a greyish mist, which obliterated all traces of the blue mist." On September 17 (fine), there was "blue mist in all directions, which became gradually thinner towards evening."

The blue mist was nearly always visible throughout September, though often thin. On September 26 (overcast) it was "again very apparent". On October 6, (wet morning, cloudy afternoon), it was very dense during the morning. On October 8 (mainly fine), there was a "thin blue mist during the morning, which disappeared as the day advanced."

This appears to have been Glaisher's final sighting of the blue mist. No further mention of it appears in the Greenwich weather record for 1866 (there was a "thin mist" on October 19, but its colour is not stated).

Miasma graph
The blue mist of 1866 at Greenwich, July 30 - October 13

Glaisher had not been the first to see the blue mist of 1866. Several observers had seen it in the week before July 30th. Glaisher said that the mist extended from Aberdeen to the Isle of Wight, and was of the same tint of blue everywhere. "This mist increased in intensity when viewed through a telescope; usually no mist can be seen when thus viewed; it increased in density during the fall of rain, although usually mist rises from rain." The blue mist was not affected by moderate winds. A gale dispersed it, but the mist returned again when the wind dropped.

Glaisher evidently thought that the blue mist had some connection with cholera, although he preserved scientific caution about it. He said, "Whatever may be its nature, the fact is very remarkable, that since the cholera period of 1854 this phenomenon has not been observed till the present time."

The peak of the cholera epidemic in London came just as the blue mist appeared, and the disease declined all through its appearance. In the week ending August 11, deaths were 781, on August 18, 455, on August 25, 265. By November 24, they were in single figures (8).

Dr. John Snow had demonstrated in 1854 that cholera was a water-borne disease, although an International Sanitary Conference in 1866 still claimed that it spread through the air. Was the blue mist of 1866 the last gasp of the miasma theory? Like many natural phenomena, it left no trace of its existence but for the records of observers. Unfortunately, colour photography had not been developed in 1866, and the blue mist might have been a normal haze whose characteristics were exaggerated by diehard proponents of the miasma. (Glaisher said that a yellow mist was sometimes 'perceptible' during epidemics of scarlatina.) When the miasma theory was discredited, the blue mist became doubtful also. This is rather unjust. The blue mist may have had no connection with cholera. There is no record of it having been seen since 1866, but perhaps that is because no one has looked. No one recorded noctilucent clouds before 1883, but it is not presumed that they suddenly appeared then.

On August 11, 1709, during an epidemic at Danzig in which 24,533 persons died, a ‘thick cloud’ or ‘stinking mist’, of a blackish-yellow colour moved over the town from one place to another, “emitting a violent stench”

On September 7, 1820. at 1 pm, in Kensington Gardens, London, during a partial solar eclipse, an “extensive yellow mist” rose from the ground. The mist was “of fine gallstone colour, and gave a beautiful cloud effect to the sky…sometimes it changed to orange.”

On February 15, 1837, (a damp, misty day with a fresh SW wind), at 5.15 pm, at Polperro in Cornwall, "there came in a mist, of a bright red colour; which attracted attention through a window by the glare of light it diffused." The observer went outside, and found that the mist had turned pink. It then passed into violet, then grey, and remained so until it was hidden by darkness. "No refraction of sunbeams can be allowed to account for this appearance; for the sun had long before been hidden by intervening hills from the valley in which this beautiful coloured mist appeared."

On September 6, 1922, at 5 pm, the town of Ross-on-Wye in Herefordshire was overshadowed by "a dense overhead fog of greenish tint…causing such intense gloom that artificial light was necessary."