Le fleuve Amazone pourrait être le plus grand sur terre, mais il y a quelque chose d'encore plus grand dans le ciel — une rivière atmosphérique. Ces "rivières dans le ciel" sont des flux massifs de vapeur d'eau, transportant plus d'eau que l'Amazone lui-même. Elles se déplacent dans l'atmosphère, entraînant souvent de fortes pluies lorsqu'elles atteignent la terre. Les scientifiques les appellent "rivières volantes", et elles jouent un rôle immense dans le cycle de l'eau de la Terre. C'est incroyable de penser que le plus grand fleuve pourrait en fait flotter au-dessus de nous ! Animation créée par Sympa.
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FunTranscript
00:00Despite popular belief, qualifying the Nile as the longest river in the world may be a mistake.
00:06To measure the length of a river, it is not enough to calculate the distance between its starting point and its arrival point.
00:13Rivers are complex systems, made up of various affluents that count for a lot in their total length.
00:20In fact, if we take into account these affluents, the total length can be multiplied by 3, as is the case for the Mississippi River.
00:27From its birth in Itasca to New Orleans, where it ends, the river measures a certain length.
00:33But if we take into account the Missouri River, things change completely.
00:38This divergence fuels many debates about the length of the Nile compared to that of the Amazon.
00:43Although Google recognizes the Nile as the longest river in the world, it may not be entirely accurate.
00:49To understand this, it is necessary to represent the rivers as the veins of a human body.
00:53It would be inaccurate to assume that an individual of 2 meters high has only 2 meters of veins, right?
00:58The veins intersect in the whole body, like a vast network.
01:02Watercourses function according to the same principle, and we cannot consider only the length of the main river.
01:08The vast network of affluents is just as important.
01:11Atmospheric rivers are fascinating meteorological phenomena.
01:15Air currents that carry an incredible amount of water vapor from the Amazon rainforest,
01:20from North America to the north of Argentina.
01:23These air routes are essential to the humidity, which is of great importance for our landscapes.
01:30Their influence extends to the regions of the Midwest, Southeast and South of Brazil,
01:36shaping the specific precipitation regimes of these areas.
01:40When these rivers meet particular meteorological conditions, such as a cold front,
01:46the latent humidity they carry can turn into precipitation.
01:51These precipitations play an essential role not only in the preservation of the various ecosystems of the region,
01:57but also in the support of the flourishing economy of Brazil, and in many ways.
02:02The importance of this phenomenon is obvious.
02:05First, it allows the irrigation of crops, guaranteeing agricultural productivity and food security.
02:11In addition, it feeds the local rivers,
02:14which allows to maintain a regular water flow and to promote aquatic life.
02:19In addition, the water collected by these rivers is sent to dams,
02:23which transform them into a reliable source of energy for the country.
02:27The amount of water transported by the air rivers can be equivalent, if not superior, to the huge flow of the Amazon.
02:33Thus, if we associate the Amazon River with its air counterpart,
02:37we get a volume of water much higher than that of an island.
02:40We also observe atmospheric rivers in the Congo Basin,
02:44where the climate is mainly shaped by the humidity emanating from the Atlantic Ocean and local forests.
02:50We witness an evaporation of 1 to 2 meters of water per year.
02:54These rivers vary in size and power,
02:57but on average, they carry a volume of water vapor comparable to the flow at the mouth of the Mississippi.
03:03In rare cases, exceptionally powerful atmospheric rivers can carry up to 15 times this volume.
03:10When these rivers reach the mainland, they release the accumulated water vapor,
03:14which leads to precipitation or snowfalls.
03:18Although atmospheric rivers are presented in different forms,
03:22those that have a strong water vapor retainer and intense winds can cause extreme precipitation and even floods,
03:29especially when they linger on hydrographic basins, subject to rain.
03:34A well-known example of an atmospheric river is the Pineapple Express,
03:38a system capable of transporting the humidity of tropical regions located near Hawaii to the west coast of the United States.
03:46Such events can disrupt transport, trigger mudslides and cause significant material and human destruction.
03:54However, all atmospheric rivers do not pose such risks.
03:59Many of them are relatively weak systems that provide precious precipitation and are essential for good water supply.
04:08In addition, they play a crucial role in the hydrological cycle of the planet.
