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THE MOISTURE OF THE AIR. 277
wind, and replaced by a new stratum of dry air, this will likewise take its
share of humidity, then those which follow will be saturated in their turn,
and the phenomenon of evaporation will advance the more rapidly, the
more violent the current of air itself is. We know with what speed the
dry winds harden the fields and wet roads; one would say that they lick
the ground, so rapidly do all the pools of water disappear.
After having thus facilitated evaporation on the sheets of water and
moist parts of the continents, the winds transport vapor into the various
countries of the earth, and mingle it with dry air, so that nowhere, even
at thousands of miles from the ocean, is the air completely destitute of
moisture. However, we easily understand that the quantity of vapor is
not at equal temperatures distributed in a uniform manner. In open sea
the atmosphere is always very near the point of saturation, even when the
clouds do not threaten to discharge rain; and consequently the vapor contained in the sea atmosphere diminishes pretty regularly from the equator
toward the poles, following the isothermal curves.* On the shores bathed
by the moist air of the oceans, the proportion of watery vapor diminishes
likewise in a normal manner on both sides of the equator. But in the interior of the continents, where the distribution of lakes, rivers, and mount-'
ains presents such a great variety, and where the winds follow such different paths, the atmospheric vapor is also distributed very unequally.
While the air is almost always either saturated with vapor, or very near
the point of saturation, above England and Ireland, in the steppes of Central Asia it is of an extreme dryness, and usually it contains only from
fifteen to twenty per cent, of the vapor which it could absorb. On an average, the atmosphere of the continents contains three-fifths of the moisture which it would hold if it were completely saturated in all its extent.f
This proportion is that which the surface of oceans or basins of evaporation, compared to that of the dry land, would have led us to suppose beforehand.
When the atmosphere contains all the moisture which its temperature
can bear, the least particle of supplementary vapor is sufficient to determine the precipitation under the form of drops of a part of the vaporized
water; either a mist or cloud is produced, and it begins* to rain. Inasmuch as the point of saturation varies in every country and at every season, according to the oscillations of heat and cold, it follows that the same
quantity of water contained in the atmosphere does not determine the
formation of rain at two different temperatures. The same proportion of
moisture which, during the winter, completely saturates the cold air, and
falls in snow to the ground again, would be very small in the heated atmosphere of summer, and the aerial mass that should contain it would
leave an impression of dryness; in the same way a wind, such as the sirocco, for example, would be dry in a warm country like Barbary, and become moist on the cold mountains of the Alps.J It is, therefore, impor-
* See below, the section entitled Climates.
t Saigey, Petite Physique du Globe. > X See above, p. 243.
| Title | The ocean, atmosphere, and life |
| Creator | Reclus, Elisée |
| Publisher | Harper |
| Place of Publication | New York |
| Date | 1873 |
| Language | eng |
| Type | Books/Pamphlets |
| Title | 00000304 |
| Type | Books/Pamphlets |
| Transcript | THE MOISTURE OF THE AIR. 277 wind, and replaced by a new stratum of dry air, this will likewise take its share of humidity, then those which follow will be saturated in their turn, and the phenomenon of evaporation will advance the more rapidly, the more violent the current of air itself is. We know with what speed the dry winds harden the fields and wet roads; one would say that they lick the ground, so rapidly do all the pools of water disappear. After having thus facilitated evaporation on the sheets of water and moist parts of the continents, the winds transport vapor into the various countries of the earth, and mingle it with dry air, so that nowhere, even at thousands of miles from the ocean, is the air completely destitute of moisture. However, we easily understand that the quantity of vapor is not at equal temperatures distributed in a uniform manner. In open sea the atmosphere is always very near the point of saturation, even when the clouds do not threaten to discharge rain; and consequently the vapor contained in the sea atmosphere diminishes pretty regularly from the equator toward the poles, following the isothermal curves.* On the shores bathed by the moist air of the oceans, the proportion of watery vapor diminishes likewise in a normal manner on both sides of the equator. But in the interior of the continents, where the distribution of lakes, rivers, and mount-' ains presents such a great variety, and where the winds follow such different paths, the atmospheric vapor is also distributed very unequally. While the air is almost always either saturated with vapor, or very near the point of saturation, above England and Ireland, in the steppes of Central Asia it is of an extreme dryness, and usually it contains only from fifteen to twenty per cent, of the vapor which it could absorb. On an average, the atmosphere of the continents contains three-fifths of the moisture which it would hold if it were completely saturated in all its extent.f This proportion is that which the surface of oceans or basins of evaporation, compared to that of the dry land, would have led us to suppose beforehand. When the atmosphere contains all the moisture which its temperature can bear, the least particle of supplementary vapor is sufficient to determine the precipitation under the form of drops of a part of the vaporized water; either a mist or cloud is produced, and it begins* to rain. Inasmuch as the point of saturation varies in every country and at every season, according to the oscillations of heat and cold, it follows that the same quantity of water contained in the atmosphere does not determine the formation of rain at two different temperatures. The same proportion of moisture which, during the winter, completely saturates the cold air, and falls in snow to the ground again, would be very small in the heated atmosphere of summer, and the aerial mass that should contain it would leave an impression of dryness; in the same way a wind, such as the sirocco, for example, would be dry in a warm country like Barbary, and become moist on the cold mountains of the Alps.J It is, therefore, impor- * See below, the section entitled Climates. t Saigey, Petite Physique du Globe. > X See above, p. 243. |
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