Video 1.2.3. The third engine: Convergence and Divergence

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Strategies for winning. Meteorology in a round the world regatta

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Universitat de Barcelona

Strategies for winning. Meteorology in a round the world regatta

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In this course you can learn about the mechanics of global weather, the foundations of ocean meteorology, predictive modeling and how sailors receive data via satellite and use high-performance navigation software. This course looks at oceanic meteorology and climatology through the lens of the sport of sailing. You will gain a basic knowledge of meteorology needed by sailors to take part in a regatta such as the Barcelona World Race, the only double-handed, round the world regatta with no stops. You will learn about the strategies employed during a round the world regatta and how these are put to use on board the latest ocean racing yachts.

À partir de la leçon

Understanding maritime meteorology

In this module you will learn the foundations for an understanding of general and in particular maritime meteorology. What are the factors and engines powering the weather? What do we need to know to understand the weather phenomena we experience every day? What do ocean sailors need to know to predict the weather? Instructors: Tomàs Molina, Santi Serrat

Video 1.2. The four local weather engines3:59

Video 1.2.1 The first engine: Orography8:03

Video 1.2.1. Case Study2:00

Video 1.2.2. The second engine: Convection8:00

Video 1.2.3. The third engine: Convergence and Divergence4:22

Video 1.2.4 The fourth engine: Fronts5:22

Rencontrer les enseignants

Tomàs Molina

[MUSIC]

The third model of weather will be convergence-divergence.

What is convergence?

Well, when the air for some reason goes in both directions in one place.

And what is divergence?

When the air for some reason flows outward from the same position.

This is convergence, this is divergence.

What kind of weather are we going to find with this convergence, divergence?

This is a bit more... On a local scale it's going to cost a bit more to find,

especially the divergence.

The convergence yes, we have seen it in fact, on surface

when we spoke of backing and veering, of the wind which was parallel.

You remember that, in some cases, that the wind tended to come together,

There we can find convergence on the surface.

So, if this is a wind that comes from here, what will happen ?.

If this is the ground, down it will not be able to go.

Because these are the current lines,

and here the wind is coming.

Where will it go?

Downwards it can not, therefore, what is doing is follow the current,

and go upwards. Therefore,

you will notice that the convergence will give us is rising air.

This rising, if we relate it to the convection, as we mentioned before,

this air is going to be forced to go up and

when it goes up, it will depend on how is the air around it.

If the air forces to continue the rise, here what will form is a

storm or there will be cloudiness and there will be precipitation.

Look here, if this air is going to leave here,

If this is being emptied, a relative vacuum will be created here,

therefore, this void will have to be filled in some way.

Typically if there is a divergence high up, what will happen is

literally sucking the air that is down the bottom.

Therefore, the divergences high up, what they generate is,

a rise of air by suction.

Here there is a kind of hole,

and it has to be filled with the air that comes from the parts below.

Therefore, anything that makes the air come together will generate, on the surface,

... it is going to generate rise at that height, and anything that makes the air

diverge will generate rises in the same way at that same height.

Notice I've said it before.

Do you remember when we had the wind parallel to the coast?

We had the strongest wind here, less strong.

Less strong and less strong, and we have said that, because of the backing effect,

Do you remember? This one is still the same, but this one, the left hand,

This from here when the wind is stronger, the more it goes here,

when it is less strong, because it goes to the other side.

So this is still the same, but this one here, as it is less strong,

it goes left, this too and this too.

What's happening here?

We have an area where, due to the backing effect,

the wind is gathering, this is called convergence.

What are we going to find here?

Well, a line of showers.

Very typically the coast of the Cantabrian, when we have winds from the west,

just parallel to the coastline of the Bay of Biscay,

there is a line of showers for two reasons.

First, because here in the Cantabrian, here is an area of mountains,

there is the whole Cantabrian mountain range that emphasizes more the effect of

of speed reduction, parallel to the coast.

And this backing is very important, therefore, in westerly situations,

right in front, on the coast line,

the line of showers produced by the backing is formed,

generated by surface convergence.

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