1、3 濕度地球的水循環過程描述濕度的物理量大氣濕度變化的基本特征一、地球水分循環過程大氣中水汽的含量雖然不多，卻是大氣中極其活躍的成分，在天氣和氣候中扮演著重要的角色。大氣中的水汽含量有很多種測量方法，日常生活中人們最關心的是水汽壓、絕對濕度和相對濕度。水汽壓（e）是大氣壓力中水汽的分壓力，和氣壓一樣用百帕來度量。以前氣壓和水汽壓常以水銀柱的毫米數來測度，1百帕0.75008毫米水銀柱。在一定溫度下空氣中水汽達到飽和時的分壓力，稱為飽和水汽壓(E)。飽和水汽壓隨著氣溫的升高而迅速增加。絕對濕度(a)指單位體積濕空氣中含有的水汽質量，也就是空氣中的水汽密度，單位為克/厘米3或千克/米3。絕對濕度不

2、容易直接測量，實際使用比較少。 相對濕度(f)指空氣的水汽壓e與同一溫度下的飽和水汽壓E之比，以百分數表示。相對濕度的大小表示空氣接近飽和的程度。當f=100時，表示空氣已經達到飽和；未飽和時，f100；過飽和時f100。相對濕度的大小不僅與大氣中水汽含量有關，而且還隨氣溫升高而降低。二、描述濕度變化的物理量比濕（q）:水汽質量與同一容積中空氣的總質量的比值，單位為克/克或者克/千克。表達式：混合比（r）：水汽質量與同一容積中干空氣質量的比值，單位為克/克或者克/千克。表達式：飽和差（d）:在某一溫度下，飽和水汽壓與實際水汽壓之差，表達式：露點溫度(Td)：. 保持空氣中的水汽含量不變，而使之

3、降低溫度，當水汽因降溫而達飽和時之溫度，即為露點溫度。露點溫度也可用來表示水汽含量的多寡，露點溫度愈高，則表示空氣中水汽含量愈多。溫度露點差： 濕度的月變化和年變化在日常生活中，與人們關系最密切的是水汽壓和相對濕度，絕對濕度用得較少。水汽壓的大小與蒸發的快慢有密切關系，而蒸發的快慢在水分供應一定的條件下，主要受溫度控制。白天溫度高，蒸發快，進入大氣的水汽多，水汽壓就大；夜間出現相反的情況，基本上由溫度決定。每天有一個最高值出現在午后，一個最低值出現在清晨。在海洋上，或在大陸上的冬季，多屬于這種情況。 但是在大陸上的夏季，水汽壓有兩個最大值，一個出現在早晨910時，另一個出現在2122時。在91

4、0時以后，對流發展旺盛，地面蒸發的水汽被上傳給上層大氣，使下層水汽減少；2122時以后，對流雖然減弱，但溫度已降低，蒸發也就減弱了。與這個最大值對應得是兩個最小值，一個最小值發生在清晨日出前溫度最低的時候，另一個發生在午后對流最強的時候。三、濕度變化的一些特征濕度的月變化和年變化 相對濕度的大小，不但取決于水汽壓，還取決于溫度。氣溫升高時，雖然地面蒸發加快，水汽壓增大，但這時飽和水汽壓隨溫度升高而增大得更多些，使相對濕度反而減小。同樣的道理，在氣溫降低時，水汽壓減小，但是飽和水汽壓隨溫度下降得更多些，使相對濕度反而增大。所以相對濕度在一天中有一個最大值出現在清晨，一個最低值出現在午后。水汽壓的

5、年變化和氣溫的年變化相似。最高值出現在78月，最低值出現在12月。 相對濕度因為與水汽壓和溫度都有關系，年變化情況比較復雜。一般情況下，相對濕度夏季最小，冬季最大。但是在季風氣候地區，冬季風來自大陸，水汽特別少，夏季風來自海洋，高溫而潮濕，所以相對濕度以冬季最小，而夏季最大。不過濕度的年、日變化，實際上比較復雜。因為除溫度以外，各個地方地面干濕不同，蒸發的水分供給有很大差異。對流運動使水汽從下層向上層傳輸，使低層水汽減少，上層水汽增加，也會影響濕度的日變化。氣流的性質也有很大影響，夏季低緯度海洋來的氣流高溫高濕，冬季高緯度大陸來的氣流寒冷而干燥，也會影響濕度的年、日變化。 水汽壓的地理分布因為

