22 2.Earth's Climate System the climate.The sulfur released to the atmos- varies with latitude).as one moves above the phere by the volcanic eruption of mount surface toward lower n ssure and density Pinatubo in the Philippines in 1991,for the vertical pressure gradient also decreases example,caused a temporary atmospheric Furthermore,because warm air is less dense cooling throughout the globe. than cold air,pressure falls off with height more Clouds have complex effects on Earth's radi- slowly for warm than for cold air. ation budget.All clouds have a relatively high The troposphere is the lowest atmospheric albedo and reflect more incoming shortwave hn does the darkert layer and contains most of the mass of the atmosphere troposphere which is a er vapor nd la d prima from om by sen ng uxes by I much om Ea of the th mperature th's surface es with h igh radiation)has a which unlike the or is h od fr reflecting incoming energy back to space.The the tor Abs oU radiation rms the ait has a warming effect by kee ing more er etrated in the stra tosnhere hecause of in the farth system from escaning to space the halance between the availability of shortwave balance of these two effects depends on the UV necessary to split molecules of molecula height of the cloud.The reflection of shortwave oxygen (O,)into atomic oxygen (O)and a high radiation usually dominates the balance in high enough density of molecules to bring about the clouds,causing cooling:whereas the absorption required collisions between atomic O and mol and re-emission of longwave radiation gener ally dominates in low clouds,producing a net warming effect 110 Atmospheric Structure ssure and density decline with 90 h 80 -Me re of the fou relatively distinet ls cha zed by thei temp erature profiles The atmosphere is high 70 com ressible.and gravity keeps most of the 60 mass of the atmo phere close to farth's sur face.Pressure.which is determined by the mass 50 of the overlving atmosphere.decreases expo nentially with height.The vertical decline in air density tends to follow closely that of pressure. The relationships between pressure,density. 20 and height can be e described in terms of the hydrostatic equation 40 Troposphere- dh=-pg (2.1) 90 % 3 press where Pis .and Temperature (C) states FIGURE 2.3.Average thermal structure of the atmos is hala phere shov du nd ng the vertical gradients in Earth's majo ss.Schlesinger 1997.) permission fror “constant' that
22 2. Earth’s Climate System the climate. The sulfur released to the atmosphere by the volcanic eruption of Mount Pinatubo in the Philippines in 1991, for example, caused a temporary atmospheric cooling throughout the globe. Clouds have complex effects on Earth’s radiation budget. All clouds have a relatively high albedo and reflect more incoming shortwave radiation than does the darker Earth surface. Clouds, however, are composed of water vapor, which is a very efficient absorber of longwave radiation. All clouds absorb and re-emit much of the longwave radiation impinging on them from Earth’s surface. The first process (reflecting shortwave radiation) has a cooling effect by reflecting incoming energy back to space. The second effect (absorbing longwave radiation) has a warming effect, by keeping more energy in the Earth System from escaping to space.The balance of these two effects depends on the height of the cloud. The reflection of shortwave radiation usually dominates the balance in high clouds, causing cooling; whereas the absorption and re-emission of longwave radiation generally dominates in low clouds, producing a net warming effect. Atmospheric Structure Atmospheric pressure and density decline with height above Earth’s surface. The average vertical structure of the atmosphere defines four relatively distinct layers characterized by their temperature profiles. The atmosphere is highly compressible, and gravity keeps most of the mass of the atmosphere close to Earth’s surface. Pressure, which is determined by the mass of the overlying atmosphere, decreases exponentially with height. The vertical decline in air density tends to follow closely that of pressure. The relationships between pressure, density, and height can be described in terms of the hydrostatic equation (2.1) where P is pressure, h is height, r is density, and g is gravitational acceleration. The hydrostatic equation states that the vertical change in pressure is balanced by the product of density and gravitational acceleration (a “constant” that dP dh = -rg varies with latitude). As one moves above the surface toward lower pressure and density, the vertical pressure gradient also decreases. Furthermore, because warm air is less dense than cold air, pressure falls off with height more slowly for warm than for cold air. The troposphere is the lowest atmospheric layer and contains most of the mass of the atmosphere (Fig. 2.3). The troposphere is heated primarily from the bottom by sensible and latent heat fluxes and by longwave radiation from Earth’s surface. Temperature therefore decreases with height in the troposphere. Above the troposphere is the stratosphere, which, unlike the troposphere, is heated from the top. Absorption of UV radiation by O3 in the upper stratosphere warms the air. Ozone is concentrated in the stratosphere because of a balance between the availability of shortwave UV necessary to split molecules of molecular oxygen (O2) into atomic oxygen (O) and a high enough density of molecules to bring about the required collisions between atomic O and mol- 110 100 90 80 70 60 50 40 30 20 10 Thermosphere Mesopause Mesosphere Stratopause Stratosphere Tropopause Troposphere Height (km) -90 -60 -30 0 30 Temperature ( o C) Mt. Everest Figure 2.3. Average thermal structure of the atmosphere showing the vertical gradients in Earth’s major atmospheric layers. (Redrawn with permission from Academic Press; Schlesinger 1997.)
