A grain in arc is a type of stormy line, or line of grain, having the form characteristic of an arc of Cercle as seen by a weather Radar. Dr. Tetsuya Theodore Fujita of the university of Chicago is the first to use the " term; Bow Echo" to indicate this type of stormy line. In a book published in 1978, the “Manual off Downburst Identification for Project NIMROD”, it describes the characteristics and the evolution of these systems. The face of gusts generated by this type of convectif Système of méso-scale can cause important occasionally damage and to be accompanied by Tornade S. the grains in arc can develop in front of a convectif Complexe of méso-scale, an ordinary line of grain or during the dissipation of a super-cellular Orage.
Formation
A grain in arc is resulted it from the spreading out of a cold Goutte which is formed in front of a storm or of a line of storms when the air of the mean levels and precipitation go down from there. When the shearing of the winds of is moderated at fort in the low levels of the atmosphere and that the direction of this change is linear, the drop is spread out in arc. Rising on the front of the drop causes the reformation of storms which will be aligned in arc. The stormy grain thus generated will be a few kilometers thickness and 20 to 200 km length, in general less long than a line of rectilinear grain. Its lifespan will be from 3 to 6 a.m. and in general will cause major damage along its way because the Courant-jet of the mean levels which goes down along the face from gusts finds concentrated. A grain in arc can be transformed into Derecho if the conditions are favorable. In 2004, a study on a great number of grains in arc coming from slightly organized stormy cells and super-cells showed that the first situation was the best with their formation. Whereas a tornado is an event of very small extents, the grain in arc can cover very long distances, cause winds of 160 km/h or give a very vast surface of damage more and thus.
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