Eventually, the mixing and modification that accompanies the movement of an airmass away from its source causes the rate of energy exchange with its surroundings to diminish, and the various associated weather phenomena tend to dissipate. This process leads to the loss of its original identity until, finally, its features merge with those of surrounding airstreams and the air may come under the influence of a new source region.
Northwest Europe is shown as an area of 'mixed' airmasses in Figures 9.2 and 9.4. This is intended to refer to the variety of sources and directions from which air may invade the region. The same is also true of the Mediterranean Sea in winter, although the area does impart its own particular characteristics to polar and other airmasses that stagnate over it. Such air is termed mediterranean. In winter, it is convectively unstable (see Figure 5.6) as a result of the moisture picked up over the Mediterranean Sea.
The length of time during which an airmass retains its original characteristics depends very much on the extent of the source area and the type of pressure pattern affecting the area. In general, the lower air is changed much more rapidly than that at higher levels, although dynamic modifications aloft are no less significant in terms of weather processes. Modern airmass concepts must therefore be flexible from the point of view of both synoptic and climatological studies.
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