04:13In addition to the risk of flooding, atmospheric rivers also contribute to the increase in snowfall.
04:19Atmospheric rivers were the cause of the violent winter storms that hit the west coast of the United States,
04:25from the state of Washington in southern California, between 2010 and early 2024,
04:31causing precipitation ranging from 28 to 63 cm in some regions.
04:36Water vapor currents can therefore be considered rivers.
04:40Is it possible to put something else under this term?
04:43Surprisingly, there is a phenomenon that, although devoid of water, falls into this same category, the weds.
04:49These are, traditionally, what are called river valleys.
04:53Weds are found in slightly inclined areas, almost flat, of the deserts.
04:58Unlike traditional rivers, weds do not have stable canals due to the intermittent presence of rivers.
05:05They therefore have a trellis structure due to a limited water supply and an abundance of sediment.
05:11The water that manages to reach the bed of these weds infiltrates the earth and causes a decrease in energy and large deposits.
05:19Weds can even form sediment dams, which modifies the configuration of watercourses during sudden rains.
05:26It is interesting to note that the wind also plays a role in the deposit of these sediments.
05:31When the weds are submerged or wet, the sediments pushed by the wind come to settle,
05:37which produces a mixture of sediment and aerial sediment.
05:41Rivers can be special not only because they are made up of sand or air, but also because of what covers them.
05:48In some conditions, the Scottish River Dee offers an intriguing spectacle.
05:53Frozen formations that look like water lilies.
05:57These formations, called ice crepes, appear naturally when the movement of the waves deforms the smooth and flat ice,
06:05which gives rise to these curiosities at the smooth and rounded edges.
06:09With this movement, these ice discs rise above the surface and take on the appearance of strange water lilies.
06:16In general, ice crepes have a consistency close to that of water lilies.
06:20Some media have suggested that the ice of the Dee River is formed from frozen foam.
06:25But scientists explain that even if this hypothesis is plausible,
06:29ice crepes always form because of the movement of the waters.
06:32Over time, these water lilies often end up forming large plates of ice.
06:37If the ice is thick enough and the water remains turbulent, it can bend, fracture and even form layers,
06:44which gives rise to a structure called ice crest.
06:48Now imagine a river made up not of water, but of methane.
06:52The idea may seem crazy, but on Titan, the moon of Saturn, it is a reality.
06:57Veed Floumina, a fascinating river on Titan, is thus made up of methane and liquid ethane,
07:04and has even been compared to the powerful Nile.
07:06Extending over more than 400 km, it gracefully throws itself into the Ijeya Maré,
07:11which is the second largest sea of hydrocarbons in Titan.
07:15Most of Titan's surface is made up of water ice,
07:18but Veed Floumina, by digging canyons in this frozen terrain,
07:22suggests that there is a rocky soil there.
07:25NASA scientists believe that this extraordinary river contains rapids,
07:29whirlwinds and even waterfalls, like those found in terrestrial rivers.
07:34When the Cassini probe reached Saturn in 2004,
07:37its main objective was to explore this giant planet and its moons.
07:42Scientists were particularly interested in the discovery of liquid methane on Titan,
07:47due to the unique temperature of its surface and its atmosphere.
07:51However, Cassini's first observations, focused on the equatorial region of Titan,
07:56disappointed researchers who found no signs of seas or rivers.
08:01But they had a nice surprise when Cassini's orbit moved,
08:05allowing the team to focus on other areas of the surface of the moon using radar.
08:11Surprisingly, the radar signals bounced on various angular and rough formations,
08:18sand dunes, cliffs and rocks.
08:21But when the radar reached the polar regions of Titan,
08:24the signals stopped, revealing mysterious dark spots
08:28resembling lakes, rivers and ripples similar to those found on Earth.
08:34The revolutionary discovery took place in December 2012,
08:38when Jean-Yves Radbeau, a scientist working with Cassini's team,
08:42presented an image taken in September of the same year.
08:45The image reveals a river that looks like the Nile and extends over 322 km,
08:50according to a relatively straight line,
08:52indicating a correlation with fault lines similar to those of other important rivers located nearby.
08:59This remarkable watercourse crosses rugged and inclined terrains,
09:03suggesting the existence of a tectonic movement similar to that observed on Earth.