The Atmospheric System 23 ecular O:to form O3.The absorption of UV surface heating,which creates convective tur bulence,and by mechanica turbulence.whicl in temperature .Ihe ozon ate ar moving damagin ule PRI etans the which e T dist The e bo the concentration of ozo ne in the stra osnhere has been declining due to the production and emis grows in height until midday. sion of CFCs,which destroy stratospheric rupted by convective activity (Fig.2.4).The ozone.particularly at the poles.This results PBL becomes shallower at night when there is in an ozone "hole,"an area where the trans- no solar energy to drive convective mixing.Air mission of UV radiation to Earth's surface is in the pBl is relatively isolated from the free increased.Slow mixing between the tropos- troposphere and therefore functions like a phere and the stratosphere allows CFCs and chamber over Earth's surface.The changes in other compounds to reach and accumulate in water vapor,CO2,and other chemical con- the ozone-rich stratosphere,where they have stituents in the PBL thus serve as an indicator long reside osphere is the al and physiochemical process osph and Harr aga crease ith 19).The of th in urban regions he the cleane ted ho the table air 1 phere has spot the cnhere' ollutants in urb total mass co mposed primarily of O and nitro ments.often reach high c tions gen(N)atoms that can absorb very shortwave because they are concentrated in a shallow again causing an increase in heating boundary layer. with height (Fig.2.3).The meso sphere and ther mosphere have relatively little impact on the biosphere. The troposphere is the atmospheric layer in which most weather occurs,including thunder- S storms,snowstorms,hurricanes,and high and low pressure systems.The troposphere is thus Ne clou the portion of the atmosphere that directly laye to an bounda height of bo the 220C 30°c re falls off st slowly with 1200 height (Eg.2.1).and at about 9km in polar ons where ospheric temperatures are FIGURE 2.4.Inct crease in the height of the planetar lowest.The height of the tropopause also varies seasonally,being lower in winter than in Amazon Basin on a day without thunderstorms.The increase in surface temperature drives evapotranspi summer. The planetary boundary laver (PBL)is the auses the dary layer to incre lower portion of the troposphere,which is influ- enced by mixing between the atmosphere and to form clouds.(Redrawn with permission from Earth's surface.Air within the PBL is mixed by Ecology:Matson and Harriss 1988.)
