Global Climate and Terrestrial Biomes

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89 Global Climate and Terrestrial Biomes Name_______________________________________________ Day/Hour____________________

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Precipita>on

Exercise A: Global Temperature and PrecipitaBon PaCerns

Solar Angle of Incidence Nearly all of the biomes on Earth are dictated by two factors: the amount of temperature and precipita>on they receive, and these are correlated. Temperature on Earth is dependent on the solar angle of incidence, or the angle at which the light waves from the sun hit the surface of the earth. Areas of earth that are perpendicular to the sun’s rays receive more energy than other parts where the sun’s rays hit the earth at an oblique angle. Over one year, the equator receives the most amount of light energy from the sun per area; whereas the poles receive the least. At the middle

Figure 1. Global varia>ons in the solar angle of incidence.

Global Temperature and Precipita9on Pa:erns The light intensity from the sun is maximized directly at the equator. This causes equatorial air masses to warm. On Earth, the warmer the air mass, the higher its water holding capacity, and therefore the more moisture (gaseous water) it can hold. Thus, air masses at equatorial regions can hold more moisture than cooler la>tudes. In addi>on, as air masses warm they expand causing warm air to have a lower density than cool air. Consequently, warm air rises. At the equator, the direct solar angle of incidence warms the air mass increasing its water holding capacity and decreasing its density. However, as the air mass rises to higher al9tudes, it begins to cool. As the air mass cools, its water holding capacity decreases which causes the gaseous water to condense into liquid, in effect causing rain. This is why the tropical forests of the world are so lush. They have the most sunlight and the most water of any other place on Earth.

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low

high

Te m pe

ra tu re

LaBtude 90˚ 0˚

1. Using the informaBon above, draw a line on each graph below indicaBng the relaBonship between temperature, water holding capacity, air mass density, laBtude and alBtude.

low

high

Te m pe

ra tu re

AlBtude high low low

high

W at er H ol di ng C ap

ac it y

Temperature high low low

high

A ir m

as s D en

si ty

Temperature high low

la>tudes there is a moderate angle of incoming light, crea>ng moderate temperature. Solar angle of incidence is the primary determinant of global temperature paVerns (see Fig. 4).

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90 Global Climate and Terrestrial Biomes

Precipita>on

Exercise B: Atmospheric CirculaBon PaCerns The Hadley Cells Differences in solar radia>on, due to varia>ons in the solar angle of incidence, are the driving force of atmospheric circula9on, the rota>on of air masses within Earth’s troposphere. Two atmospheric circula>ons occur between 30˚N and 30˚S, known as the northern and southern Hadley Cells. Solar radia>on is on-‐average greatest at the equator, which induces air masses to warm due to greater heat absorp>on rela>ve to higher la>tudes. As the equatorial air mass warms, its density decreases causing the air mass to rise. Warmer air mass also increases its gaseous water (vapor) holding capacity. As equatorial air masses rise in al>tude, temperatures decline. With falling temperatures, the water holding capacity decreases condensing gaseous water into liquid causing precipita>on. In effect, equatorial regions of Earth generate their own rain. As the rising air mass cools, it also becomes more dense, increasing air pressure. Even though higher al>tudinal air masses are more dense at the equator, they con>nue to rise due to the incoming warm air below. This phenomenon, creates an area of low air pressure close to Earth at the equator. The air mass eventually reaches the top of the troposphere, at boundary known as the tropopause, and moves poleward. As the air masses move north and south from the equator, water con>nues to condense eventually forming precipita>on. Eventually, no more gaseous water remains in the air mass hal>ng precipita>on as the air mass moves. At approximately 30˚N and 30˚S, the air mass begins to sink due to increasing air density, crea>ng an area of high pressure at the Earth’s surface. The discrepancy between the high pressure at 30˚ and the equator causes the air mass to flow toward the equator, essen>ally propelling the convec>on of the Hadley atmospheric cells. As the air mass falls, it warms. However, nearly all the moisture has been removed from the air mass. These hot, dry areas caused by the Hadley Cell convec>on phenomenon produce the world’s great deserts. The Sahara Desert (Africa), the Sonoran and Mohave (US), and the Australian outback are all oriented at 30˚ la>tude.

Figure 2. The Hadley circula>on cells. 0˚ 30˚S 30˚N

g. f.

