Horizontal wavelike oscillations in the pattern of wind flow aloft, usually with reference to the stronger portion of the westerly current in mid-latitudes. The flow is anticyclonically curved in the vicinity of a ridge line in the wave pattern, and is cyclonically curved in the vicinity of a trough line.
Any given hemispheric upper flow pattern may be represented by the superposition of sinusoidal waves of various lengths in the general westerly flow. Analysis of a typical pattern discloses the presence of prominent long waves, of which there are usually three or four around the hemisphere, and of distinctly evident short waves, of about half the length of the long waves.
Typically, each short-wave trough and ridge is associated with a particular cyclone and anticyclone, respectively, in the lower troposphere. The development and intensification of one of these circulations depends in a specific instance upon the details of this association, such as the relative positions and intensities of the upper trough and the low-level cyclone. These circulations produce the rapid day-to-day weather changes which are characteristic of the climate of the mid-latitudes.
The long waves aloft do not generally correspond to a single feature of the circulation pattern at low levels. They are relatively stable, slowly moving features which tend to guide the more rapid motion of the individual short waves and of their concomitant low-level cyclones and anticyclones. Thus, by virtue of their position and amplitude the long waves can exert an indirect influence on the character of the weather over a given region for a period of the order of weeks.
A blocking situation is one in which waves do not progress through the latitude belt of the westerlies. Blocking situations are frequently accompanied by extreme meteorological events; precipitation and cool temperatures persist near upper-level cyclones, and dry, warm weather persists near upper-level anticyclones. A blocking pattern usually consists of a ridge (anticyclone) over a trough (cyclone), a high-amplitude ridge, or flow shaped like an uppercase Greek omega (Ω). Because of the preference for blocking off the west coasts of Europe and North America, it appears that topography must play an important role in blocking. See also Atmosphere; Jet stream; Storm; Vortex; Wind.
