Jökull - 01.01.2001, Page 73

Observations on the cloud (dry fog)
which appeared in June 1783
S. P. van Swinden

Professor of Physics in the University of Franeker
Originally published in Latin under the title;
Observationes nebulam, quae mense Junio 1783 Apparuit,
spectantes in Ephemerides Societatis Meteorologicae Palatinae,
Observationes Anni 1783, Mannheim, Germany 1785
translated to English by Dr. Susan Lintleman.
Nothing memorable occurred in June until the 19
day when the haze, about which we are now con-
cerned, began to appear here in Franeker. This haze
lasted steadily from the recorded day of the 19
 un-
til the 30
 . It was distinguished from usual clouds
by its constancy, density, and especially by very great
dryness. From the 23  day, on which the haze was
the strongest, onwards the hygrometer showed a very
great dryness. This was the driest time of the whole
month. Days 19, 20, and 21, were a little damp with
sun. Neither a lightning storm on the evening of the
20
 , nor a stronger wind on the 21 
, dispersed the
haze. From the 18
 –21 
the atmosphere was thor-
oughly covered by haze; from 22–25 June it was half–
clear, clear, or mixed with little clouds. Across the
haze, the sun was perceived deep red, with brilliance
at the edge; even at midday, we were able to gaze at
the sun with our naked eyes without injury.1 Objects
scattered further were scarcely and only unintelligi-
bly perceived. These were the usual effects of this
haze, but on the 24
 day it brought with it as a com-
panion a sulfurous odor2 very readily perceived by
the senses, crawling through everything, even closed
houses. Men with delicate lungs experienced that
same sensation, as if they were turned towards a place
in the neighborhood of burning sulfur. They were un-
able to contain a cough, as soon as they were exposed
to air. I myself experienced this, and many others, first
in the city, then in the country. The heat was great
enough. The sulfurous haze of this day brought very
great loss to the vegetable realm, which I shall speak
of soon more fully. And so in June we had 11 days,
not only continuous, but notable for the presence of
the haze. Before this year, I had never seen it for more
than 3 days in this month, often none at all, and at the
lowest point sometimes not at all for subsequent years,
as from 1774-1783, years in which no haze appeared
in June.
On 3 and 9 July the genuine haze again appeared.
On other days the air was foggy, or vaporous in the
morning and the evening, as often happens when the
days are warm. In truth I hesitate to refer this type of
haze as the genuine haze, which appeared in June; but
on the 12
 to 20
 the genuine haze, similar to that in
June, was present, then in truth disappeared.3

People could gaze at the sun at midday, implying that the haze had optical depth in excess of 1.0 (e.g. Stothers, 1984).

Here van Swinden must be referring to the smell of sulfuric acid, a conclusion that is supported by his own description of what effect the
haze had on people (see below).
In Franeker the intensity of Laki haze diminished considerably around 20 July, but as will be apparent later this is not the last date of its
appearance.
JÖKULL No. 50 73