ATMOSPHERIC PHYSICS

Where do the rain and shine come from?

© Météo-France © Météo-France
Atmospheric circulation - Stratosphere - Troposphere (altitude ± 15 km) - HP = High pressure - LP= Low Pressure Atmospheric circulation - Stratosphere - Troposphere (altitude ± 15 km) - HP = High pressure - LP= Low Pressure
Meteo France weather instruments. © Météo-France Meteo France weather instruments. © Météo-France

Today’s TV and Internet weather forecasts are strewn with evocative pictograms. In a split second, viewers can form a precise idea of what the weather will be like tomorrow or the day after. But have they understood why the weather will change like this? Very unlikely. Let’s take a brief look at how our atmosphere functions.

We all know that if we open an outside window in the middle of winter, cold air rushes into the hot air zones. From this essential thermodynamic principle meteorologists are able to understand, explain and predict the present and future weather. Hot air, being lighter than cold, rises, leaving room for cold air which, being more dense, remains at the surface and spreads across the vacant space.

At the planetary level, the entire atmospheric circulation is governed by this basic principle. Large air masses present around the globe, each characterised by a homogenous temperature and humidity level, move and collide with one another owing to their difference of density (and hence of pressure).

The forces in play

Europe’s climate is influenced by five major air masses: arctic, maritime polar, continental polar, maritime tropical and continental tropical. These air masses are in constant movement, directed essentially by two forces: the pressure gradient force and the Coriolis force. The pressure gradient force is the one we have already mentioned, resulting from differences in pressure between two points, pushing air masses to adopt a movement directed from high pressure to low pressure. This force is the starting point for the movement of air masses. Without it, the atmosphere would probably be immobile, without one breath of wind. The Coriolis force, caused by the Earth’s rotational movement, pushes moving fluids rightwards of their initial movement in the northern hemisphere and leftwards in the southern hemisphere. It is this Coriolis force which gives the whirlwind-like aspect to the depressions which we can easily make out on satellite images.

Combining these forces, we get a rough idea of the movement of air masses in the northern hemisphere: turning clockwise around high pressure zones and anticlockwise around low pressure zones.

Two distinct air masses at the same altitude necessarily have different pressure levels. When these come into contact, inevitable movements between them are created by the forces in play. This gives us our weather fronts. We speak of a “cold” front when a mass of cold air moves towards a mass of warm air. Following the principles of thermodynamics, the cold air slips under the hot air, which rises. Conversely, we speak of a “warm” front when a warm air mass moves towards a cold air mass and sits on top of it. In both cases, weather fronts are at the origin of depressions. These frontal depressions are often synonymous with meteorological disturbances in the zone in question.

The journey of an air particle

In a depression, the warm air at the Earth’s surface begins to move upwards, cooling simultaneously under the effect of adiabatic transformation. This unattractively named thermal variation process is due solely to changes in air pressure: just like compressed gas escaping from a cylinder, rising air expands and cools. Inversely, when the air descends again, it is compressed under the effect of atmospheric pressure and heats up again, just like the air when you pump up a tyre. These adiabatic changes, which occur without any exchange of heat with the environment, produce a heat change of around 1°C per 100 metres of altitude.

But air that cools as it rises gradually loses its capacity to store water vapour. While any air particle (1) contains water vapour, the maximum mass of water vapour it can hold varies according to the temperature. Once this mass is reached, the air particle is saturated, and the water vapour condenses into minute droplets, around condensation nuclei – that is solid particles in suspension in the air – to form clouds. For a particle of dry air at 25 °C, the saturation point is 27.4 g of vapour per kilogramme of air. But at 15 °C, the saturation point falls to just 14.8 g. We can therefore understand that, as it cools in its ascending phase within a depression, the capacity of the air to store water vapour reduces, and the excess vapour condenses to form clouds. Sooner or later, following complex microscopic processes, the droplets contained in the clouds reach a certain size and rain is formed.

If we continue to follow the journey of air particles after rising in the centre of the depression, we notice that they expand on rising.

They spread out horizontally and reach the tops of high pressure zones, known also as anticyclones. Here, these air particles which have cooled and densified redescend, and are reheated by adiabatic compression. In this way, close to the Earth’s surface, the air particles are reheated and decrease in weight.Under the pressure of still cold descending air masses, they migrate horizontally towards low pressure zones, and start to ascend again inside the depression.

The weather related to anticyclonic situations is generally dry and fair, as heated air can contain more and more water vapour. Except under extreme conditions, clouds are unable to form.

