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the lighter rise to the top. Now, this takes place when the different parts of the layer are unequally heated. At the point of greater warmth the air expands, becomes lighter, then, pressed by the neighbouring layers which have remained colder and heavier, it rises into the higher layers. The result of this process is an ascending current, and lateral currents rushing from all sides towards the spot where the temperature is higher. Let us take an example in nature, and see what passes on an island alone in the midst of the ocean.
We must remember that the land is heated more readily than the sea. In proportion as the sun rises above the horizon, the island becomes warmer than the neighbouring sea. Their respective atmospheres participate in these unequal temperatures, the fresh air of the sea rushes from all directions under the form of a sea breeze, which makes itself felt along the whole coast, and the warmer and lighter air of the island will ascend into the atmosphere. During the night it is the reverse. The island loses heat by radiation, and cools quicker than the sea. Its atmosphere having become heavier, flows into that of the sea, under the form of a land breeze, and this interchange lasts until the temperature, and consequently the density of the two atmospheres, has again become the same. This is the pbenomenon which is observed almost daily on nearly all the seaboards.
What takes place here on a small scale passes on a great scale between an entire continent and the ocean, between the tropical regions and the temperate and polar regions. Southern Africa is fiercely heated by the rays of a summer sun, while the seas of India and Asia experience the low temperature of the winter. The temperature of the tropics is almost always the same, and constantly higher than that of the rest of the globe. To each of these differences of temperature, unequal in duration and amount, particular atmospheric currents which are their consequence correspond; to the difference of temperature between day and night, the diurnal breezes, whether along the coast or in the interior of the continents, at the foot of the mountains; to the difference of temperature between the extreme seasons, the monsoons, which one might call the season breezes; to the difference of temperature between the tropics and the poles, the trade winds, which are the great annual breeze, and the constancy of which is only the expression of the permanent inequality of the distribution of solar heat between the great atmospherical regions of our globe.
A moment's reflection will enable us to see that these differences of temperatures, which set the whole atmosphere in motion, at last connect themselves essentially with the geographical forms of our globe. It is the spherical form which causes the unequal distribution of the rays of the sun, and gives us the great zones of temperature of the astronomical climate, the torrid, temperate, and frozen zones. All the modifications of the solar climate must be referred principally to the geographical forms of the surface, to the distribution and to the relative situation of the continents and the seas.
The general or trade winds are the consequence of the general form of the globe; and their direction, as we shall see by and bye, is given by its rotatory motion. The monsoons and the breezes depend on the form and the relative situation of the lands and the seas, which govern their intensity and direction. The variable winds are due to the same causes, and to the conflict between the general currents. The primary importance of the geographical forms, which is here revealed at the first glance, will become still more evident in the course of our study.-GUYOT.
Let us consider the entire atmosphere as only a horizontal layer of air. We see that one of the principal conditions of equilibrium of the molecules does not exist, since the different parts of it are unequally heated. The regions near the equator have a high temperature, and the heat goes on gradually diminishing in proportion as we advance towards the poles. The atmosphere of the tropical zone is more dilated and constantly lighter than those of the temperate and polar regions. The height of the barometer at the level of the ocean, which measures the weight of the atmosphere, is in fact less at the equator than in the temperate regions. For 10° on each side of the equator the maximum is 80° Fahrenheit, and from thence to the tropics it decreases only 3o.7; in the middle latitudes there is an increase of temperature, the amount of which constitutes the measure of the force which incessantly impels the air of those regions towards the region of the equator.
What is the consequence of this dynamic state of the atmosphere? The denser air of the colder regions presses that of the hot on two sides, the north and the south; the tropical atmosphere rises, and here two lower currents are established, from the poles to the equator, and two superior currents, which conduct the air of the equator towards the poles, to commence again the same rotation. We ought, then, to find in the northern hemisphere a general wind coming from the north, and in the southern hemisphere a wind coming from the south. But the motion of the rotation of the earth from the west to the east exercises an influence upon the direction of these currents, which causes them to deviate from their original direction. The speed of rotation, which is almost nothing in the neighbourhood of the poles, becomes greater for any place, in proportion to its proximity to the equator. The masses of air which rush towards the equator have then an acquired speed less than that of the regions towards which they are directing themselves. At each step they are obliged to assume a greater rapidity of rotation; but as, in virtue of the law of inertia, a certain time is necessary for this to take place, they find themselves at every step a little behind, that is, they are little further towards the west than would be the case without this cir. cumstance. These successive retardations accumulating, change little by little the direction of the current from north to south of the northern hemisphere, into a south-west direction, and the direction of the current from south to north of the southern hemisphere, into a north-west current. These two general currents, of north-east and south-east, to call them according to the usage by the places from which they come, encountering each other in the tropical zone, combine together, and there results a general current from east to west, which is the great trade wind. The region where the two currents meet is in a kind of equilibrium, and it is marked by a zone of calms.
