湿度

nullnull§3 湿度地球的水循环过程 描述湿度的物理量 大气湿度变化的基本特征null一、地球水分循环过程null大气中水汽的含量虽然不多，却是大气中极其活跃的成分，在天气和气候中扮演着重要的角色。大气中的水汽含量有很多种测量，日常生活中人们最关心的是水汽压、绝对湿度和相对湿度。 　　水汽压（e）是大气压力中水汽的分压力，和气压一样用百帕来度量。以前气压和水汽压常以水银柱的毫米数来测度，1百帕＝0.75008毫米水银柱。在一定温度下空气中水汽达到饱和时的分压力，称为饱和水汽压(E)。饱和水汽压随着气温的升高而迅速增加。 　　绝对湿度(a)指单位体积湿空气中含有的水汽质量，也就是空气中的水汽密度，单位为克/厘米3或千克/米3。绝对湿度不容易直接测量，实际使用比较少。 相对湿度(f)指空气的水汽压e与同一温度下的饱和水汽压E之比，以百分数示。相对湿度的大小表示空气接近饱和的程度。当f=100％时，表示空气已经达到饱和；未饱和时，f<100％；过饱和时f＞100％。相对湿度的大小不仅与大气中水汽含量有关，而且还随气温升高而降低。二、描述湿度变化的物理量null比湿（q）:水汽质量与同一容积中空气的总质量的比值，单位为克/克或者克/千克。表达式： 混合比（r）：水汽质量与同一容积中干空气质量的比值，单位为克/克或者克/千克。表达式： null饱和差（d）:在某一温度下，饱和水汽压与实际水汽压之差，表达式： 露点温度(Td)：. 保持空气中的水汽含量不变，而使之降低温度，当水汽因降温而达饱和时之温度，即为露点温度。露点温度也可用来表示水汽含量的多寡，露点温度愈高，则表示空气中水汽含量愈多。 温度露点差： null 湿度的月变化和年变化 　　在日常生活中，与人们关系最密切的是水汽压和相对湿度，绝对湿度用得较少。 　　水汽压的大小与蒸发的快慢有密切关系，而蒸发的快慢在水分供应一定的条件下，主要受温度控制。白天温度高，蒸发快，进入大气的水汽多，水汽压就大；夜间出现相反的情况，基本上由温度决定。每天有一个最高值出现在午后，一个最低值出现在清晨。在海洋上，或在大陆上的冬季，多属于这种情况。 但是在大陆上的夏季，水汽压有两个最大值，一个出现在早晨9～10时，另一个出现在21～22时。在9～10时以后，对流发展旺盛，地面蒸发的水汽被上传给上层大气，使下层水汽减少；21～22时以后，对流虽然减弱，但温度已降低，蒸发也就减弱了。与这个最大值对应得是两个最小值，一个最小值发生在清晨日出前温度最低的时候，另一个发生在午后对流最强的时候。三、湿度变化的一些特征null湿度的月变化和年变化 相对湿度的大小，不但取决于水汽压，还取决于温度。气温升高时，虽然地面蒸发加快，水汽压增大，但这时饱和水汽压随温度升高而增大得更多些，使相对湿度反而减小。同样的道理，在气温降低时，水汽压减小，但是饱和水汽压随温度下降得更多些，使相对湿度反而增大。所以相对湿度在一天中有一个最大值出现在清晨，一个最低值出现在午后。 　　水汽压的年变化和气温的年变化相似。最高值出现在7～8月，最低值出现在1～2月。 相对湿度因为与水汽压和温度都有关系，年变化情况比较复杂。一般情况下，相对湿度夏季最小，冬季最大。但是在季风气候地区，冬季风来自大陆，水汽特别少，夏季风来自海洋，高温而潮湿，所以相对湿度以冬季最小，而夏季最大。不过湿度的年、日变化，实际上比较复杂。因为除温度以外，各个地方地面干湿不同，蒸发的水分供给有很大差异。对流运动使水汽从下层向上层传输，使低层水汽减少，上层水汽增加，也会影响湿度的日变化。气流的性质也有很大影响，夏季低纬度海洋来的气流高温高湿，冬季高纬度大陆来的气流寒冷而干燥，也会影响湿度的年、日变化。null 水汽压的地理分布 　　因为纬度、海陆分布、植被性质等等，都能够决定湿度的大小，因此地球表面湿度分布十分复杂。我们仅仅给出了水汽压的全球分布。 　　在冬季，赤道是一个水汽压特别大的地区，水汽压在30百帕以上。赤道带不但有广阔的海洋，即使在大陆上，亚马逊河和扎伊尔河流域广阔的热带雨林，都有极大的蒸发量，从赤道向两极，水汽压很快减少，亚洲东北部减少到接近于零，显然是与气温极低有很大关系。在沙漠地区，特别是撒哈拉沙漠和中亚沙漠，水汽压都很小，都在10百帕以下。 　　到北半球的夏季，虽然赤道地区仍是水汽压最大的地带，但是赤道与两极之间的水汽压差别已大大减少。例如，亚洲东北部已增加到10.7百帕，比冬季增大了100倍以上。在沙漠地区也增大到15百帕以上。null 湿度与生产活动 　　湿度作为一个重要的气象要素，引起人们广泛注意，由于湿度在说明大气水分特征上是不可缺少的，因此在气象观测和气候叙述中，都少不了湿度的观测和描述。但是湿度对工农业生产的直接影响却研究得很少。一般说，相对湿度如果在30％以下，就会加速植物的蒸腾，特别是在高温和风速较大时，农作物就会枯萎甚至死亡。 　　低相对湿度也会使地面蒸发加速，使干旱更趋严重，森林火灾在相对湿度小于30％时最容易发生。高相对湿度对于作物发芽，磨菇和木耳生产，发酵工业（酿酒、酱油、豆豉等生产）也十分重要，如果相对湿度低于70%～80%，生产就会受到影响。 　　仓库储存需要在适宜的湿度中，一般水果蔬菜是50～70%。湿度太小会加速蒸发，而使水果、蔬菜干枯；湿度太大又会加速霉烂。粮食储存仓库相对湿度最好在50％以下，以防止霉烂。当然，这些数值还随温度而变化。null§4 风一、风的表述方法 二、影响风的因子 三、局地风环流 四、全球风系null一、风的表述方法空气的流动产生风 气流由不同尺度的运动叠加而成，其中包括很多小尺度的湍流 天气中是2min的平均风速 风向以100为单位，以正北为基准，顺时针方向旋转。精度要求不高的情况下，也可以用16个方位来表示 当风速低于0.25m/s时，称为静风 风力的大小划分为12个等级nullWind compass describing the sixteen principal bearings used to measure wind direction. This compass is based on the 360 degrees found in a circle nullMeteorological instruments used to measure wind speed and direction. Wind speed is commonly measured with an anemometer（风速计）. An anemometer consists of three open cups attached to a rotating spindle. The speed of rotation is then converted into a measurement of wind speed. Wind direction is measured with a wind vane（风向标）. On the photograph above, the wind vane instrument has a bullet shaped nose attached to a finned tail by a metal bar nullBeaufort wind speed scale  null二、影响风的因子气压梯度力 Coriolis力 摩擦力null Pressure Gradient Force (PGF)null气压梯度力nullAssociation between wind speed and distance between isobars. In the illustration above thicker arrows represent relatively faster winds. nullEffect of pressure gradient on wind speedCoriolis ForceCoriolis 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 the earth’s surface.Coriolis ForceCoriolis ForceAnt and the Ice Cube:You want to smash the ant up against the spindle. What do you see? What does the ant see?Coriolis ForceCoriolis Force You see:The ice cube moves in a straight line. The ant turns with turntable. Coriolis ForceCoriolis ForceThe ant sees:The ant sees a curved path, so an