The air that we breathe comes from the atmosphere, a thin gaseous layer thatsurrounds Earth and protects it somewhat from solar radiation. Like all other matter, air has weight,

but this weight varies greatly depending on altitude and temperature. Variations in pressure cause atmospheric

movements and air masses to collide with or slide by each other. Winds, light or strong, constant or unpredictable,

contribute to the planet’s thermal equilibrium.

Earth’s atmosphere8 The atmosphere

A thin protective layer

10 Atmospheric pressureThe weight of air

12 The movement of air massesFronts and depressions

14 WindsAtmospheric circulation

16 Prevailing windsMajor atmospheric movements

18 Local windsThe result of relief features

20 TornadoesThe most violent winds on Earth

22 The power of tornadoesKiller whirlwinds

A thin protective layer

The atmosphere andclouds reflect 30% ofsolar radiation.

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ere The atmosphere, defined as the gaseous envelope surrounding Earth, does not

have well-defined edges. Half of its air molecules are concentrated in a very thinlayer, 5 km thick, but there are still traces of air at more than 1,000 km altitude.Because of their protective function, the different layers of the atmosphere play an essential role in the existence of life on Earth. All of the majormeteorological phenomena also occur in the atmosphere.

THE COMPOSITION OF AIRThe composition of the atmosphere remains stable at all altitudes: nitrogen and oxygenrepresent 99% of its volume. Other gases, including argon and neon, are also found in air,but in much smaller quantities. The proportions of water vapor and carbon dioxide in theatmosphere vary, but are always very small.

weather balloon(35 km)oxygen

(21%)

carbon dioxide(0.03%)

argon(0.93%)

nitrogen(78%)

SOLAR ENERGYIn the Sun’s core, nuclear fusion reactions maintain atemperature of 15 million degrees. This huge amountof energy, constantly radiated into space in the formof electromagnetic rays, heats Earth’s surface andenables life to develop on the planet.

supersonic jet(18,000 m)

airliner(11,000 m)

Mount Everest(8,848 m)

Solar radiation covers theentire electromagnetic

spectrum.

The atmosphere

9

The mesosphere (50–80 km) is the coldest layer of the atmosphere. At its outside edge, thetemperature is as low as –100°C.

THE LAYERS OF THE ATMOSPHEREEarth’s atmosphere is made up of layers. The lowest ones (troposphere,stratosphere, mesosphere) have a relatively homogeneous composition butwidely varying temperatures. In the thermosphere (80–500 km altitude), thetemperature rises considerably, since this layer absorbs considerable solarradiation. Above it is the exosphere, a zone where the few remainingmolecules of air escape Earth’s gravity.

The temperature of the stratosphere (15–50 km), just above thetropopause (–57°C), rises to 0°C because of absorption of solarradiation by stratospheric ozone.

The ozone layer, mainly between 20 and 30 km altitude,intercepts much of the ultraviolet radiation directed towardEarth.

The tropopause is the border between thetroposphere and the stratosphere. Its altitudevaries depending on the season, the temperatureon Earth’s surface, the latitude, and theatmospheric pressure.

The tops of cumulonimbus clouds can reach, and even go beyond, the edge of the troposphere.

15 km

At sea level, the average temperature of the atmosphere is 15°C.

Most meteorological phenomena occur in thetroposphere (0–15 km), the part of the atmospherethat contains almost all water vapor.

The sky is blue because air molecules scattermainly short-wavelength radiation, whichcorresponds to the color blue in the visiblespectrum.

50 km

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1000 800 600 400 200 0

At sea level and 15°C, atmospheric pressureis, on average, 1,013 hPa.

At the top of Mount Everest (8,488 m),the atmospheric pressure is just above300 hPa.

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14

10

8

4

2

0

6alti

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(km

)

atmospheric pressure (hPa)

THE INFLUENCE OF ALTITUDE ON ATMOSPHERIC PRESSUREThe higher we go, the less air there is above us. Atmospheric pressure therefore drops with altitude. In thelower troposphere, this drop is quite constant: about 1 hPa every 8.5 meters.

Jet planes, which fly at 11,000 m altitude,are subjected to an atmospheric pressure ofabout 200 hPa.

Because molecules obey the laws of gravity, the molecules in the gases that make upEarth’s atmosphere have a certain weight, which we bear constantly without beingaware of it. Atmospheric pressure is the force that air exerts by weighing on a givenarea. At sea level, this pressure is equivalent to an average of 1,013 hPa, or 1.013 kgper cm2.

Various factors, such as altitude and temperature, can create zones of high and low atmospheric pressure. These variations are directly linked to majormeteorological phenomena.

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vacuumAt sea level, theheight of themercury is, onaverage, 76 cm.

tube

air pressure

mercury reservoir

HOW AIR PRESSURE IS MEASUREDThe mercury barometer is used to measure atmosphericpressure. Air presses on the mercury contained in areservoir, forcing it to rise in an evacuated tube. Thepressure exerted by the air is measured according tothe level reached by the mercury. For a long time, theheight of the mercury was the unit of measurement ofatmospheric pressure. Today, the International Systemuses the hectopascal (hPa): 1,000 hPa is equivalent tothe pressure exerted by a 1 kg mass on an areameasuring 1 cm2.

