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As we have already said, it is Very difficult to establish the exact surface position of the front in the Po Valley, but its existence is ascertained by an analysis of the general charts at various altitudes. The air masses preceding the front system are potentially unstable (on the afternoon and evening of the 25th places located at high altitudes had showers and scattered storms). Later, the paSSing of the front through Southern France touched off notable vertical developments and at 12:00 noon on the 26th the western Alps and Southern Germany were hit by showers and storms.
Radio-soundings made by the Milan-Linate station at 12:00 noon on the 26th showed unstable conditions over the Po Valley. Any cause that would raise the surface air would result in condensation at about 3000 ft. by forced up-draft and at about 14000 ft. by convective up-draft, while a further use to about 6000 ft. would end the instability. FTom this point up the rismg particles remain on the peeitive side of the curve di state and the clouds may exist up to about 35,000 ft. The greatest HuT’nidity in the air (85-95%) is found in the strata between 7000 and lhDOO ft. It is therefore likely that above 111,000 ft. the cumulus clouds under development will absorb a considerable quantity of drier air and this fact W111 tend to reduce the additional developments above into less numerous but more extenSive scattered cells. The
small systems probably will be dissolved toward other locations
between 114,000 and 20,000 ft.
The thermic zero level is found at about 11,000 ft., above which there is to be eXpected some ice formation in the clouds.
In the area of the Malpensa airfield, at 00:00 hr. on the 26th, the
wind in the upper air was from the west. As it advanced from the west, it turned to the southwest and struck the chain of the Alps, causing showers and rain, especially in the mountains. Rain became more general and hit the Malpensa zone at 1:00 p.m. Although the situation in the upper air shows that a frontal system was affecting the Po Valley at
the time under examination, establishing the exact location of the front is made oifficult and uncertain by the many storms striking the zone and causing local variations in surface winds and atmospheric pressure. The greatest storm actiVity was noted at Vercelli and Govone, in the neighbor- hood of which might have been located the crest of the front's wave correlated with the formation there of a barely noticeable low-pressure center. Later this center moved toward the northeast, drawing along in that direction, and apparently ahead of the system, the major storm actiVity. Some large storm cells remained active after the front had passed by, such as at Turin and at Malpensa. However, at 6:00 pm.
most of the storm had moved over the eastern Po Valley and its hills.
maids over Northern Italy at 850 MB were lightest over Turn at 12:00 noon, while a small sacca was barely noticeable at 700 MB. This minimum was [then] later reflected by the higher altitudes and from midnight on it was reported at 500 MB. Hence, the wind over the Milan zone, at between 5,000 and 10,000 ft., was from the south with a southwest rotating motion at higher altitudes; the speed was 10 knots, going up to 20 knots at higher altitudes.
