Global Atmospheric Circulation Patterns

Variations in air characteristics are wide spread. However, a significant pattern exist on a global scale as air rises and sinks at different laitudes and is also influenced by jet streams and trade winds which force it to shift eastwards or westwards. The basic circulation of the atmosphere consists of a three-cell structure in each hemisphere which is controlled by pressure gradient force. The easiest way to imagine these patterns is by analysing the global circulation model.

A short film is good way to reinforce learning of the global circulation model. The following film link could be used at GCSE and A Level.

http://www.youtube.com/watch?v=DHrapzHPCSA

Alternatively this film clip created by the BBC is available:

http://www.bbc.co.uk/learningzone/clips/global-weather-circulation/4316.html

The BBC have a sizable archive of educational ‘Classic Clips’ accessible on the Learning Zone – the quality of production is obviously superior to many of the youtube films made by amateurs. 

Having observed how film clips are shown within the classroom, a valuable way of ensuring all abilities benefit short clips are often played a number of times. This allows pupils to concentrate on watching it completely the first time, absorbing the information, which they are then encouraged to interpret and query it themselves by answering questions directed by the teacher. Following this the film is replayed and paused intermittently if necessary, to enable each pupil to make notes and draw diagrams.  

Rossby Waves

Superimposed onto these global circulation patterns are flows that are determined by the Earth’s rotation, adding to the complexity of the system. At middle latitudes, high above the closed cyclonic and anticyclonic weather systems, smooth wave-like patterns prevail, known as Rossby waves. Temperature contrasts at the polar front where warm tropical air converges cause pressure gradients to emerge in the upper troposphere, producing strong westerly winds that follow the meandering path of the Rosgy waves. Between 5 and 8 upper Rossby waves circle the poles (more develop in the summer).

Rossby waves play an important role in the formation and evolution of surface weather conditions. Mid-latitudinal frontal depressions (high pressure systems) usually develop downwind of upper troughs, whereas anticyclones (low pressure systems) form downwind of upper ridges of Rossby waves. This is illustrated in the schematic diagram below:

Jet Streams

Embedded within these upper westerly winds are narrow currents of rapidly moving air (150-200 KM per hour) called Jet Streams. This is enhanced by the power of the Coriolis force, mentioned in blog 2. Jet streams are typically thousands of kilometres long, hundreds of kilometres wide and numerous kilometres in depth. The relationship between Jet Streams and smaller scale climate responses will be further explored in the next blog that will focus on monsoons.

This topic features on the AQA A Level syllabus. Weather does not appear in other exam board A Level syllabuses, however, climate change understandably is a compulsory topic for all Geography schemes of work.