Surface of Earth

Stratosphere

2. In figure 2, label the appropriate blanks with the following: (1) low precipitaBon, (2) decreasing temperatures, (3) tropopause, (4) troposphere, (5) low air pressure, (6) Northern Hadley Cell, and (7) Southern Hadley Cell.

91 Global Climate and Terrestrial Biomes

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Exercise B: Atmospheric CirculaBon PaCerns In each hemisphere there are three atmospheric convec>ons: the Hadley cell (from 0˚-‐30˚), the Ferrell cell (from 30˚-‐60˚), and the Polar cell (from 60˚-‐90˚). The Ferrell Cells At 30˚, the warm, extremely dry air mass descends from the tropopause earthward genera>ng high air pressure. As this pressurized air mass reaches the surface of the earth, it travels both toward the equator (closing the Hadley cell convec>on) and poleward (beginning the Ferrell cell convec>on). As the air mass moves toward the poles from 30˚, it begins to warm due to the higher temperatures at lower la>tudes. As the air mass warms, it absorbs water vapor due the increased water holding capacity of the warmer air. As the air mass reaches the 60˚ parallel, the density of the air mass decreases causing the air to rise (akin to the process at 0˚), crea>ng an area of low air pressure. As the air rises, temperatures decline dropping the water holding capacity of the air mass, forming precipita>on. This is the cause of the world’s very cold, wet taiga forests (i.e. Southern Canada, Northern Europe, and Russia and Northern China) where coniferous trees dominate. As the air reaches the tropopause, it either moves back toward the equator or toward the poles. As the air mass moves away from 60˚ the air mass con>nues to cool releasing precipita>on. This phenomenon provides rain and snow crea>ng the cool, wet temperate forests of Northern United States, Southern Europe, and China. These forests typically are dominated by deciduous trees Similar to the Hadley cell though, once the water has completely precipitated, the air mass becomes dry. As the Ferrell cells travel towards 30˚ just below the tropopause, the temperatures con>nue to decline increasing the density of the air mass, causing the air mass to descend back to Earth at 30˚, comple>ng the Ferrell cell.

0˚

30˚

60˚

90˚

60˚

30˚

Figure 3. Global atmospheric circula>on cells.

The Polar Cells The Polar cell (60˚-‐90˚) convec>on shares many similari>es with the Hadley cell. Though dry and cool rela>ve to the equator, the air mass at 60˚ has enough thermal energy to rise driving the Polar and Ferrell cells. Once the air mass reaches the tropopause at 60˚, it moves both poleward and towards the equator. In the Polar cell, the air mass moves towards the poles cooling and releasing precipita>on as snow or rain. As the convec>on reaches the poles it descends as a cold, dry high-‐pressure air mass. Very liVle precipita>on falls at extreme la>tudes due to this phenomenon. Areas of the poles that are not dominated by glaciers in extreme northern and southern la>tudes have minimal moisture availability and an extremely short growing season, and are inhabited by very short scrubby vegeta>on in a biome known as the tundra.

3. In Figure 3, label each atmospheric circulaBon cell. a.

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Exercise B: Atmospheric CirculaBon PaCerns

5. Refer to Figure 4, temperature is unimodal (a one-‐humped shape), while precipitaBon is trimodal (a three-‐humped shape). From your knowledge of global atmospheric circulaBon paCerns, explain why temperature (relaBve to laBtude) is unimodal. Provide as detailed explana9on as possible.

6. Why is global precipitaBon (relaBve to laBtude) trimodal? Provide as detailed explana9on as

possible including a discussion of the Hadley, Ferrell and Polar cells.

A ve ra ge T em

pe ra tu re (˚ C)

La>tude (˚N) 0 90 60 30 -‐30 -‐60 -‐90

500

1000

1500

2000

A verage Precipita>on (m

m )

Temperature Precipita>on

Figure 4. The rela>onship between la>tude, precipita>on and temperature.

Global Temperature and Precipita9on Pa:erns Ul>mately, differences in solar radia>on determines global posi>oning of biomes. On average, solar radia>on is greatest at the equator and lowest at the poles (Fig. 4). While precipita>on on average correlates with temperature, variability between temperature and precipita>on is due to atmospheric convec>on of the Hadley, Ferrell and Polar cells.

4. Refer to Figure 4, what property of air mass are responsible for the general correlaBon between precipitaBon and temperature?