unbalanced 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 ForceCoriolis ForceConsider a rocket fired north from the equator:Since the earth rotates as a solid body, the equator will move faster than land near the poles.The green guy will have to run faster than the orange guy to keep up.Coriolis ForceCoriolis ForceAs the rocket moves farther north, it keeps its eastward speed that it had when it left the earth. Farther north, the earth’s surface is moving slower so it will appear, to an observer on the earth, that the rocket will be moving eastward.Coriolis ForceCoriolis ForceFor a southward moving rocket, the initial eastward speed is slow compared to the rotational speed of the earth near the equator. The earth has a larger eastward motion near the equator than the rocket has so the rocket will appear to have a westward component to its motion. The rocket will have turned to the right.nullCoriolis ForceCoriolis ForceAffects direction, not speed of object Maximum at the poles Zero at the equator Proportional to wind speedAlways acts to the right of the motion in Northern Hemisphere. Always acts to the left of the motion in the Southern Hemisphere.nullThe strength of Coriolis force is influenced by latitude and the speed of the moving object Coriolis parameter f = 2sinCoriolis Force+PGFCoriolis 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 no velocity -- what are the forces on the parcel?Coriolis Force+PGFCoriolis Force+PGFLH980 mb984 mb988 mbPGFSince there is no initial speed, the Coriolis force is zero. But There 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 speed increases, the Coriolis force increases.Coriolis Force+PGFCoriolis Force+PGFLH980 mb984 mb988 mbPGFCoVThe PGF still exceeds the Coriolis force. The parcel continues to increase in speed, and the Coriolis force continues to increase in magnitude, further turning the parcel’s direction to the right (Northern Hemisphere!).Geostrophic WindGeostrophic WindLH980 mb984 mb988 mbPGFCoVEventually the Coriolis and PGF balance each other. Since there is no unbalanced force, there is no acceleration. The wind continues to move at constant speed in a straight line (remember, no Friction). This is called:Geostrophic WindnullA geostrophic wind flows parallel 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 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 othernullThe balance of forces that create a gradient wind in the Northern Hemisphere (PGF = pressure gradient force; CF = Coriolis force; Ce = centripetal force; http://regentsprep.org/Regents/physics/phys06/bcentrif/ ). In this diagram, CF = Ce + PGF for the low, and PGF = CF + Ce for the high.SummarySummaryHow the wind is started Pressure gradients How the wind is turned CoriolisnullCirculation patterns of high and low pressure systems in the North and South Hemisphere.Coriolis Force + PGF + Friction ForceRecallRecallLet’s apply this to the winds. Recall Net Force = PGF + G + Fr + Co Consider flow aloft: Away from the ground Friction  0 Net Force = PGF + G + Co Remember that the pressure gradient force is horizontal and vertical. We shall assume that the vertical pressure gradient force is exactly balanced by the gravity force.null三、局地风环流局地热力效应所产生的环流 山谷风 海陆风nullCross-section of the atmosphere with uniform horizontal atmospheric pressure nullFig. 6.22nullDevelopment of air flow in the upper atmosphere because of surface heating nullDevelopment of a closed atmospheric circulation cell because of surface heating nullDaytime development of sea breeze 海陆风nullNighttime development of land breeze 海陆风nullWinter and summer monsoon wind patterns for southeast Asia MonsoonnullDaytime development of valley breeze 山谷风nullNightime development of mountain breeze 山谷风null四、全球风系与气压带分布匹配 低纬度信风带 中高纬度西风带 极地东风带null 赤道地区受热上升的气流，流向极地；在地球自转偏向力的作用下，逐步变为偏西气流，阻滞了空气的北上，在300附近积聚下沉；下沉到达地面后一支回流赤道，形成了Hadley环流圈；另一支继续北上，与极地下沉的南流气流在600附近汇合上升；上升到高空一支南流形成中纬度Ferrel环流圈；一支北流形成极地环流圈。