Page:EB1911 - Volume 08.djvu/742

 conditions of weather that there was a daily variation in the number of particles, a maximum near the hottest part of the day and a minimum in the morning, and attributed the rise in the numbers to the impure air of the valleys rising on the sun-heated slopes of the mountain or driven up by the wind. A. Rankin, at the Ben Nevis observatory, also observed this daily variation, and his observations also indicate a yearly variation at that station, the numbers being highest in March, April and May. This may possibly be due to small rainfall in these months, but more probably to the fact that south-easterly winds blow more frequently during these months on Ben Nevis than at any other season, and these winds bring the impure air from the more densely inhabited parts of the country.

Without atmospheric dust not only would we not have the glorious cloud scenery we at present enjoy, but we should have no haze in the atmosphere, none of the atmospheric effects that delight the artist. The white haze, the blue haze, the tender sunset glows of red, orange and yellow, would all be absent, and the moment the sun dipped below the horizon the earth would be in darkness; no twilight, no after-glows, such as those given some years ago by the volcanic dust from Krakatoa; none of the poetry of eventide. Why, it may be asked, is this so? Simply because all these are due to matter suspended in the air, to dust. Water has no such effects as long as it is a vapour, and if it condensed without the presence of dust, the particles would be far too few to give any appreciable effect and too heavy to remain in suspension.

Turning now to the investigations on this point, Aitken has shown that there is no evidence to indicate that water vapour has any hazing effect, and shows that the haze is entirely due to dust, the density of the haze increasing with the increase in the number of dust particles in the air, and also with the relative humidity; but the humidity does not act as vapour, but by condensing on the dust and increasing the size of the particles, as it is not the amount of vapour present but the degree of saturation that affects the result; the more saturated the air, the more vapour is condensed on the particles, they so become larger and their hazing effect increased.

The relation of haze or transparency of the air to the number of dust particles was observed on five visits to the Rigi Kulm. The visibility of Hochgerrach, a mountain 70 m. distant from the Rigi, was used for estimating the amount of haze when the air was clear. During the visits this mountain was visible thirteen times, and it was never seen except when the number of particles was low. On eight occasions the mountain was only one-half to one-fifth hazed, and on these days the number of particles was as low as from 326 to 850 per c.c. It was seen five times when the number was from 950 to 2000 per c.c., but the mountain on these occasions was only just visible, and it was never seen when the number was a little over 2000 per c.c.

It has been pointed out that the relative humidity has an effect on the dust by increasing the size of the particles and so increasing the haze. It was therefore necessary in working out the dust and haze observations made at the different places to arrange all the observations in tables according to the wet-bulb depressions at the time. All the observations taken when the wet-bulb depression was between 2° and 4° were put in one table, all those when it was between 4° and 7° in another, and all those when it was over 7° in a third. It should be here noted that when the dust particles were counted and the wet and dry bulb observations taken, an estimate of the amount of haze was also made. This was done by estimating the amount of haze on a mountain at a known distance. Suppose the mountain to be 25 m. distant, and at the time to be one-half hazed, then the limit of visibility of the mountain under the conditions would be 50 m., and that was taken as the number representing the transparency of the atmosphere at the time. In the tables above referred to along with the number of particles was entered the limit of visibility at the time; when this was done it was at once seen that as the number of particles increased the limit of visibility decreased, as will be seen from the following short table of the Rigi Kulm observations when the wet-bulb depression was between 2° and 4°.

When the number of particles is multiplied by the limit of visibility in the tables a fairly constant number C. is obtained; see preceding table. All the observations taken at the different places were treated in a similar manner and the means of all the observations at the different humidities were obtained, and the following table gives the mean values of C. at the different wet-bulb depressions of all the observations made at the different places.

From the above table it will be seen that as the dryness of the air increased it required a larger number of particles to produce a complete haze, nearly double the number being required when the wet-bulb depression was over 7° than when it was only from 2° to 4°. To find the number of particles required to produce a complete haze, that is, to render a mountain just invisible, all that is necessary is to multiply the above constant C. by 160,930, the number of centimetres in a mile, when this is done with the observations made in the West Highlands we get the numbers given in the following table:— The above table gives the number of particles of atmospheric dust in a column of air having a section of one centimetre square, at the different humidities, required to produce a complete haze, that is, to make a distant object invisible, and is of course quite independent of the length of the column.

In making these dust and transparency observations three things were noted: 1st, the number of particles; 2nd, the humidity; and 3rd, the limit of visibility. From the results above given, it is evident that if we now know any two of these we can calculate the third. Suppose we know the limit of visibility and the humidity, then the number of particles can be calculated by the aid of the above tables.

To show the hazing effects of dust it is not, however, necessary to use a dust counter. Aitken for some years made observations on the haze in the air at Falkirk by simply noting the direction of the wind, the wet-bulb depression at the time, and the transparency of the air. Falkirk is favourably situated for such observations owing to the peculiar distribution of the population surrounding it. The whole area from west, north-west to north, is very thinly populated, while in all other directions it is densely populated. It was found that the air from the thinly inhabited parts, that is, the north-west quadrant, was nine times clearer than the air from other directions with the same wet-bulb depression, and that the density of the haze was directly proportional to the density of the population of the area from which the wind blew. These observations also showed that the transparency of the air increases with the dryness, being 3.7 times clearer when the wet-bulb depression is 8° than when it is only 2°, and that the air coming from the densely inhabited parts is about 10 times more hazed than if there were no inhabitants in the country. 