HAZE
What is haze? The dictionary says, "a fog." Well, haze is _not_ a fog. In
a fog, the dust-particles in the air have been fully clothed with
water-vapour; in a haze, the process of condensation has been arrested.
Cloudy condensation is changed to haze by the reduction of its humidity.
Dr. Aitken invented a simple apparatus for testing the condensing power of
dust, and observing if water-vapour condensed on the deposited dust in
unsaturated air.
The dust from the air has first to be collected. This is done by placing a
glass plate vertically, and in close contact with one of the panes of
glass in the window, by means of a little india-rubber solution. The plate
being thus rendered colder than the air in the room, the dust is deposited
on it.
Construct a rectangular box, with a square bottom, 1-1/2 inches a side and
3/4 inch deep, and open at the top. Cover the top edge of the box with a
thickness of india-rubber. Place the dusty plate--a square glass mirror, 4
inches a side--on the top of the india-rubber, and hold it down by spring
catches, so as to make the box water-tight. The box has been provided with
two pipes, one for taking in water and the other for taking away the
overflow, with the bulb of a thermometer in the centre. Clean the dust
carefully off one half of the mirror, so that one half of the glass
covering the box is clean and the other half dusty. Pour cold water
through the pipe into the box, so as to lower the temperature of the
mirror, and carefully observe when condensation begins on the clean part
and on the dusty part, taking a note of the difference of temperature. The
condensation of the water-vapour will appear on the dust-particles before
coming down to the natural dew-point temperature of the clean glass. And
the difference between the two temperatures indicates the temperature
above the dew-point at which the dust has condensed the water-vapour.
Magnesia dust has small affinity for water-vapour; accordingly, it
condenses at almost exactly the same temperature as the glass. But
gunpowder has great condensing power. All have noticed that the smoke
from exploded gunpowder is far more dense in damp than in dry weather. In
the experiment it will be found that the dust from gunpowder smoke begins
to show signs of condensing the vapour at a temperature of 9 deg. Fahr. above
the dew-point. In the case of sodium dust, the vapour is condensed from
the air at a temperature of 30 deg. above the dew-point.
Dust collected in a smoking-room shows a decidedly greater condensing
power than that from the outer air.
We can now understand why the glass in picture frames and other places
sometimes appears damp when the air is not saturated. When in winter the
windows are not often cleaned, a damp deposit may be frequently seen on
the glass. Any one can try the experiment. Clean one half of a dusty pane
of glass in cold weather, and the clean part will remain undewed and
clear, while the dusty part is damp to the eye and greasy to the touch.
These observations indicate that moisture is deposited on the
dust-particles from air, which is not saturated, and that the condensation
takes place while the air is comparatively dry, _before_ the temperature
is lowered to the dew-point. There is, then, no definite demarcation
between what seems to us clear air and thick haze. The clearest air has
some haze, and, as the humidity increases, the thickness of the air
increases.
In all haze the temperature is above the dew-point. The dust-particles
have only condensed a very small amount of the moisture so as to form
haze, before the fuller condensation takes place at the dew-point.
At the Italian lakes, on many occasions when the air is damp and still,
every stage of condensation may be observed in close proximity, not
separated by a hard and fast line, but when no one could determine where
the clear air ended and the cloud began. Sometimes in the sky overhead a
gradual change can be observed from perfect clearness to thick air, and
then the cloud.
A thick haze may be occasioned by an increased number of dust-particles
with little moisture, or of a diminished number of dust-particles with
much moisture, above the point of saturation. The haze is cleared by this
temperature rising, so as to allow the moisture to evaporate from the
dust-particles.
Whenever the air is dry and hazy, much dust is found in it; as the dust
decreases the haze also decreases. For example, Dr. Aitken, at
Kingairloch, in one of the clearest districts of Argyleshire, on a clear
July afternoon, counted 4000 dust-particles in a cubic inch of the air;
whereas, two days before, in thick haze, he counted no fewer than 64,000
in the cubic inch. At Dumfries the number counted on a very hazy day in
October increased twenty-fold over the number counted the day before, when
it was clear.
All know that thick haze is usual in very sultry weather. The wavy,
will-o'-the-wisp ripples near the horizon indicate its presence very
plainly. During the intense heat there is generally much dust in the
atmosphere; this dust, by the high temperature, attracts moisture from the
apparently dry air, though above the saturation point. In all
circumstances, then, the haze can be accounted for by the condensing power
of the dust-particles in the atmosphere, at a higher temperature than that
required for the formation of fogs, or mists, or rain.