93 Global Climate and Terrestrial Biomes

Exercise B: Atmospheric CirculaBon PaCerns The Coriolis Effect and Global Wind Pa:erns Every 24 hours the Earth completes one rota>on around its axis. With the North pole oriented at top, the Earth rotates counter-‐ clockwise on its axis (from leh to right in Fig 5). This causes air masses moving toward the equator to deflect towards the right (in the northern hemisphere) and toward the leh (in the southern hemisphere). Imagine a rocket being propelled directly south from the north pole (see Figure 5). If the Earth were not rota>ng, the rocket would be propelled directly toward the equator. However, since the Earth is rota>ng the rocket will not directly reach the equator perpendicular to its original trajectory. As the Earth spins, a rocket moving from the north pole towards the equator will bend to the right. What about a rocket shot from the south pole to the equator?

Figure 5. A simplifica>on of the Coriolis Effect.

Direc>on of Earth’s rota>on (counterclockwise)

Rocket fired from north pole to equator

Expected path in a non-‐rota9ng Earth

Actual path in a rota9ng Earth (clockwise)

Rocket fired from south pole to equator

This phenomenon is know as the Coriolis Effect. However, unlike the simplifica>on of the Coriolis Effect in Figure 4, the convec>on of air masses on Earth make this process more dynamic and complex. Strange at it may seem, different la>tudes rotate at different veloci>es. The circumference of the Earth at the equator (40,000km) is much larger than the circumference at the pole (effec>vely 0km).Therefore, the surface of Earth at the equator is moving approximately 1,666 km/h while directly at the pole the earth is effec>vely moving close to 0 km/h. This varia>on in rota>onal rates at the surface of Earth are responsible for global wind paVerns. This phenomenon causes terrestrial wind paVerns in the northern hemisphere to move towards the right (rela>ve to their ini>al trajectory) and wind paVerns to move leh in the southern hemisphere. Tradewinds As the warm air mass reaches the surface of the Earth at 30˚, the flow spits moving both poleward and towards the equator. As the air mass of the Hadley cell moves back towards the poles, the Earth is spinning counterclockwise. Due to the varia>on in the Earth’s circumferen>al rota>on rates, wind moving towards the equator is moving slower than the Earth’s surface. This phenomenon causes air masses moving toward the equator to simultaneously flow clockwise (to the right in the northern hemisphere and to the leh in the southern hemisphere, rela>ve to the ini>al velocity) from 30˚ to 0˚. This effect creates the Earth’s tradewinds, which move south and east (southeasterly) in the northern hemisphere (from 30˚N to 0˚) and northeasterly in the southern hemisphere (from 30˚S to 0˚). As these tradewinds near the equator, they travel across the ocean and absorb massive amounts of water (due to the increased water holding capacity of the warmer air mass). The tradewinds slam into western Africa, western South America, southern India, northern Australia and the South Pacific producing the world’s lush tropical forests.

8. In Figure 4, draw a line of the actual path a rocket shot from the south pole directly towards the equator would follow.

0˚

30˚N

30˚S

60˚S

60˚N

90˚S

90˚N

94 Global Climate and Terrestrial Biomes

Precipita>on

Exercise B: Atmospheric CirculaBon PaCerns

Figure 6. Global surface wind paVerns due to atmospheric convec>on cells and the Coriolis Effect.

Direc>on of Earth’s rota>on (counterclockwise)

Westerlies As the high pressure air mass descends at 30˚ and splits, half of the air mass travels along surface of the Earth towards the poles in the Ferrell cell. Just as the Hadley cell, the air masses will move toward the right in the northern hemisphere and to the leh in the southern hemisphere, rela>ve to the rota>on of the Earth. However, unlike the Hadley cell, the surface winds of the Ferrell cell move in a westerly direc>on. As the air mass moves toward the poles at 30˚, the ini>al speed of the air mass is faster rela>ve to the rota>onal speed of the Earth further poleward (due to the shrinking circumference of the Earth). This causes air masses (known as westerlies) travelling from 30˚ to 60˚ to move from south to west (southwesterly) in the northern hemisphere and northwesterly in the southern hemisphere. Polar Winds As the polar convec>on reaches the the poles (90˚) the cold, dry air mass descends to the surface of Earth genera>ng an area of high pressure, which moves along the surface towards the low pressure area at 60˚. As this air mass moves along the surface, it travels clockwise, due to the Coriolis effect of the counter-‐clockwise rota>on of the Earth. This effect causes polar wind to travel along the surface of the Earth from 90˚ to 60˚ southeasterly in the northern hemisphere, and northeasterly in the southern hemisphere. When the polar wind at the surface of the Earth reaches 60˚, it combines with the warming air mass of the Ferrell cell and rises, comple>ng the Polar cell. .

