Storm

Mechanism of formation

As in the case of the Downpour S, the storms are formed in a mass of unstable air when there is an important reserve of heat and moisture on the level of the ground and of drier air and cold in altitude. A piece of air hotter than the environment enters in Convection. As long as it is not saturated, its temperature changes according to the rate dry adiabatic. Starting from saturation, the steam contained in the piece of air condenses according to the laws of the Thermodynamique what slackens Latent heat and its change of temperature with the pressure is then that one calls the wet rate pseudo-adiabat. Upward acceleration continues until the piece arrives at a level where its temperature equalizes that of the surrounding air. Then it starts to decelerate and the top of the cloud is reached when the particle reached a null speed.

The Potential Énergie of Convection Available (EPCD) for this type of clouds is larger than for a downpour and makes it possible to develop tops of clouds which will reach larger Altitude. This is important because the drops which rise in the ascending Courant lose electrons by collision as in an accelerating of Van de Graff. More an high summit makes it possible to reach a temperature lower than -20 °C necessary to give a great number of crystals of ice. The latter are better producers and conveyers of load what allows a sufficient potential difference between the base and the top of the cloud to exceed the threshold of Claquage of the air and to give Foudre.

The potential instability of the air is not the only criterion, one generally needs a release. For example, the passage of a cold face or the Diurnal warming . Such a release can act on the surface or as altitude with the result that the storms can develop close it ground or be based on the average levels of the atmosphere. The storms can thus occur in any season provided that the conditions are met. Except the equatorial areas, the most active period goes from the end of spring at the beginning of the autumn because it is at this time that the atmosphere is hottest and wet.

Classification of the storms

One classifies the storms in several categories according to the Potential Énergie of Convection Available (EPCD) and the Cisaillement of the wind with altitude:

unicellular Storms
multicellular Storms
Storms supercellulaires
Lines of grain
convectif Derecho
Complex of Méso-scale

Ordinary and pulsatory storm

The unicellular storm is characterized by a weak energy (EPCD from 500 to 1000 J/Kg) with little or not of change of the winds with altitude. Thus the life cycle from approximately 30 to 60 minutes of these storms is characterized by an ascending more or less extremely and vertical current. At the beginning, we are in the presence of Cumulus mediocris who amalgamate between them. They are transformed then into Cumulus budding S (or cumulus congestus) with beginning of precipitations in their center. When crystals of ice are formed at the top of the cloud, these congestus becomes by definition of the Cumulonimbus calvus. The first electric phenomena appear then which characterize the storms.

At the mature stage, an anvil is formed at the top of the cloud which takes then the name of Cumulonimbus capillatus. This anvil is caused by the spreading out of the cloud following the inversion of temperature to the Tropopause and the presence of strong winds at this altitude. However, the heart of precipitations in the cloud, which is at a great altitude, starts to be too heavy so that the ascending current can support it. The rain interfered small let us hail then starts to go down again towards the ground, which will cause soon dissipation.

Indeed, this precipitation goes down in the ascending current and evaporates partially by cooling the air which surrounds it. This last becomes colder then than the environment, and by negative push of Archimedes, accelerates towards the ground. Gradually the current going down intensifies and supplants the ascending current. After the rain, the unicellular storm is dissipated quickly creating a fresher zone around him.

This type of storms is more frequent . It can be associated with strong a Averse and gusts with wind. The rains are almost never torrential and the falls the large one let us hail are extremely rare. In the arid areas of the sphere, evaporation can be such as the rain does not reach the ground and forms Virga under the Cumulonimbus.

Multicellular storms

When the force and the direction of the winds increase with the linear altitude of way, the ascending current of convection is not any more with the same position but the current going down with precipitation. This produced a face of gust which moves away in arc from the heart from precipitations and pushes back the zone of rise. An overhang of precipitation is thus formed generally in the south-western quadrant of the cell mother in the northern hemisphere whereas the dominant winds of surface come from this direction. As the face of gust is dissociated with time from the initial cell by forming cells girls, the multicellular form thus a line of storms at various developmental stages.

The structure radar of this type of storm is characterized by overhangs on the south-western part of a line of strong echoes and these overhangs seem to move in this direction whereas the line moves to 30 degree and 70% the speed of the winds in the layer where the storms occur.

In general, the EPCD is average in this type of storm, that is to say between 800 and 1.500 J/Kg. According to energy and moisture available, this type of storm can give violent gusts of winds, pouring rain and very seldom of the tornadoes.

Storms supercellulaires

When the shearing of the winds turns with altitude, one can arrive at a situation where one has a reinforcement of the vertical movement under the ascending current and a synchronization between the face of downward gusts and the ascending current. Moreover, if the supplied convective potential energy goes up above 1.500 J/kg, the ascending current will allow a very broad vertical extension (until more than 15 km).

