| Is usually most evident in the circulation and temperature fields of the middle troposphere and may show little or no sign on a surface chart |
| Occurs frequently in winter to the south of the Alps when this region is under the influence of cold north-westerly airstream |
| Normally disappears at night and occurs almost exclusively in summer |
| Develops usually in winter when very unstable maritime polar or maritime arctic air currents stream southwards along the eastern side of an extensive ridge of high pressure, in association with occluded systems |
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