6、緯度、海陸分布、植被性質等等，都能夠決定濕度的大小，因此地球表面濕度分布十分復雜。我們僅僅給出了水汽壓的全球分布。在冬季，赤道是一個水汽壓特別大的地區，水汽壓在30百帕以上。赤道帶不但有廣闊的海洋，即使在大陸上，亞馬遜河和扎伊爾河流域廣闊的熱帶雨林，都有極大的蒸發量，從赤道向兩極，水汽壓很快減少，亞洲東北部減少到接近于零，顯然是與氣溫極低有很大關系。在沙漠地區，特別是撒哈拉沙漠和中亞沙漠，水汽壓都很小，都在10百帕以下。到北半球的夏季，雖然赤道地區仍是水汽壓最大的地帶，但是赤道與兩極之間的水汽壓差別已大大減少。例如，亞洲東北部已增加到10.7百帕，比冬季增大了100倍以上。在沙漠地區也增大到1

7、5百帕以上。 濕度與生產活動濕度作為一個重要的氣象要素，引起人們廣泛注意，由于濕度在說明大氣水分特征上是不可缺少的，因此在氣象觀測和氣候敘述中，都少不了濕度的觀測和描述。但是濕度對工農業生產的直接影響卻研究得很少。一般說，相對濕度如果在30以下，就會加速植物的蒸騰，特別是在高溫和風速較大時，農作物就會枯萎甚至死亡。低相對濕度也會使地面蒸發加速，使干旱更趨嚴重，森林火災在相對濕度小于30時最容易發生。高相對濕度對于作物發芽，磨菇和木耳生產，發酵工業（釀酒、醬油、豆豉等生產）也十分重要，如果相對濕度低于70%80%，生產就會受到影響。倉庫儲存需要在適宜的濕度中，一般水果蔬菜是5070%。濕度太小會

8、加速蒸發，而使水果、蔬菜干枯；濕度太大又會加速霉爛。糧食儲存倉庫相對濕度最好在50以下，以防止霉爛。當然，這些數值還隨溫度而變化。4 風一、風的表述方法二、影響風的因子三、局地風環流四、全球風系一、風的表述方法空氣的流動產生風氣流由不同尺度的運動疊加而成，其中包括很多小尺度的湍流天氣報告中是2min的平均風速風向以100為單位，以正北為基準，順時針方向旋轉。精度要求不高的情況下，也可以用16個方位來表示當風速低于0.25m/s時，稱為靜風風力的大小劃分為12個等級Wind compass describing the sixteen principal bearings used to mea

9、sure wind direction. This compass is based on the 360 degrees found in a circle Meteorological instruments used to measure wind speed and direction. Wind speed is commonly measured with an anemometer（風速計）. An anemometer consists of three open cups attached to a rotating spindle. The speed of rotatio

10、n is then converted into a measurement of wind speed. Wind direction is measured with a wind vane（風向標）. On the photograph above, the wind vane instrument has a bullet shaped nose attached to a finned tail by a metal bar Beaufort wind speed scale Beaufort CodeSpeed Miles per HourSpeedKilometers per H

11、ourDescriptionEffects on the Environment0 1 75 120hurricanesevere damage to buildings and trees二、影響風的因子氣壓梯度力Coriolis力摩擦力 Pressure Gradient Force (PGF)氣壓梯度力Association between wind speed and distance between isobars. In the illustration above thicker arrows represent relatively faster winds. Effect o

12、f pressure gradient on wind speedCoriolis ForceAn apparent force that is due to the rotation of the earth.Not a real force in the sense that it cannot cause a motion.As an earth-bound observer, we are not aware of the rotation of the earth.Coriolis effect acts only on objects moving with respect to

13、the earths surface.Coriolis ForceAnt and the Ice Cube:You want to smash theant up against the spindle.What do you see?What does the ant see?Coriolis Force You see:The ice cube moves in a straightline.The ant turns with turntable.Coriolis ForceThe ant sees:The ant sees a curved path, so an unbalanced

14、 force must be acting to alter the course of the ice cube.This “force” is called the Coriolis Force.The Coriolis force turns things to the right in the Northern Hemisphere.Coriolis ForceConsider a rocket fired north from the equator:Since the earth rotates as a solidbody, the equator will movefaster

15、 than land near the poles.The green guy willhave to run faster thanthe orange guy to keepup.Coriolis ForceAs the rocket moves farthernorth, it keeps its eastwardspeed that it had when it leftthe earth.Farther north, the earths surfaceis moving slower so it will appear,to an observer on the earth, th

16、atthe rocket will be moving eastward.Coriolis ForceFor a southward moving rocket,the initial eastward speed is slowcompared to the rotational speedof the earth near the equator.The earth has a larger eastwardmotion near the equator thanthe rocket has so the rocket willappear to have a westwardcompon

