DUST AND ATMOSPHERIC PHENOMENA
Dr. Aitken has devoted a vast amount of attention to the enumeration of
dust-particles in the air, on the Continent as well as in Scotland, to
determine the effects of their variation in number.
On his first visit to Hyeres, in 1890, he counted with the instrument
12,000 dust-particles in a cubic inch of the air: whereas in the following
year he counted 250,000. He observed, however, that where there was least
dust, the air was very clear; whereas with the maximum of dust, there was
a very thick haze.
At Mentone, the corresponding number was 13,000, when the wind was blowing
from the mountains; but increased to 430,000, when the wind was blowing
from the populous town.
On his first visit to the Rigi Kulm, in Switzerland, the air was
remarkably clear and brilliant, and the corresponding number never
exceeded 33,000; but, on his second visit, he counted no less than
166,000. This was accounted for by a thick haze, which rendered the lower
Alps scarcely visible. The upper limit of the haze was well defined; and
though the sky was cloudless, the sun looked like a harvest moon, and
required no eagle's eye to keep fixed on it.
Next day there was a violent thunder-storm. At 6 P.M. the storm commenced,
and 60,000 dust-particles to the cubic inch of air were registered; but in
the middle of the storm he counted only 13,000. There was a heavy fall of
hail at this time, and he accounts for the diminution of dust-particles by
the down-rush of purer upper air, which displaced the contaminated lower
air.
At the Lake of Lucerne there was an exceptional diminution of the number
in the course of an hour, viz. from 171,000 to 28,000 in a cubic inch. On
looking about, he found that the direction of the wind had changed,
bringing down the purer upper air to the place of observation. The bending
downwards of the trees by the strong wind showed that it was coming from
the upper air.
Returning to Scotland, he continued his observations at Ben Nevis and at
Kingairloch, opposite Appin, Mr. Rankin using the instrument at the top of
the mountain. These observations showed in general that on the mountain
southerly, south-easterly, and easterly winds were more impregnated with
dust-particles, sometimes containing 133,000 per cubic inch. Northerly
winds brought pure air. The observations at sea-level showed a certain
parallelism to those on the summit of the mountain. With a north-westerly
wind the dust-particles reached the low number of 300 per cubic inch, the
lowest recorded at any low-level station.
The general deductions which he made from his numerous observations during
these two years are that (1) air coming from inhabited districts is always
impure; (2) dust is carried by the wind to enormous distances; (3) dust
rises to the tops of mountains during the day; (4) with much dust there is
much haze; (5) high humidity causes great thickness of the atmosphere, if
accompanied by a great amount of dust, whereas there is no evidence that
humidity alone has any effect in producing thickness; (6) and there is
generally a high amount of dust with high temperature, and a low amount of
dust with low temperature.