Geoscience Reference
In-Depth Information
180°
150°
150°
120°
120°
90°
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60°
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30°W
30°E
0°
Figure 6.17
The forecast pattern of upper westerlies at 500 hPa on 30 July 2000, showing the height of the 500 hPa pressure
level above a fixed datum near sea level. Winds blow parallel to the contours at a speed proportional to the gradient.
Source:
http://www.ecmwf.int
particular location southerly flows may last for a few days,
to be followed by more northerly winds as the wave
progresses eastwards.
In order to see these waves it is necessary to use a rather
different technique of presenting atmospheric circulation.
Instead of mapping the actual wind directions or speeds,
the height at which a particular pressure surface is reached
can be plotted. This may seem a strange way of depicting
winds, but, as we know, the geostrophic winds blow
parallel to the isobars, at a speed inversely proportional
to the distance between the isobars. Similarly the winds
blow parallel to the contours of the pressure surface.
Where the contours are close together, the winds are
rapid. Irregularities in the pressure surface indicate local
patterns of wind movement.
Figure 6.19
shows a mean pressure surface (500 hPa)
map for December-February. The projection of the map
may make it difficult to appreciate the direction of flow
immediately. What is clear is that the flow is not perfectly
circular around the north pole. Areas occur, even on this
