AND one of the phenomena most feared by aviators, precisely because of its unpredictable violence. Yesterday evening on Lake Maggiore there was a “downburst”: an “explosion” of air that descends at full speed from the cloud of a storm. “Technically it’s a descending gust that comes from a thundercloud. When it hits the ground, it spreads with strong winds in all directions,” he explains. Guido Guidi, Air Force lieutenant colonel, meteorologist. “When it has an extension of 3-4 kilometers, as it probably happened on Lake Maggiore, it is called a microburst”.
We have no data on the speed reached by the wind, but the weather station at Malpensa airport, which is about fifty kilometers away, at 7.20 in the evening – the peak time of the downburst on Lake Maggiore – indicates a strong storm with hail. “The visibility, compared to the previous bulletin, had dropped from more than 10 kilometers to 1.2. It means that a wall of water was coming down” says Guidi.
“A downburst is a fairly common phenomenon when there is a severe thunderstorm. It occurs when very cold air at the top of a cumulonimbus cloud plummets downwards. As soon as it touches the surface of the air or water, it with all its weight and density, it starts radiating in every direction with gusts of up to a hundred kilometers an hour,” he explains Julius Bettimeteorologist and climatologist of the Cnr, of the Lamma consortium of Florence and of Ampro, (Professional Meteorologists Association).
Seeing a downburst isn’t difficult: it appears as a sort of dark cylinder descending from the cloud towards the ground. “These days, with 15-20 thunderstorms a day, we count several,” explains Guidi. “But they hardly have the disastrous consequences we’ve seen on Lake Maggiore.” The overturned boat probably found itself in the wrong place at the wrong time, during a particularly strong gust and located right on the stretch of water.
“We know how to predict severe thunderstorms. But we can’t say exactly whether there will also be a violent downburst,” says Betti. “The factors that favor this phenomenon are the very developed vertical clouds. The downburst occurs right in the center of the cloud and is different from the wind we perceive when the storm is advancing towards us”. The descent of the air is generally accelerated by rain or hail: the drops or grains drag the very cold air of the upper parts of the cloud downwards.
For those who fly, these currents are a bete noire. “That’s why we issue a warning whenever a thunderstorm forms within 5 kilometers of an airport. Landings and takeoffs during a downburst should be avoided in any way,” explains the Air Force expert.
Downburst is a different phenomenon than tornadoes, where the wind moves in a circular fashion. Both have the ability to destroy anything in their path, but the whirlwind comes with a cone that descends from the cloud and spins in the air. There are no indications that one has formed on Lake Maggiore. “The conditions of these days can still favor both downbursts and tornadoes. There is a lot of instability” according to Guidi.
All the conditions for the “perfect downburst” were on the table yesterday evening: “At the end of May, the sun is already high,” explains Guidi. “The temperatures reach 25-26 degrees. The heat causes the air to rise at great speed up to very high altitudes. Yesterday’s cloud over Lake Maggiore reached a height of 12 kilometers and even exceeded the tropopause, which does not happen frequent. The satellites clearly showed it precisely at 7:20 in the evening. The higher the ascending currents arrive, the faster they fall back down. With the unstable atmosphere of these days it is easy for the currents to reach great heights and great accelerations”.