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CHAPTER I

THE ATMOSPHERE

1. CLOUDS have been correctly defined by Dickson as "portions of the atmosphere which, from natural causes, have become temporarily visible." They are aggregates of particles floating in the air, generally, but not necessarily, particles of water or ice. It will therefore be useful, before proceeding to analyse the various forms of cloud and their bearing on weather, to try and fix in our minds some ideas of the nature of this atmosphere and of the processes taking place in it which affect the formation of cloud.

2. For the actual constitution of ordinary impure air it will be sufficient to remind the reader that it contains by volume about 78 per cent of nitrogen, 20 per cent of oxygen, a variable quantity of aqueous vapour (usually about 1 per cent), a little carbonic acid and other gases, and lastly, a variable but important amount of dust of various kinds.

Now the most important of these constituents, as regards the subject of which we are going to treat, are the variable ones, viz. the aqueous vapour

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and the dust particles. The powers which are possessed by these two factors in altering the conditions and affecting the character of weather and elimate are perhaps not sufficiently realised. But before dealing with these it will be necessary to say a few words on the variation of temperatures and pressures through the atmosphere, even at the cost of repeating universally known and understood facts.

3. The pressure of the atmosphere at any point is simply the weight per unit of area of the whole column of air which rests above that point. Thus the higher we rise in the atmosphere the less the pressure will be. And if the pressure and density of the atmosphere were the same throughout as at sea-level, and the temperature were also uniform, the whole height of the atmosphere would only be about 5 miles. But these three factors all decrease rapidly with increase of altitude, and react upon one another in such a manner as to make it impossible to absolutely determine the height of the atmosphere. From other considerations we suspect that this height must be at least 100 miles or more, although, of course, we cannot say that it ends at any particular level, for it grows gradually more and more rarefied until it disappears in space. The fact that the envelope of air exerts a weight on the earth's surface of about eighty billions of tons may give us some notion of the vastness of this envelope. 4. For small altitudes and in still atmosphere we may take the pressure as decreasing at the rate

of 1 in. per 1000 feet increase in elevation, and the temperature as decreasing at the rate of 216° Fahrenheit for every 100 feet increase. But these rules are

liable to considerable variation. For instance, in the cold winter nights of the temperate zones, when the earth loses heat rapidly by radiation into space, the temperature distribution may become inverted, the base of a column of air becoming colder than the upper portion. On the other hand, during the afternoon of a hot summer's day, and in some other conditions, the decrease of temperature with elevation. is much greater than that given above. The pressure distribution also varies considerably. There are extensive permanent areas in different parts of the globe in which the pressures at the earth's surface are less, and decrease at a less rapid rate with altitude, than those on either side of these areas. There are also local wandering areas of low pressure called cyclones, and local areas of high pressure called anticyclones. These will be treated of in later chapters. It will be sufficient here to state that the unequal distribution of pressure and temperature over the globe disturbs the atmospheric equilibrium to such an extent as to cause strong permanent as well as local currents.

These movements are not only horizontal, but also more or less vertical, interchange taking place between the upper and the lower layers of air. It is necessary to mention one result of this interchange which plays a conspicuous part in some of those phenomena which are the subject of the present work.

5. Whenever a gas is subject to rapid compression, whatever the nature of the force which produces the compression may be, the temperature of the gas is raised. And conversely, whenever a gas undergoes a sudden expansion, its temperature is correspondingly lowered. Consequently, if a current of dry air is ascending rapidly, its decrease of temperature is very rapid, not only on account of its increasing distance from the earth, but also on account of its expansion due to decrease of pressure. The rate of this decrease in such a current is found to be about 54° Fahrenheit for each 100 feet of ascent (98° C. per 100 m.). It follows from this that if the distribution of temperature with altitude in quiet air be about that mentioned in § 4, a column of ascending air will be quickly cooled down to a point below the temperature of the surrounding quiet air, and its density becoming greater than that of this air, the column will quickly fall back again. We may for convenience call such a condition one of “stable equilibrium." On the other hand, if, as in the example given above of a hot summer's day, the temperature of the still air decreases abnormally with increase of altitude, the ascending column may lose heat less rapidly than the still air around it, and consequently it will continue to ascend. To such a state we can apply the title "unstable equilibrium." It will be obvious that this state is not likely to exist at a great altitude above the earth.

It will also be seen that in the state of stable

equilibrium a downward motion commencing in any layer of the atmosphere would, like an ascensional movement, be speedily arrested, while in a state of unstable equilibrium it will tend to continue downwards. It should be borne in mind that while the atmosphere is undisturbed, whatever the change of temperature dependent on altitude may be, no vertical movement of the atmosphere will take place, whether the atmosphere be in a state of stable or unstable equilibrium, until some disturbing force begins to act in one direction or another. Any approximately vertical current which tends to turn a portion of the atmosphere upside down or to effect an interchange between its lower and higher strata may be called an "inversion movement" or "current of inversion."

6. We now have to consider one of the primary causes of these disturbances in our atmosphere, viz. aqueous vapour. This vapour is a compound gas, and were it similar to the simple gases in the atmosphere, its effects would not be nearly so complicated. But in the first place, its density is only about half that of dry air, so that if it were to remain a gas, it would ascend to a height much greater than that of our atmosphere. In the second place, with decrease of temperature it does not remain a gas, but becomes condensed into water and congealed into ice.

Now the greater the temperature of any portion of air, the greater its capacity for holding water-vapour, or the amount of water-vapour which such a portion of

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