Multidisciplinary
Interpretation of Precipitation: Principles and Concepts (Part I)
Etymologically
derived from the Latin expression "praecipitatio" which literally
means "fall". Precipitation, according to the meteorological sense,
takes another dimension of concept to designate a particular natural phenomenon
of the climate called "rain". This last one has the possibility to
occur in a region, in several states (liquid, solid... etc.). With the
breakthrough of scientific research in the nineteenth and twentieth centuries,
the most abundant element in the Universe and on Earth are coincidentally the
elements that constitute water, that is, hydrogen in the Universe and oxygen on
Earth. We owe this important discovery to the precursor of modern chemistry, Antoine Lavoisier (1743-1794),
who succeeded in identifying the components of water in a world where
technology was sorely lacking to carry out scientific research.
From the well-known characteristic
forms attributed to precipitation, it is also known that there is a secondary
specificity that categorizes precipitation, depending on the area of coverage;
intensity and duration. It seems quite reasonable to assume that less extensive
cloud cover will tend to result in precipitation of higher intensity and
shorter duration. The latter is defined as convective precipitation, as opposed
to stratiform precipitation. In the seven layers that make up the atmosphere,
the different cloud classifications are generally located at an altitude
between 1000 and 12000 m, barely exceeding the troposphere (13000 m).
The real cradle of precipitation begins
with the partial blocking of stellar radiation by the ozone layer of the
stratosphere. In fact, once they reach the surface of the earth, the emitted
radiations cause, through the temperature, the warming that results in the
evaporation of the surface of the seas (which is none other than the
evapotranspiration). Thus, the water vapor generated as a result of this
phenomenon, causes an upward movement under the impulse of the atmospheric
pressure which gradually decreases in altitude. A lower pressure due to a low
density of molecules present in the air with a lower temperature leads in turn
to the condensation of water vapor around a core, consisting of charged and neutral
particles. These combine to reach the
saturation point of humidity (high density of water vapor in the air) and marks
the appearance of the cloud in its irregular forms that we observe during the
day. With its almost negligible weight compared to its immense size, the
quantity of visible clouds manages however to remain suspended in the
atmosphere and thus escapes, for a time, the gravitational attraction of the
terrestrial gravity.
Drawn at will by wind circulations, the
new cooling and condensation improvise the cloud to take more weight in the
process of supersaturation and favors the phenomenon of precipitation and
previously conditioned by the humidity of the air of the coverage area. Once
again, between the humid area and the dry air, the temperature and the equinox
play in favor or against a precipitation phenomenon that can give life to the
ground without vaporizing into the atmosphere. Although essential for the
supply of groundwater, which is sometimes found almost in the fourth layer of the
earth's crust (lower mantle), the chemical composition of precipitation can be
altered by the polluting components present in the air. Therefore, it would not
be a prejudice to consider precipitation as a major factor of a rapid response
disease in a locality.
Due to the rotation, revolution, tilt
and sphericity of the Earth, the unevenly distributed global temperature
results in the occurrence of wind circulations in multiple directions. For
example, a warm air mass gaining altitude cools and creates convection to
interact with water vapour as it condenses and, on the other hand, causes wind
to be driven. In addition, the
horizontal force of atmospheric pressures and the perpendicular force of
inertia known as its precursor Coriolis (Gaspard-Gustave
Coriolis (1792-1843)) explain in the
literature to a large extent the direction and intensity of wind in atmospheric
circulations, which is a major means of locomotion of clouds across continents
and oceans.
On a global scale, there are three
distinct zones of wind circulation: the Hadley cell, the Ferrel convection and
the polar circulation. For the first, the Hadley cell, located from the Equator
to 30°N and 30°S, winds blow from the Northeast to the Southwest and from the
Southeast to the Northwest (called Trade Winds) and marks the low pressure
intertropical convergence zone. The second, the Ferrel zone, is characterized
by transient low pressure areas with winds blowing generally towards the West.
North and south of the 60th parallel is the polar zone with eastward wind
speeds. These three zones of the globe are interconnected by the so-called
"fast air" atmospheric current (polar and subtropical type) whose
directions are almost sinusoidal over the globe.
Through these atmospheric currents,
oceanic circulations are also affected by seasonal variability as pointed out
by Walker (1868-1958)
and Humboldt (1769-1859).
For example, the decrease of the circulation in the Hadley cell, can displace
the Walker cell and favor the displacement of warm surface waters of the South
Pacific: this meteorological phenomenon is known under the famous expression
"El Niño", which in turn influences the precipitation, just as
"La Niña" would disrupt the temperature of other lands.
In addition, the topographic nature of
the Earth's surface also interacts with the climate. For example, by blocking
the circulation of temperature at high altitudes and weather, mountainous areas
contribute to the supply of precipitation over several kilometers and change
the climatic conditions of the surrounding area.
Eccentrically, the Paleocene-Eocene
period (56 million years) has been identified as a breakthrough in the
knowledge of paleoclimatic variability. This highlights the existence of a
major factor, beyond a simple meteorite impact on the Earth, to upset the
climate balance. The change in the
Earth's orbit, or the deformation of the Earth's ellipse, according to
scientists at the University of Pennsylvania, marked the warmest period since
the Earth's birth 4.6 billion years ago. Such warming has certainly traced the
history of precipitation evolution on Earth.
From a theological point of view, the
links between atmospheric circulations and precipitation have been mentioned
since the seventh century in, for example, the Holy Quran of Islam. In
particular, it is reported in Sura 30, verse 48:
« It is Allah Who sends the winds,
which then stir up ˹vapour, forming˺ clouds, which He then spreads out in
the sky or piles up into masses as He wills, from which you see rain come
forth. Then as soon as He causes it to fall on whoever He wills of His
servants, they rejoice, ».
And from Sura 24, verse 43:
« Do you not see that Allah gently
drives the clouds, then joins them together, piling them up into masses, from
which you see raindrops come forth? And He sends down from the sky mountains ˹of clouds˺ loaded with hail, pouring it on
whoever He wills and averting it from whoever He wills. The flash of the
clouds’ lightning nearly takes away eyesight. ».
In general, in the process of
precipitation formation, several different atmospheric phenomena according to
specific standards, some known, others unknown, and exo-atmospheric phenomena
contribute directly and indirectly.
Abdi-Basid ADAN, 2022.