This gives stormy cells independent in steady balance between the entry and the exit of the currents which enable them to live very a long time. They can produce large hail, destroying winds and torrential rains. Moreover, if a horizontal shearing of the wind on the surface is transformed into vertical swirl by the ascending current, these supercellules can produce Tornade S if rotation is accentuated by the downward current.

On the image of right-hand side, one sees a representation of such a cumulominbus which includes/understands:

a Enclume is formed with the Tropopause which is a barrier with the vertical development of the cloud. It extends far from the thorough original cell by very strong horizontal winds.
a top in dome which overhangs the anvil where the ascending current finds and indicates that it is enough strong to overcome the inversion of temperature to the Tropopause.
Of the Mammatus under the anvil, of the cloudy protuberances formed by the cold air of altitude going down by negative push from Archimedes in the cloud. They are sign of instability.
In the side right back, behind precipitations, a tornado under the cloud-wall (Wall-cloud).

From the radar point of view, one notices a vault without echoes (images opposite on the right), where ascending fort running allows moisture pieces of air in convection to very condense only with high level. This gives a form to low level the shape in hook (left) with the echoes radar and a fort Gradient of Réflectivité close to the hook. From the circulation point of view air, the zones in blue on the figure of left show where the air goes down in this type of cloud giving from the gusts on the ground. It is noticed that the southern side, the current going down enters in interaction with the ascending current (yellow) and it is at this place that the tornadoes can occur.

Types

One notes four types of storms supercellaires, classified according to their intensity of precipitation or their vertical extension:

traditional Supercellule : it is the most typical form of a supercellule described previously.
Minis-supercellule (LT for Low Topped in English):

characterized by a lower height of tropopause and generally a EPCD (Potential Energy Convective Available) more moderate. They occur in general under colder atmospheric conditions as in spring and the autumn. The shearing and the presence of a mésocyclone are on the other hand quite present because the shearing of the winds is then more important. They are also called microphone-supercellules.

Supercellule with weak precipitation (English LP for Low Precipitation):

characteristic of the drier places like the Canadian Meadows and the Large Plains American, they has a very high base above the ground and a great vertical extension but their horizontal dimension is weak. The rate of precipitation seen to the radar, in the cloud and under this one, is relatively low and it is often difficult to see a rotation there. However, it can occur a fall of large hail which generates echoes radar little. The column of rain is separated from the zone in rotation and that of hail. These stormy cells can give all the elements violent ones mentioned before but more probale is hail;

Supercellule with strong precipitation (HP for Hight Precipitation in English):

they are formed in an environment rich in moisture. They are wider horizontally, their base is generally darkened by the rain and one often does not distinguish the zones from rain, hail and of rotation. They give especially torrential rains, downward gusts and weak tornadoes with moderate, but are very dangerous because the tornadoes in a supercellule HP are drowned in the heart of precipitations, which makes the tornado almost invisible. Hail is less probable there.

Lines of grain and Derecho

Complex convectif of mésoéchelle

See also: Complex convectif of méso-scale

Complex stormy being generally formed in end-of-day starting from dispersed storms. He reaches his apogee during the night whereas he organizes himself like a broad circular zone. One definite like having them:

* Summit of the clouds having a temperature lower than -32°C and a surface of approximately 150.000 km ².

* Lasted of more than 6 hours.

* Rapport between the North-South and East-West diameters must approach 1.

These systems are frequent in the American plains during the summer. They derive during the night in flow from altitude and give mainly intense precipitations causing of the floods on broad areas. At the end of April at October 1993, the floods which prevailed all along the basin of the river the Mississippi, of the Big lakes to the New-Orleans, were mainly caused by CCM with repetition during several weeks at the beginning of the summer (Inondation of American Midwest of 1993)

Effects

The storms are potentially dangerous because they are the place of important vertical movements, the lightning, strong winds and precipitations of various types. Their appearance is very fast and can take by surprised the animals and the human ones.

The lightning

Rain

The quantity of rain under a storm is variable according to its type but occurs always quickly. However, the Relief of the area where it tomb can influence the effect of this one. In the mountainous areas, the streaming on the slopes can bring floods in the valley by concentrating the quantities received towards a restricted area. The Deforestation and the saturation of the grounds will accentuate the effects of a rain under a storm. The rain can cause a liquifaction of the ground under certain conditions what will give flow mud.

In aviation, there exist examples of crushings under storms, of which that of the Vol 358 Air France with Toronto (Canada) in August 2005, or the rain seems to have also led to Aquaplanage what made him miss braking and leave track.

Wind and tornado

Hail

Hail is formed under certain storm and can destroy the cultures, damage the vehicles and the houses like harming circulation. The planes, sailplanes and airships are very likely to incur damage when they pass near these clouds. Indeed, not only they will be struck in the cloud but also to certain distance this one by the ejection of hail. Moreover, the latter will be often larger than those found on the ground since the aircraft fly on a level of temperature or the cast iron did not have time yet to reduce hail them.