The Atmospheric System 23 ecular O2 to form O3. The absorption of UV radiation by stratospheric ozone results in an increase in temperature with height. The ozone layer also protects the biota at Earth’s surface from damaging UV radiation. Biological systems are sensitive to UV radiation because it can damage DNA, which contains the information needed to drive cellular processes. The concentration of ozone in the stratosphere has been declining due to the production and emission of CFCs, which destroy stratospheric ozone, particularly at the poles. This results in an ozone “hole,” an area where the transmission of UV radiation to Earth’s surface is increased. Slow mixing between the troposphere and the stratosphere allows CFCs and other compounds to reach and accumulate in the ozone-rich stratosphere, where they have long residence times. Above the stratosphere is the mesosphere, where temperature again decreases with height. The uppermost layer of the atmosphere, the thermosphere, begins at approximately 80 km and extends into space. The thermosphere has a small fraction of the atmosphere’s total mass, composed primarily of O and nitrogen (N) atoms that can absorb very shortwave energy, again causing an increase in heating with height (Fig. 2.3).The mesosphere and thermosphere have relatively little impact on the biosphere. The troposphere is the atmospheric layer in which most weather occurs, including thunderstorms, snowstorms, hurricanes, and high and low pressure systems. The troposphere is thus the portion of the atmosphere that directly responds to and affects ecosystem processes. The tropopause is the boundary between the troposphere and the stratosphere. It occurs at a height of about 16 km in the tropics, where tropospheric temperatures are highest and hence where pressure falls off most slowly with height (Eq. 2.1), and at about 9 km in polar regions, where tropospheric temperatures are lowest.The height of the tropopause also varies seasonally, being lower in winter than in summer. The planetary boundary layer (PBL) is the lower portion of the troposphere, which is influenced by mixing between the atmosphere and Earth’s surface. Air within the PBL is mixed by surface heating, which creates convective turbulence, and by mechanical turbulence, which is associated with the friction of air moving across Earth’s surface. The PBL increases in height during the day largely due to convective turbulence. The PBL mixes more rapidly with the free troposphere when the atmosphere is disturbed by storms. The boundary layer over the Amazon Basin, for example, generally grows in height until midday, when it is disrupted by convective activity (Fig. 2.4). The PBL becomes shallower at night when there is no solar energy to drive convective mixing. Air in the PBL is relatively isolated from the free troposphere and therefore functions like a chamber over Earth’s surface. The changes in water vapor, CO2, and other chemical constituents in the PBL thus serve as an indicator of the biological and physiochemical processes occurring at the surface (Matson and Harriss 1988). The PBL in urban regions, for example, often has higher concentrations of pollutants than the cleaner, more stable air above. At night, gases emitted by the surface, such as CO2 in natural ecosystems or pollutants in urban environments, often reach high concentrations because they are concentrated in a shallow boundary layer. Previous day's cloud layer New cloud layer Height above canopy (km) Local time 1.5 1.0 0.5 0 22o C30o C 0600 0800 1000 1200 Figure 2.4. Increase in the height of the planetary boundary layer between 6:00 a.m. and noon in the Amazon Basin on a day without thunderstorms. The increase in surface temperature drives evapotranspiration and convective mixing, which causes the boundary layer to increase in height until the rising air becomes cool enough that water vapor condenses to form clouds. (Redrawn with permission from Ecology; Matson and Harriss 1988.)
24 2.Earth's Climate System Atmospheric Circulation surface air to expand and become less dense than surrounding air so it rises as air rises the The fundamental cause of atm lation is the une decrease in atmospheric pressure with height The equator receives more inc ing solar radi causes continued expansion (Eq.2.1),which ation than the poles because earth is spherical. decreases the average kinetic energy of air mol- At the equator,the sun's rays are almost per- ecules.causing the rising air to cool.The dry pendicular to the surface at solar noon.At the adiabatic lapse rate is the change in tempera lower sun angles experienced at high latitudes. ture experienced by a parcel of air as it moves ar)ine vertically in the atmosphere without exchang rgy with the surr g als 10 ceived per unit ground area.In addition, the sun's rays have a longer path through t and precip air has a l hold atmosphere,so more of the incoming sola tha t re which e at which h hy e ansion.This release of latent heat can air to ho than sur the s tha an at the po roundin air so it continues to r e.The result ulation has both ing moist adiabatic lapse rate is about 4Ckm ical near the surface rising to 6 or 7Ckm-i in the and horizontal components (Fig.2.6) The transfer of energy from Earth's surface to the middle troposphere.The greater the moisture atmosphere by latent and sensible heat