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5 amazing facts you never knew

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In this article we will discuss five common places that could be described as popular frequently encountered in meteorology. The ambition here is to boost information sharing in line with modern knowledge acquired in extratropical meteorology. Each point is explained via a dedicated paragraph. In addition, for each common place, a link to a more in-depth article is given in addition. The bibliographical references which were used in the construction of this article can be found at the end of the articles given in link.

1. Cold air does not force warm air to rise to the level of the foreheads.

How does a large-scale atmospheric front work (front dit synoptic), such as the cold and warm fronts that bring rain and wind so often? The explanation found in most general weather books indicates that cold (heavier) air forces warm (lighter) air to rise in altitude and condense into clouds. This pattern also seems to explain why the cold front is often more active and moves faster than the warm front.

Representation of a meteorological disturbance with warm front (red), cold front (blue) and occlusion (purple). Credits: @SimonLeeWx.

However, the mechanism actually at work is very different from this image of Épinal. Indeed, on the scale of the synoptic fronts, the rotation of the Earth imposes a constraint which does not allow an operation of the type gravity current where cold air jostles hot air on its way. Consequently, this mechanics is observed only at scales where the rotation of the Earth becomes not very influential, in other words, at the level of phenomena of small dimension like the thunderstorm cells. A realistic explanation of how a synoptic front works can be found in the link given below.

Complement : Meteorology: the functioning of an atmospheric front, much more subtle than we think!

2. A high pressure system does not block depressions

Who has never heard or read that, apart from the phenomena of low clouds in winter, an anticyclone guaranteed good weather by blocking or rejecting Atlantic disturbances, thus preventing them from entering the territory? In this respect, the popular conception of the high pressure is akin to that of a protective dome which, depending on its presence or absence, prevents or not depressions from circulating in our regions.

Here too, the reality is very different. Several criticisms can be made of this view of things, but we will content ourselves with mentioning the most important. And it is once again the rotation of the Earth which comes into play. Indeed, by the counterclockwise rotation (cyclonic) of our planet, the process of formation and development of a vortex of opposite direction (anticyclonic, d ‘where the name) is severely limited. Conversely, cyclonic eddies are strongly accentuated, because they have a vorticity of the same sign.

High pressure depression
Reduced pressure at sea level (black isolines) and wind around 1500 meters (shades of colors) predicted by the European model IFS for July 9, 2018. A high pressure area affects the near Atlantic and part of Western Europe. Credits: ECMWF.

Earth’s rotation therefore induces an asymmetry between cyclonic eddies, with high inertia, and anticyclonic eddies, with low inertia. In fact, the latter have little or no ability to influence synoptic flow. In reality, it is the cyclones and their dynamics that have this capacity and the anticyclones ultimately tend to organize themselves according to the configuration of the rail of the depressions, which can for example balance out at high latitudes, leaving us in a high pressure regime. hot and dry.

Complement : Meteorology: understanding how a high pressure system works and its (amazing!) Characteristics

3. A cloud is never blown by the wind

Seen from a distance, the clouds seem to us to be pushed by the winds like large heaps of cotton. When viewed up close, however, the keen observer will find that the clouds are in reality far from being passively transported by atmospheric flow. And for good reason, they are an integral part of the latter. More precisely, they concretize its saturated parts, where the water vapor changes into water droplets or ice crystals. There is therefore no real sense in designating cloud formations as entities undergoing an external wind: the air continuously enters and leaves these regions.

It remains that we perceive well these things which move in the sky and which one calls “clouds”. Like shadows, however, they have no material reality in the sense that they are not clearly defined objects. They simply materialize the spatial dimension of a volume of air which, due to specific thermodynamic conditions, has the particularity of being visible. In the end, the perceived movement is in reality that of conditions and not of entities. He is intangible, like a shadow that persists in escaping us.

Complement : Contrary to appearances, the wind does not push the clouds

4. Coriolis force is more than an optical illusion

In most popular works and even in some high level textbooks, the Coriolis force is described as a simple consequence of mathematical manipulations related to the change of frame of reference or worse, as an optical illusion. However, gravity (in other words, a real force) is involved in the mechanism of the Coriolis effect. It is therefore not a simple effect of perspective, but a real physical phenomenon, a fact recalled and detailed many times by the late Anders Persson, expert on the subject at SMHI, in various articles published in the specialized literature.

In meteorology, the Coriolis effect is due to the rotation of the Earth. Credits: flickr.

It goes without saying that inadequate explanations prevent any detailed understanding of the dynamics of the atmosphere and the ocean. Of all the processes involved, the Coriolis effect is certainly the most irritating from a teaching point of view. At the origin of various misleading and ambiguous interpretations, one probably finds a historical influence, but also the persistence of a bad approach of the concept perpetuated over the generations.

Complement : Understanding the Coriolis force and its role in the dynamics of the atmosphere and the ocean

5. Lows and highs do not move together

Observation of pressure maps at the sea surface reveals the existence of barometric entities moving globally from west to east. These are our lows and highs so familiar with daily weather reports. Also, it is common to assimilate these pressure systems to physical objects for the simple reason that they seem to move as individuals, that is to say in block, a bit like travelers comfortably embarked on a river.

Nevertheless, when one delves into the physics of the phenomena in question, astonishing things appear. Take the case of a depression advancing eastward. The observation of the atmospheric parameters will demonstrate in an undeniable way that the structure evolves in a coherent way in space. Pressure, wind, clouds, etc. will move as a functional unit. Yet the dynamics show that the air parcels are constantly entering and leaving the low pressure area. Near the surface, they circulate less quickly than the depression and cross it from east to west and then escape. At altitude, on the contrary, they travel faster and cross it from west to east.

front depression meteorology
Low-pressure winding in the northwest Pacific on January 9, 2019. We can easily guess the location of the low and that of the cold front. In addition, a trolling regime is clearly identifiable behind the cloud comma. Credits: Wikimedia Commons.

Therefore, at maturity, the vortex may be composed of air parcels totally different from those involved 48 hours previously. In other words, the displacement of the depression is not that of the vortex air which composes it. So the concept of block movement akin to that of a physical object does not hold. The question is therefore to know what really moves and why what moves does so in a coherent way, that is to say by keeping a particularly well preserved structure. Answers to these questions can be found in the additional link given below.

Complement : Lows and highs: more amazing weather systems than you might think


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