Meteorology Part 3

CHAPTER XI

IT ALWAYS RAINS

All are familiar with the answer given by the native of Skye to the irate tourist on that island, who, for the sixth day drenched, asked the question: "Does it always rain here?" "Na!" answered the workman, without at all understanding the joke; "feiles it snaas" (sometimes it snows).

Yet, strange to say, the tourist's question has been answered in the affirmative in every place where a cloud is overhead, visible or invisible.

Whenever a cloud is formed, it begins to rain; and the drops shower down in immense numbers, though most minute in size--"the playful fancies of the mighty sky."

No doubt it is only in certain circ.u.mstances that these drops are attracted together so as to form large drops, which fall to the earth in genial showers to refresh the thirsty soil, or in a terrible deluge to cause great destruction. But when the temperature and pressure are not suitable for the formation of what we commonly know as the rain, the fine drops fall into the air under the cloud, where they immediately evaporate from their dust free-surfaces, if the air is dry and warm. This is, in other words, the decay of clouds.

It is a curious fact that objects in a fog may not be wetted, when the number of water-particles is great. It seems that these water-particles all evaporate so quickly that even one's hand or face is not sensible of being wetted. The particles are minutely small; and they may evaporate even before reaching the warm skin, by reason of the heated air over the skin.

There is a peculiarly warm sensation in the centre of a c.u.mulus cloud, especially when it is not dense. A glow of heat seems to radiate from all points. Yet the face and hands are quite dry, and exposed objects are not wetted; but it is really _always raining_. That is a curious discovery.

It is radiant heat that is the cause of the remarkable result. The rays of the sun, which strike the upper part of the cloud, not only heat that surface but also penetrate the cloud and fall on the surface of bodies within, generating heat there. These heated surfaces again radiate heat into the air attached to them. This warm air receives the fine raindrops in the cloud, and dissolves the moisture from the dust-particles before the moisture can reach the surfaces exposed. That a vast amount of radiant heat rushes through a cloud is clearly shown by exposing a thermometer with black bulb _in vacuo_. On some occasions, a thermometer would indicate from 40 to 50 above the temperature of the air, thus proving the surface to be quite dry.

These observations have been corroborated on Mount Pilatus, near Lucerne--1000 feet higher and more isolated than the Rigi. The summit was quite enveloped in cloud, and, though one might naturally conclude that the air was dense with moisture, yet the wooden seats, walls, and all exposed surfaces were quite dry. Strange to say, however, the thermometers hung up got wet rapidly, and the pins driven into the wooden post to support them rapidly became moist. A thermometer lying on a wooden seat stood at 60, while one hung up read only 48. This difference was caused by radiant heat.

It is well known that, when bodies are exposed to radiant heat, they are heated in proportion to their size; the smaller, then, may be moist, when the larger are dry by radiation. The effect of the sun's penetrating heat through the cloud is to heat exposed objects above the temperature of the air; and if the objects are of any size they are considerably heated, and retain their heat more, while at the same time around them is a layer of warm air which is quite sufficient to force the water-vapour to leave the dust-particles in the fine rain.

Hence seats, walls, posts, &c., are quite dry, though they are in the middle of a cloud. They are large enough to throw off the moisture by the retained heat, or by the large amount of surrounding heat; whereas, small bodies, which are not heated to the same degree and cannot therefore retain their heat so easily, have not heat-power sufficient to withstand the moisture, and they become wetted. Hence, by the radiant heat, the large exposed objects are dry in the cloud; whereas small objects are damp, and, in some cases, dripping with wet.

The fact is, then, that whenever a cloud overhangs, _rain is falling_, though it may not reach the earth on account of the dryness of the stratum of air below the cloud, and the heat of the air over the earth. So that on a summer day, with the gold-fringed, fleecy clouds sailing overhead, it is really raining; but the drops, being very small, evaporate long before reaching the earth. As Ariel sings at the end of "The Tempest" of Shakespeare, "The rain, it raineth every day." It rains, but much of the melting of the clouds is reproduced by a wonderful circularity--the moisture evaporating, seizing other dust-particles, forming cloud-particles, falling again, and so on _ad infinitum_, during the existing circ.u.mstances.

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 gla.s.s plate vertically, and in close contact with one of the panes of gla.s.s 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 gla.s.s 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 gla.s.s 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 gla.s.s. 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 gla.s.s. 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 gla.s.s 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 gla.s.s. Any one can try the experiment. Clean one half of a dusty pane of gla.s.s 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 circ.u.mstances, 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.

CHAPTER XIII

HAZING EFFECTS OF ATMOSPHERIC DUST

The transparency of the atmosphere is very much destroyed by the impurities communicated to it while pa.s.sing over the inhabited areas of the country. Dr. Aitken devoted eighteen months to compare the amount of dusty impurities in different ma.s.ses 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 pa.s.sage 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 37 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.

CHAPTER XIV

THUNDER CLEARS THE AIR

The phrase "thunder clears the air" is familiar to all. It contains a very vital truth, yet even scientific men did not know its full meaning until just the other day. It came by experience to people who had been for ages observing the weather; and it is one of the most pointed of the "weather-lore" expressions. Folks got to know, by a sort of rule-of-thumb, truths which scientifically they were unable to learn. And this is one.

Perhaps, therefore, we should respect a little more what is called "folk-lore," or ordinary people's sayings. Experience has taught men many wonderful things. In olden times they were keener natural observers. They had few books, but they had plenty of time. They studied the habits of animals and moods of nature, and they came wonderfully near to reaching the full truth, though they could not give a reason for it. The awe-inspiring in nature has especially riveted the attention of man.

And no appearance in nature joins more powerfully the elements of grandeur and awe than a heavy thunder-storm. When, suddenly, from the breast of a dark thunder-cloud a brilliant flash of light darts zigzag to the earth, followed by a loud crackling noise which softens in the distance into weaker volumes of sound, terror seizes the birds of the air and the cattle in the field. The man who is born to rule the storm rejoices in the powerful display; but kings have trembled at the sight.

Byron thus pictures a storm in the Alps:--