CHAPTER XII

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° Fahr. above the dew-point. In the case of sodium dust, the vapour is condensed from the air at a temperature of 30° 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.