The hail is a type of Précipitation which results sometimes from Orage S particularly strong. It takes the shape of balls of ice ( let us hail ) whose diameter can vary few millimetres with ten centimetres. The code METAR of hail is GR.

Origin

Let us hail grow when the drops of Pluie contained in a storm are raised by the ascending current forts which characterize the Orage S. These drops can freeze whereas they are subjected to the Température S under the freezing point but also to remain in Surfusion until -39°C, according to whether they contain or not a core of condensation.

As soon as a drop freezes, especially in the higher levels of the Troposphère where the temperature is lower than -10°C, it becomes such a core. The Steam pressure of Saturation of the ice being less than that of water at temperatures under the freezing point, let us hail them grow more quickly than the drops in a wet atmosphere like that of the storm. Moreover, they cannibalisent the steam of the superfused drops in their entourage thanks to this difference. The whole allows hail to grow quickly in the areas of the cloud with strong liquid contents. Growth rate is particularly important around -13°C.

A transverse section of large let us hail watch which they have a structure in onion skin, that is to say thick and translucent layers of growth alternate with thin, white and opaque layers. The theory wanted before that hail them were prone to several return tickets, falling down in the wetland then freezing again in a new ascending phase, which would have generated the successive layers. However, theoretical research and on the ground showed that it was not the case.

In fact, the grêlon in rise crosses zones of the cloud where the concentration of moisture and droplets in Surfusion varies. Its growth rate changes according to the variations met. The rate of Accrétion of the droplets is another growth factor. These last agglomerate by contact with the grêlon. Thus when the grêlon passes in a zone rich in droplets, it will acquire a translucent layer by capturing them, whereas in the areas of the storm where it is especially of the steam which is available, it will be formed a layer of white white frost opaque.

Finally, the grêlon is driven vertically at a variable speed which depends on its position in the ascending current as well as its weight. It is what will vary the thickness of the layers because the rate of capture of the superfused droplets (accretion) depends on relative speeds between those and the grêlon, certain speeds of rise supporting it. The growth of let us hail brings the relaxation of Latent heat what can keep the outside of the grêlon liquid, returning it more " collant". Let us hail can then agglomerate to two or several, according to the collisions, to form some larger, with the strange forms.

The thus high grêlon until its weight cannot be supported by the ascending current any more, which takes at least about thirty minutes if one judges of it the force of these currents in a storm with hail whose top is generally with more than 10 km height. Then it starts to go down again towards the ground while continuing its growth by the same processes. This single way in the storm is thus sufficient to explain the configuration in layers of hail. The only case where one can speak about multiple ways is that of the multicellular storms where a grêlon can be ejected top of the cell-mother and to be included in the ascending current of a more intense cell-girl but it is an exceptional case.

The maximum size of let us hail in the cloud is not that which one finds on the ground. Indeed, once it leaves the cloud, the grêlon starts to be sublimated because the air is not there any more with saturation. When it passes in the layer where the temperature exceeds the point of Congélation, it starts to melt and to evaporate. What one finds on the ground is thus what could not change and depends on the height of the level of congelation.

The budding Cumuli (cloud of downpours), with an ascending current much weaker and a less cold top, can give very small hail (less than 5 mm) by a similar process. This small hail is sometimes named Grésil.

Damage

Hail is a destroying phenomenon for harvests and the property. Largest let us hail are also dangerous for the people and the animals.

Mitigation

One uses sometimes the Ensemencement of the clouds to try to minimize the damage caused by hail. By increasing the number of cores of Congélation one hopes to increase the number of hail at the expense of their size. The money iodide is generally used for that.

A belief of the 19th century wanted that the noise of the guns to the battle field can influence the place and the release of hail. This belief is not founded on scientific bases. That does not stop salesmen of " gun with hail " to make excellent bargains with this myth.

Records

  • largest grêlon listed in the United States measured 44,5 cm of circumference or 7 cm of ray (size of a watermelon!) and 750 G weighed.

  • on July 11th, 1984, a series of storms violent ones, sometimes accompanied by tornadoes and especially by hail, bursts of Burgundy in the North-East. In Dijon, let us hail 5 cm in diameter fall. Locally, largest let us hail reach 7 cm in diameter. The following day, it is with the turn of Munich in Germany to find itself under hail.

  • on August 11th, 1958, a storm of hail devastates the area of Strasbourg. Largest grêlon reaches 972 G.

  • In 1959, in Kazakhstan, one weighs a grêlon… 1,9 kg!! (all these records were approved by OMM).

  • Not approved by OMM: 2 kg in Bangladesh on April 14th, 1986. 92 people are killed. In 1829 in Cordoue in Spain, still a grêlon of 2 kg.

References

  • has Shorts Race in CLOUD PHYSICS , by Rogers and Yau. Published by Butterworth-Heinemann, Massachusetts, 1989 ISBN 0-7506-3215-1

See too

Article related

External bonds

  • hail by MétéoFrance
  • Principal storms of hail in Canada

Beats-smg: Kroša Simple: Hail

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