fluxes content of rising air.the more latent heat is and longwave radiation generates strong released to drive convective uplift,which con heating at the surface.This warming causes the tributes to the intense thunderstorms and deep boundary layer in the wet tropics.The average lapse rate varies regionally,depending on the Atmosphere strength of surface heating but averages about air ris es most strongly a se of equ orial the atin nd the tropop quatorial air als ates a hor natorial air aloft to tropopause toward the poles (Fig.2.6).This poleward-moving air cools due to emission of longwave radiation to space.In addition,the air arth converges into a smaller volume as it moves poleward because Earth's radius and surface area decrease oward the poles ng o the air and its sun's rays show the depth of the atmosphere tha nto smalle mu pen etrate.The arrows pa volume,the density air to oside 110 High-latitude ecosystems receive less radiation than ising Had or to replac this a onger path thr n the atmosphere and odel quatorial in 1735, that arg
24 2. Earth’s Climate System Atmospheric Circulation The fundamental cause of atmospheric circulation is the uneven heating of Earth’s surface. The equator receives more incoming solar radiation than the poles because Earth is spherical. At the equator, the sun’s rays are almost perpendicular to the surface at solar noon. At the lower sun angles experienced at high latitudes, the sun’s rays are spread over a larger surface area (Fig. 2.5), resulting in less radiation received per unit ground area. In addition, the sun’s rays have a longer path through the atmosphere, so more of the incoming solar radiation is absorbed, reflected, or scattered before it reaches the surface. This unequal heating of Earth results in higher tropospheric temperatures in the tropics than at the poles, which in turn drives atmospheric circulation. Atmospheric circulation has both vertical and horizontal components (Fig. 2.6). The transfer of energy from Earth’s surface to the atmosphere by latent and sensible heat fluxes and longwave radiation generates strong heating at the surface. This warming causes the surface air to expand and become less dense than surrounding air, so it rises. As air rises, the decrease in atmospheric pressure with height causes continued expansion (Eq. 2.1), which decreases the average kinetic energy of air molecules, causing the rising air to cool. The dry adiabatic lapse rate is the change in temperature experienced by a parcel of air as it moves vertically in the atmosphere without exchanging energy with the surrounding air and is about 9.8°C km-1 . Cooling also causes condensation and precipitation because cool air has a lower capacity to hold water vapor than warm air. Condensation in turn releases latent heat, which reduces the rate at which rising air cools by expansion. This release of latent heat can cause the rising air to be warmer than surrounding air, so it continues to rise. The resulting moist adiabatic lapse rate is about 4°C km-1 near the surface, rising to 6 or 7°C km-1 in the middle troposphere. The greater the moisture content of rising air, the more latent heat is released to drive convective uplift, which contributes to the intense thunderstorms and deep boundary layer in the wet tropics. The average lapse rate varies regionally, depending on the strength of surface heating but averages about 6.5°C km-1 . Surface air rises most strongly at the equator because of the intense equatorial heating and the large amount of latent heat released as this moist air rises and condenses.This air rises until it reaches the tropopause. The expansion of equatorial air also creates a horizontal pressure gradient that causes the equatorial air aloft to flow horizontally from the equator along the tropopause toward the poles (Fig. 2.6). This poleward-moving air cools due to emission of longwave radiation to space. In addition, the air converges into a smaller volume as it moves poleward because Earth’s radius and surface area decrease from the equator toward the poles. Due to the cooling of the air and its convergence into a smaller volume, the density of air increases, creating a high pressure that causes upper air to subside, which forces surface air back toward the equator to replace the rising equatorial air. Hadley proposed this model of atmospheric circulation in 1735, suggesting that there should be one large circuAtmosphere Sun's rays Earth Axis Figure 2.5. Atmospheric and angle effects on solar input at different latitudes.The arrows parallel to the sun’s rays show the depth of the atmosphere that solar radiation must penetrate. The arrows parallel to Earth’s surface show the surface area over which a given quantity of solar radiation is distributed. High-latitude ecosystems receive less radiation than those at the equator because radiation at high latitudes has a longer path through the atmosphere and is spread over a larger ground area
