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LAND TRADE-WINDS. 235
CHAPTER VI.
THE TRADE-WINDS OP THE CONTINENTS.—THE MONSOONS.—ETESIAN WINDS.
The trade-winds, as we said, have not the same regularity on the continents as over the sdas. On the surface of the ocean the masses of moving air are not arrested by any obstacle; they are propagated freely toward the equatorial zone, and can scarcely.be turned from their route by
the attraction of any marine centre of heat, as the temperature of the water only increases or diminishes very slowly, and the oscillations of the
thermometer from day to night do not attain thirty-six degrees Fahrenheit. In the midst of the large islands and the continents it is no longer
so. Their mountain chains oppose the course of the winds, and cause
them to change their direction; forests, prairies, sheets of inland waters,
plateaux with long slopes, hilly countries, large plains, and the innumerable variations of topographical relief, are variously heated by the sun, and
by this very circumstance turn aside or repel the wind which blows from
the neighboring seas. In the higher regions the current can, it is true,
continue its normal movement above the plateaux and the mountains;
but below the uneven surface of the country is traversed by irregular
winds. Here the band of equatorial calms is completely obliterated, there
it is enlarged in an abnormal manner; the winds are deflected variously
on one side or the other, and are directed toward that country whose air
is most expanded by the rays of the sun. Nevertheless, it must be said
that only a very insufficient number of meteorological observations have
as yet been made in the greater part of tropical countries.
Still we can not doubt but that the trade-winds blow over vast continental tracts, as well as over the surface of the seas. In fact, the want of
rain and the almost complete absence of vegetation in all that part of Africa known by the name of the Desert of Sahara, prove in an indubitable
manner the existence of a regular wind from the north-east. After having passed the high plateau of Asia, and having discharged itself of the
greater part of its watery vapor, this atmospheric current traverses obliquely the whole of Africa from the banks of the Nile to those of the Niger. On this enormous*tract of nearly 3000 miles it only lets rain fall on
some mountain summits, such as the Djebel-Hoggar, and scarcely casts a
single cloud on the unchanging azure of the sky. On the western coast
ofthe Sahara, the burning wind called the Harmattan is nothing else than
the north-east trade-wind more or less turned from its course because of
the neighborhood of the sea. Toward the seventeenth degree of north
latitude, on the southern frontiers of Soudan, clouds are at last formed in
space, abundant rains penetrate the soil, and the aridity ofthe desert gives
place to a fine vegetation; this is, because the domain ofthe permanent
| Title | The ocean, atmosphere, and life |
| Creator | Reclus, Elisée |
| Publisher | Harper |
| Place of Publication | New York |
| Date | 1873 |
| Language | eng |
| Type | Books/Pamphlets |
| Title | 00000260 |
| Type | Books/Pamphlets |
| Transcript | LAND TRADE-WINDS. 235 CHAPTER VI. THE TRADE-WINDS OP THE CONTINENTS.—THE MONSOONS.—ETESIAN WINDS. The trade-winds, as we said, have not the same regularity on the continents as over the sdas. On the surface of the ocean the masses of moving air are not arrested by any obstacle; they are propagated freely toward the equatorial zone, and can scarcely.be turned from their route by the attraction of any marine centre of heat, as the temperature of the water only increases or diminishes very slowly, and the oscillations of the thermometer from day to night do not attain thirty-six degrees Fahrenheit. In the midst of the large islands and the continents it is no longer so. Their mountain chains oppose the course of the winds, and cause them to change their direction; forests, prairies, sheets of inland waters, plateaux with long slopes, hilly countries, large plains, and the innumerable variations of topographical relief, are variously heated by the sun, and by this very circumstance turn aside or repel the wind which blows from the neighboring seas. In the higher regions the current can, it is true, continue its normal movement above the plateaux and the mountains; but below the uneven surface of the country is traversed by irregular winds. Here the band of equatorial calms is completely obliterated, there it is enlarged in an abnormal manner; the winds are deflected variously on one side or the other, and are directed toward that country whose air is most expanded by the rays of the sun. Nevertheless, it must be said that only a very insufficient number of meteorological observations have as yet been made in the greater part of tropical countries. Still we can not doubt but that the trade-winds blow over vast continental tracts, as well as over the surface of the seas. In fact, the want of rain and the almost complete absence of vegetation in all that part of Africa known by the name of the Desert of Sahara, prove in an indubitable manner the existence of a regular wind from the north-east. After having passed the high plateau of Asia, and having discharged itself of the greater part of its watery vapor, this atmospheric current traverses obliquely the whole of Africa from the banks of the Nile to those of the Niger. On this enormous*tract of nearly 3000 miles it only lets rain fall on some mountain summits, such as the Djebel-Hoggar, and scarcely casts a single cloud on the unchanging azure of the sky. On the western coast ofthe Sahara, the burning wind called the Harmattan is nothing else than the north-east trade-wind more or less turned from its course because of the neighborhood of the sea. Toward the seventeenth degree of north latitude, on the southern frontiers of Soudan, clouds are at last formed in space, abundant rains penetrate the soil, and the aridity ofthe desert gives place to a fine vegetation; this is, because the domain ofthe permanent |
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