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We will now see the second engine of local weather,
which we will call convection.
What is convection?
It is an effect of the energy that comes from the Sun, and there is a law,
which is called Stefan-Boltzmann's law, here it is - this strange thing.
This is the temperature raised to the fourth power and this is a constant,
meaning that the temperature,
on a surface, raised to its fourth power will give us the
amount of radiation emitted by that surface.
The higher your temperature, more radiation, more energy is emitted and,
that energy is what is going to heat the air.
This air is heated and as a result
its density decreases, and therefore it rises up.
So the temperature will give us
indication of a tendency for the ascent of the air
near the ground because of its temperature.
1:25
This can be studied with indexes, this can be seen with radiosonde,
and has to do with how the temperature is structured high up.
Basically it is going to give us situations associated with storms,
situations of instability.
To look at this more easily we would typically look at index maps,
like the K index,
and there is also the Lifted Index,
or the total index called TT.
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You will see that each one has assigned the possibility of storms
or intense storms, and basically what they indicate is
how the atmosphere is structured in height, and its temperature.
What is this going to become?
In storms! And how are we going to face the storms?
Let's try to see what a storm is like in itself.
Note that if I see a storm, typically
you know that they have this form so, this is called thunderhead,
and it is a very clearly defined vertical structure, that if we are at sea
we see it from far away.
If here we have the surface,
weather the sea or the earth, why does a storm form?
It is formed because there is a significant rise of air,
so we will have an area of ascent and here is basically going to be connected
3:11
Notice that I have put it this way because the air,
we have mentioned many times veering and backing for speed,
but air also has a tendency to rotate high up,
because the surface wind and the wind high up do not have the same direction.
When air enters the surface, it is wet and warm,
enters as a storm and has a tendency to rotate on itself,
it is a like a tape that is rolled onto itself.
When there is precipitation, precipitation is nothing more than something that goes down.
Something that descends into the cloud, therefore within
the cloud we will also find a zone of descent of the air.
And this descent is also a like a tape
rotating on itself.
Notice that the air enters, turns, rises and there is a moment when it descends,
It turns again and goes back out at a ground level.
So you notice that the storms are going to have an inflow and an outflow,
A zone that enters and an area that leaves.
Depending on how these two "belts" rotate, it may happen that the rotating part
of the wave that ascends and the one that descends intertwine with each other.
This could generate, within the storm,
a cycle that unites the descent and the ascent.
If this occurs, this would be a kind of rotor inside
that can generate hailstones because
imagine a rising particle, on a certain level in this storm
hits a zero degrees Celsius.
From there it begins to freeze, freezes, falls with the outflow,
but for some reason, because they crossover, it rises again.
This particle is going to refreeze more, it will increase its volume of ice
and it will come back down, and it can be circling until the
the one that rises can not raise it again and it will go down.
This would be the stone cycle,
when these two belts are intertwined between themselves.
Regarding the wind, notice that there is a part,
typically in the outflow area, where there is a descent
and here is where the precipitation is going to be.
Perhaps one of the clouds that we who sail have to look for the most
would be so called shelf-cloud.
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It is a kind of cloud that has a flat shape from the bottom
of the storm and most of the times indicates that,
below this cloud, the air is falling.
If you see a storm cloud, and next to it
a cloud that looks like an octopus leg, be careful.
Because that can be an area of strong windfall, and intense winds.
This happened to us once while sailing,
we went downwind, and all the spinnaker went flying,
torn off, literally, only the bolt ropes were left.
Because it falls, this is documented,
and it's called downburst.
It is a very marked descent of the air
which produces a shock wave, here there would be an intake and when it reaches
the ground, it dispersed like a fan, falls and disperses.
And this is a strong wind zone, it is curious because most of the times
you would see this form of octopus leg and here it would be rounder.
This would be a round cloud, this would be just as rounded,
because this wind will mark a rounder shape.
When you see these clouds, if you see a storm cloud on one side or
from your front you see that there is a kind of shelf,
or it goes to a side a kind of octopus, this would be a shelf cloud, watch out.
Well, if you want a lot of wind, get in there, but it can probably be an
area of strong descending winds, and that cause some difficulty.
Tomàs Molina