The Atmospheric System 25 Cold subsidinga Warm rising ai 609 Polar front Cold subsiding air NE SE bropica high pressure Cold subsiding ai Polar front_ Warm rising ai Cold su g a FIGURE 2.6.Earth's atitu atmospheric circula her poleward to replace air that ha and at the pol forces produce three maior cells of vertical atmos shown are the horizontal patterns of atmospheric cir pheric circulation (Hadley,Ferrell,and polar cells). culation.consisting of th Air warms and r prevailing surface winds (the ra h Tpics and n thes sither low tudes where it descends and either returns to the of rising air (the intertropical conversion zone pole ITCZ.and the pola moving air at about 6 latitude.There lation cell in the Northe ern Hemisph 2.6).The Ferrell cell is actually the long-tern weat tin an ce at the 1865th he cnao th The actual dynamics are r auch more complex.This cels subdivide the atmosphere into three model describes atmospheric circula tinct circulations: e series of thtee circulation cells in each hemi. sphere.(1)The Hadley cell is driven by expan sion and uplift of equatorial air.(2)The polar air masses between 60N and S and the poles cell is driven by subsidence of cold converging (Fig.26).The latitudinal location of thes air at the poles.(3)The intermediate Ferrell cell cells moves seasonally in response to latitudi is driven indirectly by dynamical processes(Fig. nal changes in surface heating by the sun
The Atmospheric System 25 lation cell in the Northern Hemisphere and another in the Southern Hemisphere, driven by atmospheric heating and uplift at the equator and subsidence at the poles. Based on observations, Ferrell proposed in 1865 the conceptual model that we still use today, although the actual dynamics are much more complex. This model describes atmospheric circulation as a series of three circulation cells in each hemisphere. (1) The Hadley cell is driven by expansion and uplift of equatorial air. (2) The polar cell is driven by subsidence of cold converging air at the poles. (3) The intermediate Ferrell cell is driven indirectly by dynamical processes (Fig. 2.6). The Ferrell cell is actually the long-term average transport caused by weather systems in the mid-latitudes rather than a stable permanent atmospheric feature. The chaotic motion of these mid-latitude weather systems creates a net poleward transport of heat. These three cells subdivide the atmosphere into three distinct circulations: tropical air masses between the equator and 30° N and S, temperate air masses between 30 and 60° N and S, and polar air masses between 60° N and S and the poles (Fig. 2.6). The latitudinal location of these cells moves seasonally in response to latitudinal changes in surface heating by the sun. ITCZ Polar cell Cold subsiding air Cold subsiding air Ferrell cell Hadley cell Hadley cell Cold subsiding air Warm rising air 60o 30o 0o Subtropical high pressure Warm rising air Warm rising air Cold subsiding air Ferrell cell Polar cell Westerlies NE tradewinds SE tradewinds Subtropical high pressure Westerlies Polar front Polar front Figure 2.6. Earth’s latitudinal atmospheric circulations are driven by rising air at the equator and subsiding air at the poles. These forces and the Coriolis forces produce three major cells of vertical atmospheric circulation (Hadley, Ferrell, and polar cells). Air warms and rises at the equator due to intense heating. After reaching the tropopause, the equatorial air moves poleward to about 30° N and S latitudes, where it descends and either returns to the equator, forming the Hadley cell, or moves poleward. Cold dense air at the poles subsides and moves toward the equator until it encounters polewardmoving air at about 60° latitude. There the air rises and moves either poleward to replace air that has subsided at the poles (the polar cell) or moves toward the equator to form the Ferrell cell. Also shown are the horizontal patterns of atmospheric circulation, consisting of the prevailing surface winds (the easterly trade winds in the tropics and the westerlies in the temperate zones). The boundaries between these zones are either low-pressure zones of rising air (the intertropical conversion zone, ITCZ, and the polar front) or high-pressure zones of subsiding air (the subtropical high pressure belt and the poles)
2.Earth's Climate System Earth's rotation causes winds to deflect to the circulation in the Northern Hemisphere and right in the Northern Hemisphere and to the counterclockwise circulation in the left in the Southern Hemisphere.Earth and Hemisphere. its atmosphere complete one rotation about The interaction of vertical and horizontal Earth's axis every day.The direction of rotation motions of the atmosphere create Earth's is from west to east.Because the atmosphere in prevailing winds (i.e.,the most frequent wind equatorial regions is farther from Earth's axis directions).As air in the Hadley cell moves of rotation than that at higher latitudes,there poleward along the tropopause,it is deflected is a corresponding ward decreas in the by the Corioli ahYAN trave hat is,it the wes 2.6).T move nts th pol ard- or south moving air wa momentum( on-cellcirulatio ppose tum when you try to stop or a bel 13n icy auses the air to sink.So M。