Grain (meteorology)

A grain is a weather event during which the speed of the Vent increases in a way abrupt and marked with a Net change of management (45 with 90°), and which lasts only a few minutes. This phenomenon, particularly dreaded by the sailing ships, is frequently accompanied by Averse S of rain, of snow or of Orage S. According to the definition of the World Meteorological Organization, the gusts of wind must be at least 15 nodes (28 km/h) higher than the mean velocity of the wind for one ten minutes period.

The grains can be caused by the descent of air of the mean levels at the time of the passage of a cold face, a hollow or clouds convectifs. They can be isolated under a storm or extend along a line.

Synoptic grain

Jacob Bjerknes and the school of meteorology of Bergen, which developed the frontal model , noted that the passage of a barometric Creux could be marked by a sudden change of the direction of the winds and strong gusts when the change of pressure was very important with before and with the back of this one. This is due to strong a convergence in the hollow which generates a strong movement to the top with the approach of this one and a strong descent of air to the back.

In general, these strong movements were associated with a change Masse of air and one finds a cold Front in the hollow. However, in the hot sector of a depression one can find hollows giving of the similar grains because the air is unstable there.

Stormy grain

The air in a stormy environment is hot and wet in bottom, but dry and cold in altitude. When a piece of air becomes hotter than this environment on a given level, it is thorough upwards. Condensation forms a Cumulus budding or Cumulonimbus in which Précipitation develops.

Possibly the heart of rain becomes too heavy to be constant by the ascending Courant which creates the cloud. It is then put to go down. One sees on the image the life cycle of a storm and the arrows show the direction of the movement of the air. In an ordinary storm, that gives a face of more or less strong gusts. However, in a storm where precipitation is very intense and instability (Potential Énergie of Convection Available) important, the downward gust becomes extreme.

It is to some extent about a sudden collapse of the roadbases of the atmosphere, true a Avalanche of Air which involves violent atmospheric turbulences, force of powerful a hurricane, and which lasts of a few seconds to several tens of minutes. It is this folding wind of an extreme violence that one calls downward gust.

These are the downward gusts which cause the grains. One classify them in macro-gusts (more than 2,5 km) or microphone-gusts (less than 2,5 km) according to the width of the corridor of damage.

Black grain

A wet Rafale downward occurs when the air is wet in all the layer, between the ground and the mean levels of the atmosphere, where the descent occurs. Moreover, the air of the high levels, which can enter the storm, is colder than this one. It is thus in negative balance (Poussée of Archimedes) and also will go down to him. The wind thus folded back is accompanied then by rain. It is what is called a black grain .

White grain

Line of grains or grain on line

When isolated storms gather in a line and that this line moves with the moderate wind in the atmosphere, one deals with line of grains whose extreme is the Derecho. In general, these stormy lines develop in front of a cold face or of a very marked barometric hollow and can move more quickly than this one.

Such a grain produces a Front of gusts which is organized on line in front of the convection. It is reinforced by the Subsidence Courant-jet of the mean levels which is folded back towards the ground. Indeed, the entry of this last in the cloud brings cold air and surrounding dryness to it, which forms a negative balance according to the Poussée of Archimedes.

The horizontal cut through such a line, in the top of the image, thus shows forts Gradient S of reflectivity (rate of precipitations) on before line. On the part of bottom, one sees a horizontal cut where notches behind the line gives a form undulated to this one. These notches are created where the jet drains precipitation while going down. There are generally reformations of storms upstream of the principal line with the downward Rafale. The vertical cut shows that the storms are followed from a continuous and less intense zone associated with precipitations Stratiforme S and the position of the jet of mean level going down towards the ground.