9. In Figure 6, the black dots at 30˚ and 90˚ represent descending air masses hiang the surface of the Earth. Based on your knowledge of global convecBon paCerns and the Coriolis Effect, draw the expected trajectory of each air mass responsible for the wind paCerns on the surface of the earth.

95 Global Climate and Terrestrial Biomes

Precipita>on

Exercise B: Atmospheric CirculaBon PaCerns Seasonality and Axial Tilt Rela>ve to its (annual) orbital axis around the sun, the (daily) rota>onal axis of Earth is >lted 23.5˚. This is known as Earth’s axial 9lt. As the earth orbits the sun throughout the year, its axial >lt affects regional temperature paVerns genera>ng seasonality. Due to Earth’s axial >lt, the equator of Earth is not always directly perpendicular (90˚) to the solar radia>on angle of incidence. On Jun 21st, day length is at its maximum in the northern hemisphere and at its minimum in the southern hemisphere (and is known Northern Summer Sols9ce and the Southern Winter Sols9ce). On this day, the solar radia>on angle of incidence is perpendicular (90˚) to the la>tude 23.5˚N. Thus on the Northern Summer Sols>ce, areas surrounding 23.5˚N la>tude receive the most direct amount of solar radia>on than any where else on the planet (Fig. 7c), responsible for northern summers and southern winters. The la>tude of Earth that receives a perpendicular solar radia>on angle of incidence (and therefore the most solar radia>on) is known as the solar equator. As Earth rotates around the sun, the posi>on of the solar equator changes (Fig. 7a). As Earth orbits from the Northern Summer Sols>ce the solar equator moves southward. On September 21st, the solar equator and Earth’s equator align, on a day known as the Northern Autumnal Equinox and the Southern Vernal Equinox. On this day, day length in the northern and southern hemispheres are equal with night length.

Figure 7. The constant 23.5˚ axial >lt of Earth alters annual solar radia>on paVerns genera>ng seasonality.

c) Northern Summer Sols>ce, Southern Winter Sols>ce (June 21st)

23.5˚

b) Equinoxes (September 21st and March 21st)

Equator

N So la r Ra

di a>

So la r Ra

di a>

on Solar Radia>on

(a) Rota>on of Earth around the Sun

d) Northern Winter Sols>ce, Southern Summer Sols>ce (December 21st)

June 21

Sept. 21

Dec. 21

March 21

96 Global Climate and Terrestrial Biomes Exercise B: Atmospheric CirculaBon PaCerns

Seasonality and Axial Tilt As Earth moves beyond the Northern Autumnal Equinox, the constancy of Earth’s axial >lt causes the south pole to move closer to the sun, while the north pole moves further away. Days in the north become shorter while days in the south become longer. Likewise, the solar equator begins to move south. On Dec. 21st (the Northern Winter Sols9ce and the Southern Summer Sols9ce), the solar equator reaches la>tude 23.5˚S. On this day, day length is minimized in the northern hemisphere and maximized in the southern hemisphere. As Earth moves beyond the Northern Winter Sols>ce the solar equator move back towards Earth’s equator, and days become longer in the north and shorter in the south. On March 21st (the Northern Vernal Equinox and the Southern Autumnal Equinox), day length is equal to night length in both hemispheres.

10. From your understanding of the Earth’s axial Blt relaBve to the rotaBon around the sun, draw a line indicaBng the posiBon of the solar equator during one annual cycle.

June 21 Sep. 21 Dec. 21 March 21 June 21

23.5˚N

23.5˚S

0˚

Ea rt h La B tu de

S A O N D A F M M J J J

So la r Ra

di a>

90˚N 60˚N 30˚N 0˚ 30˚S 60˚S 90˚S .

Figure 8. Solar radia>on at varying la>tudes waxes and wanes with the solar equator due to Earth’s axial >lt.