=mv (2.2) ns te ward the ed at the the Hadl where m is the mass.y is the velocity.and r is 26).The interaction between motions induced the radius of rotation.If the mass of a parcel of by the pole-to-equator temperature gradient air remains constant,its velocity is inversely and the Coriolis force explains why there are related to the radius of rotation.We know.for three atmospheric circulation cells in each example,that skaters can increase their speed hemisphere rather than just one,as Hadley of rotation by pulling their arms close to their had proposed.At the boundaries between the bodies,which reduces their effective radius.Air major cells of atmospheric circulation,there pre th angula gen ore rapidly (i. e.,m e in ere wes btropical pol tor di west to radius of rota nd Ea axis and,to conserve At the surface,the direction of moves more slowly lie n ves fron east to winds de nds on whether air is m west)relative to farth's surface This causes the or away from the equator in the tropics surfac air to be deflected to the right relative to air in the hadley cell moves from 30n and s Earth's surface.in the northern Hemisphere toward the equator.In the Northern Hemi and to the left in the Southern Hemisphere sphere,this air is deflected to the right by the (Fig.2.6).creating clockwise patterns of atmo Coriolis force and forms the northeast trad spheric circulation in the Northern Hemisphere winds (1e.,surface winds that blow from the from 30 cons angula an Iorms th momentum th ts dire nin low pre h the No to Earth' surfac converge Her 1 on an currents, idity(Fig. an are
26 2. Earth’s Climate System Earth’s rotation causes winds to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Earth and its atmosphere complete one rotation about Earth’s axis every day.The direction of rotation is from west to east. Because the atmosphere in equatorial regions is farther from Earth’s axis of rotation than that at higher latitudes, there is a corresponding poleward decrease in the linear velocity of the atmosphere as it travels around Earth. As parcels of air move north or south, they tend to maintain their angular momentum (Ma), just as your car tends to maintain its momentum when you try to stop or turn on an icy road. Ma = mvr (2.2) where m is the mass, v is the velocity, and r is the radius of rotation. If the mass of a parcel of air remains constant, its velocity is inversely related to the radius of rotation. We know, for example, that skaters can increase their speed of rotation by pulling their arms close to their bodies, which reduces their effective radius. Air that moves from the equator toward the poles encounters a smaller radius of rotation around Earth’s axis. Therefore, to conserve angular momentum, it moves more rapidly (i.e., moves from west to east), relative to Earth’s surface, as it moves poleward (Fig. 2.6). Conversely, air moving toward the equator encounters an increasing radius of rotation around Earth’s axis and, to conserve angular momentum, moves more slowly (i.e., moves from east to west),relative to Earth’s surface. This causes the air to be deflected to the right, relative to Earth’s surface, in the Northern Hemisphere and to the left in the Southern Hemisphere (Fig. 2.6), creating clockwise patterns of atmospheric circulation in the Northern Hemisphere and counterclockwise patterns in the Southern Hemisphere. This conservation of angular momentum that causes air to change its direction, relative to Earth’s surface, is known as the Coriolis force. The Coriolis force is a pseudo-force that arises only because Earth is rotating, and we view the motion relative to Earth’s surface. Similar Coriolis forces act on ocean currents, creating clockwise ocean circulation in the Northern Hemisphere and counterclockwise circulation in the Southern Hemisphere. The interaction of vertical and horizontal motions of the atmosphere create Earth’s prevailing winds (i.e., the most frequent wind directions). As air in the Hadley cell moves poleward along the tropopause, it is deflected by the Coriolis force to a westerly direction— that is, it blows from the west (Fig. 2.6). This prevents the poleward-moving air from reaching the poles, as it was supposed to do in Hadley’s one-cell circulation model. This results in an accumulation of air and a belt of high pressure at about 30° N and S latitude, which causes the air to sink. Some of this subsiding air returns toward the equator at the surface, completing the Hadley circulation cell (Fig. 2.6). The interaction between motions induced by the pole-to-equator temperature gradient and the Coriolis force explains why there are three atmospheric circulation cells in each hemisphere rather than just one, as Hadley had proposed. At the boundaries between the major cells of atmospheric circulation, there are relatively sharp gradients of temperature and pressure that, together with the Coriolis force, generate strong winds over a broad height range in the upper troposphere. These are the subtropical and polar jet streams. The Coriolis force explains why these winds blow in a westerly direction (i.e., from west to east). At the surface, the direction of prevailing winds depends on whether air is moving toward or away from the equator. In the tropics, surface air in the Hadley cell moves from 30° N and S toward the equator. In the Northern Hemisphere, this air is deflected to the right by the Coriolis force and forms the northeast trade winds (i.e., surface winds that blow from the northeast). Equatorward flow from 30° S is deflected to the left and forms the southeast trade winds. Thus equatorial winds blow predominantly from the east. The region where surface air from the Northern and Southern Hemispheres converges is called the intertropical convergence zone (ITCZ). Here the rising air creates a zone with light winds and high humidity (Fig. 2.6), an area known to