17、ent to its motion.The rocket will have turned tothe right.Coriolis ForceAffects direction, not speed of objectMaximum at the polesZero at the equatorProportional to wind speedAlways acts to the right of the motion in Northern Hemisphere.Always acts to the left of the motion in the Southern Hemispher

18、e.The strength of Coriolis force is influenced by latitude and the speed of the moving object Coriolis parameter f = 2sinCoriolis Force+PGFApply this knowledge to wind forced by horizontal pressure gradient force and balanced by the Coriolis force:Net Force = PGFH + CoLH980 mb984 mb988 mbStart with

19、no velocity - what are the forces on the parcel?Coriolis Force+PGFLH980 mb984 mb988 mbPGFSince there is no initial speed, the Coriolis force is zero. ButThere is PGF. As soon as the parcel starts to move, the Coriolis force is no longer zero.The Coriolis force is small at first, but as the speedincr

20、eases, the Coriolis force increases.Coriolis Force+PGFLH980 mb984 mb988 mbPGFCoVThe PGF still exceeds the Coriolis force. The parcel continuesto increase in speed, and the Coriolis force continues to increase in magnitude, further turning the parcels directionto the right (Northern Hemisphere!).Geos

21、trophic WindLH980 mb984 mb988 mbPGFCoVEventually the Coriolis and PGF balance each other. Sincethere is no unbalanced force, there is no acceleration.The wind continues to move at constant speed in a straightline (remember, no Friction). This is called:Geostrophic WindA geostrophic wind flows parall

22、el to the isobars. In this model of wind flow in the Northern Hemisphere, wind begins as a flow of air perpendicular to the isobars (measured in millibars) under the primary influence of the pressure gradient force (PGF). As the movement begins, the Coriolis force (CF) begins to influence the moving

23、 air causing it to deflect to the right of its path. This deflection continues until the pressure gradient force and Coriolis force are opposite and in balance with each otherThe balance of forces that create a gradient wind in the Northern Hemisphere (PGF = pressure gradient force; CF = Coriolis fo

24、rce; Ce = centripetal force; /Regents/physics/phys06/bcentrif/ ). In this diagram, CF = Ce + PGF for the low, and PGF = CF + Ce for the high.SummaryHow the wind is startedPressure gradientsHow the wind is turnedCoriolisCirculation patterns of high and low pressure systems in the North and South Hemi

25、sphere.Coriolis Force + PGF + Friction ForceRecallLets apply this to the winds. RecallNet Force = PGF + G + Fr + CoConsider flow aloft:Away from the groundFriction 0 Net Force = PGF + G + CoRemember that the pressure gradient force is horizontal andvertical. We shall assume that the vertical pressur

26、e gradientforce is exactly balanced by the gravity force.三、局地風環流局地熱力效應所產生的環流山谷風海陸風Cross-section of the atmosphere with uniform horizontal atmospheric pressure Fig. 6.22Development of air flow in the upper atmosphere because of surface heating Development of a closed atmospheric circulation cell beca

27、use of surface heating Daytime development of sea breeze 海陸風Nighttime development of land breeze 海陸風Winter and summer monsoon wind patterns for southeast Asia MonsoonDaytime development of valley breeze 山谷風Nightime development of mountain breeze 山谷風四、全球風系與氣壓帶分布匹配低緯度信風帶中高緯度西風帶極地東風帶 赤道地區受熱上升的氣流，流向極地；在

28、地球自轉偏向力的作用下，逐步變為偏西氣流，阻滯了空氣的北上，在300附近積聚下沉；下沉到達地面后一支回流赤道，形成了Hadley環流圈；另一支繼續北上，與極地下沉的南流氣流在600附近匯合上升；上升到高空一支南流形成中緯度Ferrel環流圈；一支北流形成極地環流圈。與三圈環流相對應，是所謂的“三風四帶”，即：極地東風(Polar Easterlies)中緯度西風(Westerlies)低緯度信風(Trades)極地高壓帶(Polar High)副極地低壓帶(Subpolar Low)副熱帶高壓帶(Subtropical High)赤道低壓帶(Equatorial Low)赤道低壓帶又稱為熱帶輻

29、合帶(InterTropical Convergence Zone)，簡稱ITCZ，它是南北半球兩支信風在赤道地區匯合而形成的低氣壓區1、熱帶地區和極地低層為東風2、中緯度為寬廣的西風帶3、冬季中緯度西風增強且范圍擴大4、夏季高層熱帶東風加強且范圍擴大The “Trade Winds” and Oceanic Trade RoutesThe zones of the Earths major winds: the trade winds and the westerlies.There were two situations that the sailing captains of old h