The incredible fragility of weather forecasts

With this knowledge, acquired over almost a century, meteorologists are able to give more or less detailed weather forecasts. Once the humidity, temperature and pressure parameters of numerous contiguous air particles (2) are known, it is possible to apply the laws of thermodynamics and fluid mechanics, and calculate what the weather will be like at a particular place once the air particles which have been analysed reach this place. Which is precisely what meteorologists and their computers do on a daily basis.

But things are of course not that simple. Because within an air particle, whatever scale we take, parameters are never totally uniform. This presents us with a far from negligible and constantly present source of error, given the physical impossibility of analysing at every moment the parameters of every air molecule, right around the planet.

This difficulty takes a totally different dimension once one realises that the Earth’s atmosphere is chaotic, in the mathematical sense of the term. This means that a tiny variation in the initial conditions of the calculation can produce very considerable variations in the final outcome. Biased analysis of a single air particle can make the whole forecast wrong. It was American meteorologist Edward Lorenz who demonstrated atmospheric chaos in 1963, suggesting that the beating of a butterfly’s wings in Brazil could, by the displacement of air it provokes, produce a tornado in Texas… In other words, and simplifying to the extreme, to forecast the tornado in Texas, one would have needed to observe this wingbeat.

Data gathering, the key to forecasting

All this tells us just how important the stage of observing initial situations is to forecasters. Biased starting information can produce a forecast which is significantly inaccurate for more distant dates. In this respect, meteorology has made constant progress. The tools originally used to measure the state of the atmosphere were simplified versions of those still used today in around 12 000 land weather stations: a thermometer for temperature, a pluviometer for rain, a weather vane and anemometer for wind speed and direction, a barometer for air pressure, a hygrometer for humidity, and a luxmeter for the intensity of the sun's radiation.

To these 12 000 land stations we should add another 800 or so ocean stations, on fixed or floating buoys. But the disadvantage of these stations is that they take measurements at most a few metres above the Earth’s surface. To circumvent this problem, meteo - rologists have invented the balloon sensor, a sort of weather station suspended from a balloon rising into the air and moving with the wind. Fitted with a radio transmitter and a GPS, the station sends out real-time information every 10 seconds both on its environment and its location. The balloon itself, generally inflated with hydrogen, rises at around 5 metres a second, and ending up always bursting, at around 30 000 metres, owing to the difference between internal and external pressure. The sensor has then finished its work and falls to Earth on a little parachute.

The big weak point of forecasting is rain. Even if there is still a long way to go (see pages 14–16), the introduction of meteorological radars has permitted attractive advances. At regular intervals, every 5–10 minutes, they send out electromagnetic waves which, when they encounter precipitation, are reflected with an intensity which varies proportionally to the size and intensity of these precipitation. From this return path the forecaster can visualise the type of rain, its geographical position, and its direction.

A huge step forward for meteorology

It was the advent of satellites in the second half of the 20th century that allowed meteo - rology to take its largest step forward. These satellites are either stationary with respect to Earth, constantly flying over the same place 35 800 km up, or else orbiting the planet, around 1 000 km up, filming strips several thousand kilometres wide. Together they enable us to observe the atmospheric system as a whole. On-board radiometers and interferometers sense, in the same way as radars, the different layers of the atmosphere down to the Earth’s surface, in the visible spectrum to observe cloud positions, in the infrared spectrum to observe temperatures, and in the “water vapour” spectrum to observe air content and humidity.

The European Union’s Eumetsat agency places it at the forefront of weather satellite observation. In 1977 it launched its first satellite, Meteosat1. Today it is working with Meteosat9, which began its mission in 2005, and will complete it in 2014. In 2006, the first European orbiting satellite, MetOP-A, was launched from the Baikonur launch site. Twofurther orbiting satellites should follow over the next eight years. Together, these meteo - rological satellites are contributing to the global atmospheric observation system set up by the World Meteorological Organization.

Given that the atmosphere knows no frontiers, this international cooperation at a planetary level is proving more necessary than ever in order to understand meteorological and, more generally, climatic issues.

Matthieu Lethé

  1. An air particle is a more or less tiny portion by volume of the atmosphere within which the parameters of temperature, pressure and humidity are deemed to be homogenous.
  2. Meteorologists view the atmosphere as being parcelled out, in all three dimensions, into a large number of “boxes”. Depending on how fine a forecast is desired, these boxes vary in size from a few to several hundred kilometres in length and from a few metres to several tens of metres in height.


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