The same cause makes the upper currents, which set from the equator towards the poles, swerve, but in the opposite way. They arrive successively in the higher latitudes, with a velocity of rotation greater than that which they find there, and are always a little in advance of the Earth's motion in each place;
that is, they swerve always more and more to the east. There will then result a current bearing to the north-east, or a south-west wind, in the northern hemisphere, and a current bearing to the south-east, or a north-west wind, in the southern hemisphere.-GUYOT.
The winds of the Indian Ocean experience greater perturbations than those of the other two oceans of the tropics. If we have elsewhere called the Pacific the most oceanic of the oceans, the Atlantic the most maritime, we will call the Indian Ocean the most mediterranean. It is, in reality, only a half ocean, a great gulf, surrounded on the sides by huge continental masses : the mighty Asia, with its peninsulas and its table-lands, on the north, Africa on the west, Australia on the east. Asia prevents the oceanic trade wind of the north-east from arriving there, and the influence of the lands and of the vast plateaus remains greatly preponderating. Thus the movements of the atmosphere depend upon the unequal heating of the neighbouring continents during the extreme seasons of summer and winter, which are opposite in the continents situated in the north and in the south. The eastern trade wind in this way changes into a sort of double semi-annual breeze, blowing regularly six months in one direction, and six months in another: this is called monsoon, from the Arabic word moussin, signifying season. It will be easy to understand this effect, if you call to mind what we have said of the land and sea breezes that spring up on the islands and along the sea-shores.
While Africa, south of the equator, receives the vertical rays of the southern summer sun, in December, January, and February, Southern Asia, on the north of the equator, and the neighbouring seas, are feeling the low temperatures of winter. The air rushes in from the colder regions of the Indies and of Upper Asia towards the warmer regions of Southern Africa, and the trade wind is transformed into a north-easter, which blows as long as this difference of temperature lasts. It is for India the winter or north-east mon
The reverse takes place when India and Asia are heated by the burning sun of the northern summer, and when Africa is cooled by the southern winter. The air blows towards the places of which the temperature is more elevated; it is for India the summer or south-west monsoon.
Hence, in place of a constant current setting from east to
west, the relative position of the lands, combined with the action of the Earth’s rotation, gives occasion to two periodical winds: the monsoon of the south-west, blowing from April to October during the northern summer, and the northeast monsoon, blowing from October to April during the southern summer. In the southern part of the Indian Ocean, which is not under the influence of the lands, the south-east trade wind blows quite regularly through the whole year.
The transition from one monsoon to another, depending upon the course of the Sun, does not occur at the same period in places situated under different latitudes; but the approach of this critical season is always announced by variable winds, succeeded by intervals of calm, and by furious tempests and whirlwinds, proving a general disturbance of the atmosphere.
A determinate volume of air-a cubic foot, for example, at a given temperature—has the property of receiving a certain quantity of vapour, of water in an invisible state, or, as we call it, humidity. When it contains all the humidity it is capable of receiving, it is said to be saturated. If you increase the temperature, it will be able to hold more ; if, on the contrary, you lower the temperature, you diminish its capacity for vapour, and, in the given case, a part of the vapour would be condensed and deposited in small drops of rain along the outside of the vessel. The moist air here is like a sponge filled with water ; reduce its volume by pressure, there will run out a certain quantity of water; in the air laden with moisture the diminution of the temperature takes the place of
pressure. We can easily conceive the application of this principle in meteorology.
A warm and moist wind, the south-west of the Atlantic, for example, setting from the tropics, comes in contact with the colder air of the temperate regions ; its temperature is lowered; it can no longer contain as great a quantity of vapour. A portion of its humidity is immediately condensed into clouds, then falls in rain.
Or the opposite : a wind charged with clouds arrives in a warmer and drier air, comes, for example, from the Mediterranean to the Sahara, as is the case during three-fourths