HAZING EFFECTS OF ATMOSPHERIC DUST
The transparency of the atmosphere is very much destroyed by the impurities communicated to it while passing over the inhabited areas of the country. Dr. Aitken devoted eighteen months to compare the amount of dusty impurities in different masses of air, or of different airs brought in by winds from different directions.
He took Falkirk for his centre of observations. This town lies a little to the north of a line drawn between Edinburgh and Glasgow, and is nearly midway between them. If we draw a line due west from it, and another due north, we find that, in the north-west quadrant so enclosed, the population of that part of Scotland is extremely thin, the country over that area being chiefly mountainous. In all other directions, the conditions are quite different. In the north-east quadrant are the fairly well-populated areas of Aberdeenshire, Forfarshire, and the thickly populated county of Fife. In the south-east quadrant are situated Edinburgh and the well-populated districts of the south-east of Scotland. And in the south-west quadrant are Glasgow and the large manufacturing towns which surround it. The winds from three of these quadrants bring air polluted in its passage over populated areas, whereas the winds from the north-west come comparatively pure.
The general plan of estimating the amount of haze is to note the most distant hill that can be seen through the haze. The distance in miles of the farthest away hill visible is then called “the limit of visibility” of the air at the time. For the observations made at Falkirk, only three hills are available, one about four miles distant, the Ochils about fifteen miles distant, and Ben Ledi about twenty-five miles distant—all in the north-west quadrant. When the air is thick, only the near hill can be seen; then the Ochils become visible as the air clears; and at last Ben Ledi is seen, when the haze becomes still less. After Ben Ledi is visible, it then becomes necessary to estimate the amount of haze on it, in order to get the limit of visibility of the air at the time. Thus, if Ben Ledi be half-hazed, then the limit of visibility will be fifty miles. In this way all the estimates of haze have been reduced to one scale for comparison.
As the result of all the observations it was found that, as the dryness of the air increases, the limit of visibility also increases. A very marked difference in the transparency of the air was found with winds from the different directions. In the north-west quadrant the winds made the air very clear, whereas winds from all other directions made the air very much hazed. The winds in the other three areas are nearly ten times more hazed than those from the north-west quadrant. That is very striking.
The conclusion come to is that the air from densely inhabited districts is so polluted that it is fully nine times more hazed than the air that comes from the thinly inhabited districts; in other words, the atmosphere at Falkirk is about ten times thicker when the wind is east or south than it would be if there were no fires and no inhabitants.
It is interesting to notice that the limit varies considerably for the same wind at the same humidity. This is what might have been expected, because from the observations made by the dust-counter the number of particles varied greatly in winds from the same directions, but at different times. This depends upon the rise and fall of the wind, changes in the state of trade, season of the year, and other causes. During a strike, the dearth of coal will make a considerable diminution in the number of dust-particles in the air of large towns. With a north wind, the extreme limits of visibility are 120 to 200 miles; and with a north-west wind, from 70 to 250 miles. An east wind has as limits 4 to 50 miles, and a south-east wind 2 to 60 miles.
One interesting fact to be noticed, after wading through these tables, is this—that, as a general result, the transparency of the air increases about 3·7 times for any increase in dryness from 2° to 8° of wet-bulb depression. That is, the clearness of the air is inversely proportional to the relative humidity; or, put another way, if the air is four times drier it is about four times clearer.