Atmospheric Circulation: Jet Stream and Surface Pressure Systems

Thermodynamic Factors in Atmospheric Circulation

The thermodynamic factors are responsible for the succession of atmospheric circulation and air masses, which determines the different types of weather. Atmospheric circulation is governed by the jet stream at height and by surface centers of action, air masses, and fronts.

2.1 Circulation in Height: The Jet Stream

In the temperate zone where Spain is located, atmospheric circulation at height is governed by the jet stream. This is a strong, tubular current that runs from west to east. The jet stream separates the low pressure near the pole from the high pressure located toward the tropics. The jet stream is responsible for surface weather.

The current's velocity varies. When it travels fast, it features a gently undulating surface corresponding to the polar front and its storms. However, when its velocity decreases, it describes deep undulations that cause valleys of low pressure and ridges of high pressure.

These movements determine seasonal changes affecting Spain, principally in winter. In summer, they affect northern latitudes and usually only impact the Cantabrian fringe.

2.2 Surface Circulation

Centers of action are areas of high and low pressure. Atmospheric pressure is the weight of air above a unit area; it is measured in millibars with a barometer and plotted on maps using isobars (lines connecting points of equal pressure).

High and Low Pressure Systems

• A high pressure area, or anticyclone, is an area of high pressure surrounded by lower pressures. It produces stable weather.
• A low pressure area surrounded by high pressure produces unstable weather.

A thermal anticyclone forms when an air mass is cooled; cold air weighs more and descends, exerting high pressure. A thermal low forms when air rises; it weighs less and results in low pressure. Dynamic centers of action form at certain peaks.

Dominant Action Centers

The action centers driving circulation over the peninsula include:

• Anticyclonic centers of action: The Azores High, which moves northward in summer and southward in winter.
• Depressionary centers of action: The Icelandic Low and the Gulf of Genoa depression.

Air masses are large bodies of air with specific characteristics of temperature, humidity, and pressure. Fronts are surfaces separating two air masses of different characteristics.