The winds are expected to be strong this Saturday morning. Météo France is planning a yellow vigilance in the Somme and the Oise. SNCF announces the suspension of part of its traffic.
Strong gale expected in the second part of the night from Friday to Saturday and Saturday during the day, linked to the depression “DARRAGH” circulating over England and the North Sea.
Winds up to 120 km/h
The south-westerly winds, quite strong in the second part of the night from Friday to Saturday, freshen from the south-west to the west strong on Saturday morning, and will remain very strong at least until the end of the day on Saturday.
During the morning, wind gusts of around 100 to 120 km/h are expected on the extensive coastline of Hauts-de-France. In the interior of the region, gusts should reach 80 to 90 km/h, locally 100 km/h.
A change in the intensity of these gusts is still possible in the coming hours and a worsening of vigilance cannot be ruled out.
Trains will not run due to wind
The violent winds from the Darragh depression are now disrupting train traffic in the region.
“In order to guarantee the safety of travelers and staff, a total interruption of traffic is planned on the Saint-Pol star and the lines Amiens-Compiègne, Amiens-Abancourt, Beauvais-Le Tréport, Creil-Beauvais, Paris-Beauvais , Crépy-Laon and Amiens-Boulogne on Saturday December 7” announces the SNCF.
What is a weather depression?
In common parlance, an atmospheric disturbance refers to cloudy, rainy and windy weather. But what is it exactly?
Disturbance refers to an interruption in the balance of the atmosphere and is therefore accompanied by turbulent conditions (wind, precipitation).
At temperate latitudes in Western Europe, disturbances are often likened to relatively moist, soft air on the ground that stretches several thousand kilometers and wraps around a low pressure center, an area of low pressure . We can visualize a disturbance as a zone of soft, water-laden air bounded on both sides by cooler, drier air.
How does a disturbance form?
A disturbance follows three stages during its life: birth, maturation and dissipation. These stages depend on the state of the atmosphere in which the disturbance is evolving.
A disturbance most often originates at sea because it draws in both relatively mild and above all humid air because water is an essential component in forming clouds.
In addition, a low pressure center is necessary because it will generate vertical movements between the ground and the atmosphere and thus allow the formation of clouds by condensation of the water contained in the air. The low pressure center is in some ways the essential engine for a disturbance to come to life.
The disturbances are better structured during winter while in summer, they more often take on a stormy and diffuse character.
The different parts of a disturbance
The margin
The first warning signs of the arrival of a disturbance are clouds located at high altitude and which thicken over the hours. This is called the margin of a disturbance. A few showers may already occur.
The warm front
The real rains occur when the warm front passes, an initial delineation between the mild/warm air contained in the heart of the disturbance and the cooler air ahead. In addition to rain, strong gusts of wind are possible.
The hot sector
The heart of the disturbance is called the hot sector. It is a more or less large homogeneous area where the sky is mainly gray but the rains are generally light.
The cold front
The warm sector is followed by the cold front. Like the warm front, the cold front can be accompanied by intense but brief rain and strong gusts of wind. Also, it demarcates two very distinct air masses, on one side the disturbed body and on the other the trail.
L’occlusion
The occlusion corresponds to the part of the disturbance which wraps around the low pressure node. At this location, the warm air on the ground is propelled higher. The occlusion is in a way the extension of the meeting between the warm front and the cold front (regular rain sometimes punctuated by thunderclaps, heavy skies, windy weather).
In textbook cases, a disturbance seen from the sky looks like a large cloud scarf in the shape of the Greek letter lambda.
The role of a disturbance
The Earth is not heated uniformly. The sun's rays responsible for this warming are concentrated on the equatorial belt of the globe and oscillate during the year between the Northern and Southern hemisphere. Thus, it is on average warmer towards the central regions than towards the pole regions.
Nature always seeks to balance things and this is where disturbances come into play. They are the direct consequence of this thermal contrast and their aim is to constantly restore temperature differences on the planet.
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