11. As the solar equator waxes and wanes due to the axial Ble of Earth, solar radiaBon hiang Earth at varying laBtudes is altered (Figure 8). Why does the amount of solar radiaBon hiang the equator (0˚) vary throughout the year.

11. Why is the variaBon in solar radiaBon throughout the year greater for higher laBtudes than lower laBtudes?

12. As air rises (Fig. 10), do the following phenomena increase, decrease or remain the same?

________________________

Air pressure

Dew point

Temperature

CondensaBon

PrecipitaBon

97 Global Climate and Terrestrial Biomes Exercise C: Regional Effects on Temperature and PrecipitaBon

Evapora9on and Transpira9on Precipita>on is caused by two forces: (1) increasing water vapor and (2) decreasing air temperatures to its dew point. Increasing water vapor happens in a variety of ways. The dominant source of atmospheric water vapor is evapora9on. As a warm air mass moves across a cooler body of water, liquid water evaporates into gaseous water increasing the air mass’s humidity, or the amount of water in an air mass. Secondly, when plants absorb water from their roots, the water is carried through their vascular system and is released to the atmosphere through small pores in their leaves, a process known as transpira9on.

12. As warm air moves across a cool ocean (Fig. 9), do the following phenomena increase, decrease or remain the same?

________________________

EvaporaBon

Humidity

TranspiraBon

Figure 9. A warm air mass moving over cool water.

Figure 10. Condensa>on and precipita>on via adiaba>c cooling.

Condensa9on As air masses cool, water vapor condenses to liquid water, forming clouds. Eventually, this water falls back to Earth as precipita9on (as rain, snow, or ice), when the atmosphere becomes saturated with water vapor. Dew point is the temperature at which water vapor condenses into liquid water at the same rate it evaporates. At temperatures below the dew point, water in the atmosphere condenses and precipitates; while at temperatures above the dew point, atmospheric liquid water vaporizes into

16. In Figure 11, label the windward and leeward sides of the mountain.

98 Global Climate and Terrestrial Biomes Exercise C: Regional Effects on Temperature and PrecipitaBon

Figure 11. The rain shadow effect creates some of the world’s driest deserts.

gaseous water. One mechanism of producing condensa>on by reducing the air mass to its dew point is known as adiaba9c cooling Fig. 10). As air masses rise, air pressure and temperature decrease, and the rela>ve humidity increases. Once this air mass reaches the dew point, water vapor condenses forming liquid water droplets we see as clouds. This process is ini>ated at condensa9on nuclei, where condensing water collides with dust or ice crystals. As more condensa>on occurs, developing water molecules aVach to other condensa>on nuclei or existent water droplets (or ice crystals). Alterna>vely, condensa>on can occur when a wet warm air mass encounters a cooler air mass, a process known as conduc9ve cooling. As the warmer air mass collides with a cooler air mass, its temperature declines to its dew point causing condensa>on. Precipita9on Though water is denser than air, air currents keep water and ice aloh un>l they reach a mass no longer capable of keeping them sustained in air. At this point, the water droplets (or ice crystals) fall to the surface of Earth as precipita>on. Adiaba>c cooling by rising air masses occurs in one of two ways: upward convec>on (Fig. 10) or movement up a physical barrier (Fig. 11). The global scale atmospheric convec>ons cells (Hadley, Ferrell, and Polar) cause air masses to rise, genera>ng precipita>on due to adiaba>c cooling. As an air mass reaches the con>nent from an ocean (or other large body of water), the air mass is forced upward, in a process known as orographic liM. If the rise of con>nent is great enough, the air mass will lose all (or nearly all) of its moisture as precipita>on on the windward side of the mountain chain. As the air passes over the mountain chain, it is stripped of its moisture crea>ng very dry condi>ons on the leeward side of the mountain. This process is known as the rain shadow effect, and is responsible for crea>ng some of the world’s driest deserts. Chile’s Atacama Desert is the driest desert in the world due to a double rain shadow effect from two very tall mountain chain (some rain sta>ons have never received measurable precipita>on). The Sahara Desert is actually made much drier due the the presence of two rain shadow effects, as well. Rain shadow deserts in N. American include the Mohave, Great Basin, Sonoran and Chihuahuan deserts. 14. How does condensaBon from orographic lif and convecBve cooling differ? 15. What similariBes do orographic lif and convecBve cooling share that promote condensaBon?