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Ascending current, or even ascending, very useful for those who fly, plane, bird or those who are fond of gliding and free flight, but also dangerous if it gives rise to a violent storm. In this last case the ascending current does form clouds called cumulonimbus clouds, often fast and not easy to predict. This means sudden and even heavy rainfall unexpected difficulties for those who are crossing the sky with a plane, with your own wings or something similar. There are also times when thunderstorms thus formed are characterized by internal eddies of ice and hail.
Ascending current: where it forms
There ascending current can form in presence of mountains, when a mass of moving air encounters a relief as it advances and is forced to climb to the top to continue. When it drops in altitude, once it has passed over the obstacle, it is a question of forced convection.
There are other occasions when one can form rising current and it is because of the density, the difference in density. Let's imagine a mass of hot air, less dense than cold, which therefore rises by convection.
If we are in unstable conditions the moving mass rises more and more until it finds a stable layer of air. It often happens between 8,000 m and 20,000 m, otherwise it is located at the beginning of the stratosphere, which is stable.
Rising current: what it is
As imaginable it is one mass of air rising in altitude and as it rises it expands and cools, usually with a gradient, a “speed” therefore, “dry adiabatic”, that is with a rate of 0.7 ° C / 0.8 ° C every 100 m traveled.
Let's see what happens when it goes up. Self it has colder air around it, and that of the ascending current cools more slowly than it, rises more and more and we are in a unstable regime. On the contrary, if the mass of hot air that rises cools down faster than the air it is in, it stops and we are in a regime of stable air.
Let's not forget the'humidity which is a feature to always keep in mind when it comes to air. The rising hot mass is also moist and when its relative humidity will reach 100%, the water vapor will begin to condense giving life to a cloud, in particular to a heap. However, something special happens inside a cloud: the ascending current is more powerful and continues even if the air is stable.
Of course, if it is already unstable, the cumulus becomes a cumulonimbus, enlarging, and giving rise to a thunderstorm. When we evaluate whether the air is stable or not, it is necessary that we also take into account the presence of nearby mountain ranges.
Ascending and descending current
There ascending current and the descending one occurs in the presence of a storm which in turn originates, as we have just seen, from a cumulonimbus. We therefore have masses of air that rise and others that descend, also called respectively inflow and outflow.
The first is hot and humid and feeds the thunderstorm, then it stays at its sides and supports it. However, when a mass of hot air rises, cools and becomes heavier, until it is "forced" to become a descending current.
There are, in thunderstorms, inside the cloud, and they are called "Downdraft", as they descend they are colder and colder until they get close to the ground, to theirs maximum speed, creating storm phenomena often also characterized by violent gusts of wind. Sometimes gusts with speeds exceeding 100km / h are also recorded which are confused with tornadoes!
Ascending and convective current
The convective currents are those relating to convective cells which in turn originate from convective motions. These motions, these movements of air are linked to the "innate" instability of the atmosphere which is nothing more than a huge mass of air resting on the Earth.
Given that the surface, heated by the sun, emits heat that the part of the atmosphere closest to Earth's crust, absorbs, then there is movement between masses of air that form convective cells. In practice, they rotate, “mixing” masses of more or less hot air. In general the ascending current is that formed by hot air and that goes upwards, very useful to birds.
Among the living beings who use it the most are the vultures who even do without flapping their wings and use this current to take flight, even the albatross they are not doing badly at all.
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