This phenomenon is most frequently observed where low to thick frontal cloud bands cross a W - E orientated mountain range from W to E. The Alpine mountain range is again a good example of such a situation.

A frontal cloud band approaching such a mountain range from the west is retarded by the mountains but not to the north. Consequently, cloudiness propagates much faster north of the mountain range than within.

Such a phenomenon can be mostly observed in the lower to middle level cloud, with cloud top temperatures (from (-15)°:C to (-20)°C). The loop below shows an example. The effect is most often observed in an area from Switzerland to central Austria leading to cloudiness in areas north of the mountains while within the mountains there may still be fair weather. In E. Austria the tilt of the frontal cloudband becomes more vertical and, consequently, cloudiness north of and within the Alps form one comtinuous line. The different displacement velocities north of and within the Alps are reflected in the Atmospheric Motion Vectors (AMVs) which are computed from the cloud displacement between successive images. In the image below, the fastest displacement vectors lie over Bavaria (north of the Alps) and much slower vectors within and south of the Alps over W. Austria, N. Italy and Switzerland. In the frontal cloud which has not yet reached the Alps (over France) higher speed vectors can be observed again. The situation described here does not take place with every frontal cloud band crossing the Alps from W to E; from 15 cases which were analysed in detail, only 6 cases showed the faster propagation effect north of the Alps.

---

---