Convergence and divergence occurs when masses of air have a tendency to come together or separate. They cause air to rise or sink.

On this page you will find:

1. A video with Tomàs Molina in which he explains the phenomena of convergence and divergence.
2. A synopsis of the video with the basic concepts in further detail.
3. A case study: The effects of convergence associated with backing on the Northern Spanish coast.

1. Convergence and divergence

2. Basic concepts

Lets' take convergence and divergence as the third weather engine.

Convergence is when the air comes in horizontally from both directions to a specific area.

When air convergence takes place at ground level on Earth, it can only move upwards and so an upward flow of air masses is produced.

Depending on the characteristics of the air that it finds high up, this rising air will produce certain weather phenomena. If it finds conditions favouring a further ascent, cloud may form, or even storms.

Divergence happens when the air, for whatever reason, moves horizontally outwards from a position. When divergence happens at surface level on Earth, a descent of masses of air above the area occurs. This descent of air is called subsidence. It is the opposite to convergence. When upper level divergence occurs, the air below the divergence zone tends to move upwards, sucked by the relative vacuum produced. So, upper level divergence generates an upward flow of air due to suction.

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© Cris Barredo/Latitud42/FNOB

**3. Case study: Convergence caused by backing along the northern Spanish coast and the Spanish coast of Catalonia**

Westerly winds on the northern Spanish coast

This video also includes a case study on the convergence occurring along the coast of northern Spain, when the winds blow from the west, which is to say parallel to the coast, which is to the right of the wind. This situation is built on the concept of the backing of wind due to the effects of orography, as studied in Module 1.2.1.

In this particular case the diversion of the flow of current due to the effects of backing (shifting to the left, due to a loss of speed) generates this wind convergence phenomenon, which further out to sea does not experience this deflection.

This convergence zone usually produces a zone of showers opposite the coast, as the westerly winds are more moist and the height of mountain range along this part of the northern Spanish coast accentuates the loss of wind speed and therefore also backing.

Northeasterly winds (gregal) on the Catalan coast

In the lower photograph, the same wind convergence effect can be seen, with winds blowing in from the NE, locally known as 'gregal' winds, parallel to the Catalan coast. In this situation the air is a lot less moist than the winds on the northern Spanish coast and the mountains on the coast are not as high, so the chain of clouds which forms isn't as significant and doesn't tend to cause precipitation.

Photo caption: A line of convergence clouds parallel to the Catalan coast due to NE winds. © Cris Barredo / Latitud 42