Meteosat-6 rapid scan images showing the severe hailstorm in Po Valley in August 2002.
24 May 2022
04 August 2002
Hail, wherever it occurs, is a much cursed and unwelcome form of precipitation causing much damage to crops and sometimes to livestock, property and people. With a few exceptions, it is a phenomenon of middle latitudes and occurs over a wide belt.
Hail is the most variable of the hydrometeors in its distribution and recurrence, and for most people a hailstorm is a singular and memorable event. For example, for people living in Bavaria, the famous Munich hailstorm of 12 July 1984 will be remembered for a lifetime. The enormous impact of the hailstones from this major convective storm, combined with very strong winds and rain, produced losses on an unprecedented scale.
In some European areas hail is altogether too regular in frequency and becomes a determining factor in the success or failure of many farming operations. Areas of high occurrence of hail damage tend to be situated near mountain ranges: e.g. southern France, northern Italy, Switzerland, southern Germany, Austria, Hungary and the Caucasus region.
Of all the European countries where hail poses a significant problem for agriculture, Italy, and more specifically, the Po Valley of northern Italy, suffers the greatest losses.
This is because of the unique geographical situation of the Po Valley. Surrounded on three sides by high mountains, the Po Valley is an area with high humidity and rather light winds at lower atmospheric levels, which provides favourable conditions for the formation of line storms and Mesoscale Convective Systems (MCS).
According to the static stability criteria, the lower troposphere in the Po Valley is, on average, markedly unstable during summer afternoon and evening hours. However, dynamical forcing, induced by synoptic or mesoscale disturbances, plays an even more important role than purely thermodynamic causes in the triggering and maintenance of thunderstorm activity in northern Italy. In this region thunderstorm activity is more determined by whether or not some upward forcing will lift parcels of air up to the Level of Free Convection (LFC), rather than by whether or not the vertical profile is unstable (which it almost constantly is).
Low-level convergence of local winds can play a key role in carrying moist surface air to the LFC or higher. However, this lifting may not necessarily start from close to the surface, but may initiate above the boundary layer leading to elevated thunderstorms. This is not a particularly unusual occurrence in northern Italy since the frequency of thunderstorms often has a secondary maximum during the night.
The night-time hailstorm of Sunday, 4 August 2002, is a good example for a synoptic disturbance combined with the polar jet stream triggering the formation of deep convective storms in northern Italy. Following a short period of high-pressure with a corresponding heat wave over Central Europe at the end of July 2002, a cut-off low moved slowly from Brittany,
France to the eastern part of the English Channel during the first few days of August (see Meteosat-6 visible images below). The associated occlusion moved slowly from eastern France to eastern Germany triggering the formation of severe convection over Germany. On 1 August 2002 the city of Hamburg was hit by severe storms that resulted in more than 60mm of precipitation in 45 minutes. Large parts of the city were flooded within a few minutes.
On 3 August the occlusion took the form of a huge spiral over western Europe, and its south-eastern part was approaching the French Alps. At the same time, the polar jet running along the south-eastern side of the cut-off low (well visible in the WV loop) moved across the Alps into northern Italy.
Around this time, a second jet stream, coming from northern Africa, was also directed towards northern Italy. Surface conditions in the Po Valley were quite typical for the time of the year with weak south-easterly surface winds and high equivalent potential temperatures (there were dew point temperatures of up to 24°C in the Milan area).
At around 20:30 UTC a first convective storm developed in the Milan area and moved in an easterly direction along the slope of the Alps. A second, more severe, storm developed two hours later in the eastern part of Piedmont, following the movement of the first storm. This storm reached the area of Brescia between 00:30 and 01:00 UTC causing severe damage to property, mainly through hail and wind gusts.
In the following hours, the storm continued all the way along the Alps with maintaining its severity and leaving a trail of destruction in its path. Severe damage to trees, crops and buildings was reported from the provinces of Verona, Vicenza, Padova, Treviso, Venezia, Pordenone and Udine.
According to newspaper reports, the area most badly hit was around Lake of Garda, where a camping area was destroyed and people sleeping in tents were seriously injured by large hailstones falling at speeds of up to 100km/h.
Meteosat-6 visible images
Meteosat-6 IR and WV images
The images below show the development and movement of the hailstorm over northern Italy in the infrared and water vapour channels taken by Meteosat-6.
Consequences of the hailstorm
Meteorological facts and enviromental impacts
- Hailstones up to 700g in weight.
- Wind gusts of 36.7m/s.
- Precipitation amounting to 37.8mm in 20 minutes.
- More than 30 persons injured by large hailstones and falling objects. One person seriously injured by a tree falling onto his car.
- Up to 20% of fruit (peaches, kiwis, pears, apples) and olive harvest destroyed.
- About 300 MEUR damage to crops and buildings.
- Particularly hit were the vineyards in the Tri-Veneto area and in the area of Lake Garda. Many cars, campers and RVs damaged.
- The football stadium in Vicenza was damaged and electricity supplies interrupted.
- Thousands of fallen or damaged trees and consequential blockage of streets and railway lines.
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