30、ad to avoid at all costs. One was to be captured by a pirate. The other was to have the wind die down to nothing and have to sit around in the doldrums, sails flapping, with no prospect of getting fresh water or meat or vegetables any time soon. Fortunately, in the zone of the trade winds the second

31、 problem arose but rarely. The trade winds (named centuries ago by sailors on trade ships) are quite reliably blowing from the east at an angle to the equator such that they bring air from higher latitudes to the equatorial zone of convergence. Thus, the smart captains sought out the trades to go we

32、st (and they sailed fast). In the high pressure regions at the eastern edge of the ocean basins where the trade winds originate, the climate is typically hot, sunny, and dry (Baja California, for example); as the winds move westward across the oceans, they gain moisture, which is eventually dumped a

33、t the western side of the ocean basins.輔助材料一“貿易風”及Hadley環流的發現Discovery of the Trade Winds and the Voyage of ColumbusIt was the Genoese seaman, explorer and adventurer Christopher Christopher Columbus (1451-1506), who discovered the trade winds. These winds carried his three modest-size sailing vesse

34、ls all across the Atlantic at its widest, from the Canary Islands to the Bahamas, a distance of 5400 miles, in 36 days, in 1492. Centuries later (in 1970), the Norwegian seaman and amateur archeologist Thor Heyerdahl showed that the trade winds are capable of blowing a sailing vessel built of reeds

35、(made to resemble an Egyptian craft) from Morocco to the Caribbean. He suggested that the idea of building pyramids could have reached Central America by Egyptians traveling in this fashion in ancient times. (What do you think? Is this a scientific hypothesis?) In any event, the trade winds powered

36、transcontinental trade for centuries, A diagram illustrating Columbus first route to the New World (in red) and the trade winds he used to get there (in black). Columbus return trip was powered by the Westerlies. and they are the most conspicuous part of the wind system on the planet and the most st

37、eady (demonstrated by the phrase, the wind blows trade, that is, on track). When the trade winds hit the western edge of an ocean basin, the winds turn first toward the poles, and then loop back east to become prevailing westerlies (winds flowing to the east from the west). These westerlies, for exa

38、mple, are what powered Columbus sailing vessel on the return trip to Europe. The westerlies are also responsible for the far better surfing on the Pacific side of North America compared to the Atlantic side. On the Pacific side, the westerlies blow in the same direction as waves rolling toward shore

39、 from storms out at sea, building up their height. In the Atlantic, the prevailing winds blow against the incoming waves, shrinking them down to sizes that are less than adequate for surfers. The Discovery of Hadley CellsThe trade winds are part of a circulation of air, a cell when seen in profile,

40、which starts with rising air in the tropics. This rising air is driven by the energy received from the Sun, which is virtually overhead at the equator all year. The airs vertical motion implies convergence, that is, the air rising from the bottom of the atmosphere is replaced by winds blowing from h

41、igher latitudes. This mechanism for making trade winds was first postulated by the famous astronomer and atmospheric physicist Edmond Halley, in 1686. There was, however, a problem with Halleys proposition: he did not explain why the winds actually blow as much from the east as is observed. What kee

42、ps the trade winds from going straight to the convergence zone at the equator? This conundrum was solved by the English meteorologist George Hadley (1685-1768) and this is why we talk about a Hadley Cell rather than a Halley Cell.” Hadley realized that wind particles moving toward the equator would

43、come from a region of lower eastward velocity and enter a region of higher eastward velocity as they moved toward the equator. Thus, the wind would have a westward motion, as indeed observed. George Hadley published his theory in a famous paper Concerning the Cause of the General Trade Winds, in 173

44、5. This was exactly one hundred years before Gustave-Gaspard Coriolis (1792-1843) produced the equations describing motions in a rotating coordinate system. Thus, Hadley had it right, but we now credit Coriolis for the description of how the winds bend toward the west of their path when they move to

45、ward the equator. Either way, the trade winds have a strong easterly component, and they feed the convergence zone and the rise of the air there. Diagram illustrating how Hadley cells create the trades. The rising warm air in the tropics creates a “void” that is filled by air coming from higher lati

46、tudes, thus giving rise to the trade winds.The compensating air flow for the trade wind is a kind of anti-trade wind in the uppermost troposphere, located above the trades, where the flow of air is going east and away from the convergence. The compensating flow for the rise of the air in the convergence is the downward motion of air in the desert zone, centered between 20 and 30 latitude.