null与三圈环流相对应，是所谓的“三风四带”，即： 极地东风(Polar Easterlies) 中纬度西风(Westerlies) 低纬度信风(Trades) 极地高压带(Polar High) 副极地低压带(Subpolar Low) 副热带高压带(Subtropical High) 赤道低压带(Equatorial Low) 赤道低压带又称为热带辐合带(InterTropical Convergence Zone)，简称ITCZ，它是南北半球两支信风在赤道地区汇合而形成的低气压区null1、热带地区和极地低层为东风 2、中纬度为宽广的西风带 3、冬季中纬度西风增强且范围扩大 4、夏季高层热带东风加强且范围扩大nullThe “Trade Winds” and Oceanic Trade Routes The zones of the Earth’s major winds: the trade winds and the westerlies.There were two situations that the sailing captains of old had 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 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 west (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 at the western side of the ocean basins. 辅助材料一 “贸易风”及Hadley环流的发现nullDiscovery of the Trade Winds – and the Voyage of Columbus It 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 vessels 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 (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 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 steady (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 example, 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 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. nullThe Discovery of Hadley Cells The trade winds are part of a circulation of air, a "cell" when seen in profile, 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 air’s vertical motion implies "convergence," that is, the air rising from the bottom of the atmosphere is replaced by winds blowing from higher 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 Halley's proposition: he did not explain why the winds actually blow as much from the east as is observed. What keeps 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 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 1735. 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 toward 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 latitudes, 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. This falling air heats up as the pressure increases and is therefore greatly under-saturated with water vapor. This produces clear skies, evaporation at the sea surface (or soil) and general aridity. (To verify this check the positions of the major deserts on a map.) Surface air from subtropical regions returns towards the equator (as the Trade Winds) to replace the rising air, so completing the cycle of air circulation within the Hadley cell. nullnullHorse Latitudes Between about 30° to 35° north and 30° to 35° south of the equator lies the region known as the horse latitudes or the subtropical high. This region of subsiding dry air and high pressure results in weak winds. Tradition states that sailors gave the region of the subtropical high the name "horse latitudes" because ships relying on wind power stalled; fearful of running out of food and water, sailors threw their horses and cattle overboard to save on provisions. (It's a puzzle why sailors would not have eaten the animals instead of throwing them overboard.) The Oxford English Dictionary claims the origin of the term "uncertain." Major deserts of the world, such as the Sahara and the Great Australian Desert, lie under the high pressure of the horse latitudes. The region is also known as the Calms of Cancer in the northern hemisphere and the Calms of Capricorn in the southern hemisphere辅助材料二 什么是“Horse Latitudes”null思考题1、天气报告中的风速是（ ）的平均风速。 a. 15min. b. 10min. c. 5min. d. 2min. 2、天气报告中的风向是以（ ） 以正北为基准，顺时针方向旋转 b. 以正北为基准，逆时针方向旋转 c. 以正南为基准，顺时针方向旋转 d. 以正南为基准，逆时针方向旋转 3、影响风的因子有哪些？ 4、地球表面有哪些盛行的风带？