Fall of 1999 a difficult awakening was waiting France as well as much of rambolitans people. The storm had stroke during the night , leaving an impressive silage on the solid mass of the rambouillet’s forest. Flashback on this exceptional strong gale stigmata.

What is a storm ?

Storms and depressions assures the climatic temperatures stability that we observe in the atmosphere. They are the forms that take in the latitudes, beyond the tropics, the necessary exchanges of heat between the equator, which to simplify, receives too much energy from the sun, and the poles, which lacks them.
Storms are more active on the winter hemisphere because the polar deficit to be filled requests more exchange.Amongst other roles, the depressions assure mostly the essential fresh water resources of the tempered regions.

Where does the devastating strength of the storms come from?

A simple analogy summarizes the essential.
Depressions are a natural and provisional engine which transforms into winds the essence contained in the rail of the depressions.
The idea is identical as for a car.
With a gasoline reservoir posed on wheels, one hardly does not go far.
The energy (chemical) contained in the gasoline only constitutes a potential that will not spontaneously convert into kinetically energy (speed).
The wheels speed and the car one is created by the mixing of the gasoline and the air inside a particular arrangement of mechanical pieces that an engine is made of.
The gasoline alone does not explain the movement of the car, nor even affirming that the strong thermal contrast inside the depression’s rail explains storms but the thermal contrast influences nevertheless the engine’s rendering .
We cannot here get into the operation of the engine’s details.
Let’s put simply that a good engine depends on the sparkle and piston’s synchronisation, the increase of the wind within a depression depends of the synchronisation, inside the rail, of a modest precursor swirl towards an altitude of 9km and another, shifted towards east, near the ground.
It is not then the hot or cold air feeding that makes the depression, it is the depression, source of the winds and of the displacement of air, that makes circulate around itself the air that will amplify it or not.
The simplistic analogy with a car is useful, but limited.
However the energetic considerations put forward here should never be occulted.


 

26 December 1999, a strong gale that will mark the spirits.

Few are astonished by the extend of the regions touched by the 1999 storm, which was huge. It is because we are not used to be in the centre of these meteorological depressions (they usually go north-west, we only know the south-east part of them)

Also, we associate the violent winds and the damages caused by them to the storms and to the grains isolated or organised, typical in The summer or beginning of autumn times. In this case the touched regions are fewer. To the meteorologist, on the other hand, this storm was rather smaller.

The characteristic size of a storm is usually 1500 to 2000km (in diameter), where as the characteristic size for France is 1000km. This size (1500 to 2000 km) is important for their very functioning; It plays a role in their trajectory orientation.

A few precisions from Milene Gentils, Rambouillet’s Territorial Unit manager or the last two years.

Gael Cotonnec for Ramboliweb:
Can you tell us what impact the storm has had on the rambouillet’s forest ?

Milène Gentils : L'One of the consequences will have been to revise the forest management plan in an anticipated way.
Of a 25 years length before, it spreads now on a 20 years duration, and will be revised again in 2025.
It is our ‘bible’, a document which frame the forestry management for the years to come.
But in the first place it has been to clear the forest and to sell the woods in the best commercial and financial conditions possible.
In Rambouillet the worst effect was the quantity of wood fallen on the ground which was not always the ones mature enough for commercialisation, the storm did not sort them at all.
A lot of coniferous tree (Sylvester pine mostly) have been touched, and one must know that they need over a century to come to their best dimensions.
But taking the fact that they all had been planted at the same epoch ,there is nearly none left in the forest.

GC : What is the amount of wood that has fallen? Can you give us a scale to get all the measurement of it?

M.G. :
In fact it represents only 4%of the overall surface of the forest solid mass, but the visual impact was so that it could give the impression that it was much more important.
Nevertheless in measurement of the wood exploitation resources the storm has put down in one night the equivalent of 10 years exploitation.
That is what can explain the drop of the wood price, knowing that other European countries have also been touched.
The O.N.F (national forest office) to this day, is slowly recovering from the aftermath of the storm.
Since one year, the financial balance of the O.N.F. is restoring, the oak tree price is getting back to normal and comes to the equivalent market price it was before the storm, which is not the case for the beech tree, and it is becoming a serious problem.
But unfortunately for the oak tree as an example we do not have the volumes to sell that we had before the storm.

G.C. :
How long is it going to take to get back to normal operation?

M.G. :
For the provisioning of the wood channel as an example we harvested between 2004 and 2006 a volume of wood of 47000m3 a year whereas we expect now 30000m3.
Without forgetting that we sell less, we got to invest more to replant, and these are the reasons why we are confronted to those difficulties.

G.C. :
On the ecological concern what has been the impact of the storm?

M.G. :
It has not been the announced catastrophe.
Few species have been threatened, the deer tribes have nearly not been touched, but it is interesting to note that the clearings, openings and vast wide that the storm has produced have allowed the apparition of new species such as bats for example.
For the other animals, no real changing happened in this forest that contains numerous protected species.
In conclusion, if for the O.N.F. the strike has been financially tough, for the nature and the forest it is far from catastrophic.
but a second equivalent storm should not happen in the next few years, because even if for the forest it would be another hard strike, it would recover much faster than us.

Texte & photos : Gaël Cotonnec

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