The weight of airAtmospheric pressure

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0Q 0W

0Q 0W 0E

0R 0T 0Y

air molecule

air parcel

HOW TEMPERATURE AFFECTS ATMOSPHERIC PRESSURE

When an air parcel cools Q, the molecules in itslow down. The air parcel contracts, becomesdenser and thus heavier, and descends toward theground W, where it creates a high-pressure zone(anticyclone) E.

high-pressurezone

low-pressurezone

The heating of surface air R causes the oppositeeffect: molecules become more agitated and movefarther away from each other, which makes the airparcel less dense. As it is lighter than the surroundingair, the warm air parcel rises T, leaving a low-pressure zone (depression) at ground level Y.

PacificOcean

AzoresIslands Africa

Asia

Australia

Antarctica

Africa

Siberia

Iceland

NorthAmerica

SouthAmerica

Antarctica

In January Q, strong anticyclones settle over thecontinents in the Northern Hemisphere, sittingalongside the depressions over the northern oceans.In equatorial regions, warm air rises and creates abelt of depressions, particularly marked over thecontinents. In the Southern Hemisphere, wherethere is less land, tropical anticyclones are confinedto the oceans, while depressions sit over thesubpolar regions.

In July W, the heat that reigns in Asiamaintains a huge low-pressure zone that extendsas far as Africa. The oceans in the NorthernHemisphere are under high-pressure zones (the Azores and Pacific anticyclones), but thesubpolar depressions have almost disappeared. In the Southern Hemisphere, a broadanticyclonic belt settles over all the tropicalregions, both continental and oceanic. The low-pressure zone that sits above the Antarctic coastvaries little.

THE DISTRIBUTION OF ANTICYCLONES AND DEPRESSIONSAROUND THE PLANETIn general, anticyclones (high-pressure zones) and depressions (low-pressure zones) alternate around Earth in wide, relatively parallelbands. This distribution, however, is affected by the presence of thecontinents, whose land accentuates the warming or cooling of the airmasses above them.

ATMOSPHERIC PRESSURE (hPa)

> 1032

1026–1032

1020–1026

1014–1020

1008–1014

1002–1008

996–1002

< 996

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Fronts and depressions

An air mass is an enormous atmospheric volume that stays in a specific region andhas acquired climatic characteristics. As winds push the air masses, they comeinto contact with each other and thus help to distribute humidity and heat aroundthe surface of the planet.

When two air masses with different temperatures and humidity levels meet, they donot mix, but collide along a line called a front. This encounter causes the formationof clouds and precipitation.

The movement ofai r masses

cold air mass

cumulonimbus cloudswarm air mass

heavy precipitation

cold front

cold air masswarm front

precipitation

warm air mass

nimbostratusclouds

cold air mass

warm air mass

cold airmass

precipitation

occluded front

OCCLUDED FRONTAn occlusion (or occluded front) occurswhen a cold front overtakes a warmfront: two cold air masses join,surrounding the warm air mass andpushing it upward.

COLD FRONTA cold air mass that overtakes a

warm air mass produces a cold front.The denser cold air slides under the

warm air, which is forced to riserapidly, forming cumulonimbus

clouds. Heavy precipitation,sometimes with storms, develops.

WARM FRONTWhen a moving warm air mass overtakes a coldair mass, it creates a warm front. The warm airrises, since it is lighter, becoming cooler as itrises. The humidity that it contains condensesin the form of nimbostratus clouds. Thisconfiguration is often associated withmoderate precipitation.

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0Q0W

0E0R

0U

0I

0O

0T

0Y

AIR MASSESAir masses are divided into six categories, according to the climatic characteristics (temperature andhumidity) of the places where they are formed. As they are pushed by the winds, they directly influence theweather in the regions over which they move. However, their characteristics slowly change, sometimes tothe point where they become unrecognizable.

HOW A DEPRESSION FORMS AND DISSIPATESWhen cold air masses from polar regions collide with warm air masses from the tropics, their meeting produces afront on which, at a particular point, the pressure begins to drop: a depression is born. Clouds, precipitation, andwinds develop there until the depression dissipates.

cold front

warmtropical air

cold polar air

warm front

low-pressure zone

Lighter warm air Q rises above the cold air W,which creates a low-pressure zone E around whichwarm and cold fronts form.

warm front

precipitationclouds

cold front

Sucked in by low pressure, the cold air begins tomove in a circular direction. The cold front R

approaches the warm front T. At a high altitude,the warm air condenses and forms clouds Y, whichdrop precipitation.

warm airwarm air

cold windsoccluded point

When the cold front overtakes the warm front, thewarm air U is pushed to a higher altitude, abovethe occlusion I. The weather is unstable andwindy O.

The occlusion finally blocks the inflow of warm air.The depression then begins to dissipate: the windsubsides and precipitation stops.

TYPES OF AIR MASSES

maritime tropical

continental tropical

maritime arctic

continental arctic

maritime polar

continental polar