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1、Like a fish in the ocean, man is confined to a very shallow layer of atmosphere.The gaseous envelope of the Earth is physically inhomogeneous in both the vertical and horizontal directions, although the horizontal inhomogeneity is much less marked than the vertical inhomogeneity. Various criteria ha
2、ve been devised for dividing the atmosphere into layers. This division can be based on the nature of the vertical temperature profile, on the gaseous composition of the air at different altitudes, and the effect of the atmosphere on aircraft at different altitudes, etc. The division based on the var
3、iation of the air temperature with altitude is used most commonly in the meteorological literature. According to a publication of the agrological commission of the World Meteorological Organization (WMO) in 1961, the Earths atmosphere, is divided into five main layers: the troposphere, the stratosph
4、ere, the mesosphere, the thermosphere and the exosphere. These layers are bounded by four thin transition regions: the tropospause, the stratospause, the mesospause, the thermospause . The troposphere is the lower layer of the atmosphere between the Earths surface and the tropopause. The temperature
5、 drops with increasing height in the troposphere, at a mean rate of 6.5 per kilometer (lapse rate). The upper boundary of the troposphere lies at a height of approximately 8 to 12 km in the polar and troposphere contains about 75% of the total mass of atmospheric air, while in the tropics it contain
6、s about 90%. The tropoause is an intermediate layer in which either a temperature in version or an isothermal temperature distribution is observed. The stratosphere is the atmospheric layer above the troposphere. In the stratosphere the temperature either increases with height or remains nearly cons
7、tant. In the lower part of the stratosphere (up to approximately 20 km above the Earths surface) the temperature is practically constant (about 56 ). While further up the temperature increases with altitude at a rate of about 1 /km at heights of 20 to 30 km and about 2.8 /km at altitudes from 32 to
8、47 km. Under the standard conditions the temperature at the 47 km level is normally -2.5 . This increase in temperature with height is due to the absorption of UV solar radiation by ozone molecules. It should be noted that about 99% of the total mass of atmospheric air is concentrated in the troposp
9、here and stratosphere, which extend up to an latitude of 30 or 35 km. The stratopause is an intermediate layer between the stratosphere and the mesosphere (in the altitude region from 47 to 52 km ), in which the temperature remains constant at about 0.The thermosphere is the atmospheric layer above
10、the mesopause. The temperature in this layer increases with increasing altitude, reaching about 2000 at about 450km, the mean height of the upper boundary of the thermosphere. The temperature increase in this layer is mainly caused by the absorption of UV solar radiation by oxygen molecules, which d
11、issociate as a result of this.The exosphere is the furthest out and the least studied part of the upper atmosphere. It is located above 450km altitude. The air density in the exosphere is so low that atoms and molecules can escape from it into interplanetary space.Finally, along with the above divis
12、ion of the atmosphere, we will also make use of a division based on the extent of atmospheric interaction with the Earths surface. According to this principe,the atmosphere is usually divided into a so called boundary layer (sometimes also called the friction layer) and the free atmosphere. The atmo
13、spheric boundary layer(up to 1 or 1.5 km) is influenced considerably by the Earths surface and by eddy-viscosity forces. At the same time, we can neglect, as a first approximation, the influence of eddy-viscosity forces in the free atmosphere.Of all the above atmospheric layers, only the troposphere
14、(especially its boundary layer) is characterized by a marked instability of the vertical distribution of the meteorological parameters. It is in this layer that both temperature inversions and superadiabatic temperature variations with height are observed.The Earths atmosphere is a mixture of gases
15、and aerosols, the latter being the name given to a system comprised of small liquid and solid particles distributed in the air. Air is not a specific gas :rather, it is a mixture of many gases. Some of them, such as nitrogen, oxygen, argon, neon, and so on, may be regarded as permanent atmospheric c
16、omponents that remain in fixed proportions to the total gas volume. Other constituents such as water vapor, carbon dioxide, and ozone vary in quantity from place to place and form time to time. The principal sources of nitrogen, the most abundant constituent of air, are decaying from agricultural de
17、bris, animal matter, and volcanic eruption. On the other side of the ledger, nitrogen is removed from the atmosphere by biological processes involving plants and sea life. To a lesser extent, lightning and high temperature combustion processes convert nitrogen gas to nitrogen compounds that are wash
18、ed out of the atmosphere by rain or snow. The destruction of nitrogen is in the atmospheres in balance with production. Oxygen, a gas crucial to life on Earth, has an average residence time in the atmosphere of about 3000 years. It is produced by vegetation that, in the photosynthetic growth process
19、, takes up carbon dioxide and releases oxygen. It is removed from the atmosphere by humans and animals, whose respiratory systems are just the reverse of those of the plant communities. We inhale oxygen and exhale carbon dioxide. Oxygen dissolves in the lakes, rivers and oceans, where it serves to m
20、aintain marine organisms. It is also consumed in the process of decay of organic matter and in chemical reactions with many other substances. For example, the rusting of steel involves its oxidation. From the human point of view, the scarce, highly variable gases are of great importance. The mass of
21、 water vapor, that is,H2O in a gaseous state, in the atmosphere is relatively small and is added to and removed from the atmosphere relatively fast. As a result ,the average residence time of water vapor is only 11 days. Water vapor is the source of rain and snow, without which we could not survive.
22、 From common experiences it is well known that the water vapor content of air varies a great deal. In a desert region the concentration of water vapor can be so low as to represent only a tiny fraction of the air volume. At the other extreme, in hot, moist air near sea level, say over an equatorial
23、ocean, water vapor may account for as much as perhaps 5 percent of the air volume.There are large variations of atmospheric water vapor from place to place and from time to time, but the total quantity over the entire Earth is virtually constant. The same can not be said about carbon dioxide (co2).T
24、he concentration of this sparse but important gas has been increasing for the last hundred years or so. Carbon dioxide is added to the atmosphere by the decay of plant material and humus in the soil ,and by the burning of fossil fuels: coal, oil, and gas. The principal sinks of co2 are the oceans an
25、d plant life that uses co2 in photosynthesis. In the middle 1980s,atmospheric chemists were still debating about the effects on atmospheric co2 of burning, harvesting and clearing of forests. The oceans take up large amounts of co2,about half the amount released by fossil fuel combustion. It is expe
26、cted that this fraction will diminish with the passing decades whereas the total mass of co2 released will increase ,at least through the early part of the next century. During the 1980s atmospheric co2 was accumulating at a rate of about 1 part per million (ppm)of air per year, but it is expected t
27、o increase more rapidly in decades to come .In1983 it averaged about 340 ppm of air. Ozone(o3),another important, highly variable gas, occurs mostly at upper altitudes ,but it is also found in urban localities having a great deal of industry and automotive traffic and a generous supply of sunshine.
28、In cities such as Los Angeles, ozone concentration may be more than 0.1ppm in extreme cases. Most atmospheric ozone concentrations often exceed 1.0ppm and may be large as 10 ppm. They vary greatly with latitude, season ,time of day, and weather patterns. The high-altitude ozone layer is maintained b
29、y photochemical reactions. The ozone layer is important because, by absorbing UV radiation in the upper atmosphere, it reduces the amount reaching the surface of the Earth, exposure to increased doses of ultraviolet rays would cause more severe sunburns and increase the risk of skin cancers. Biologi
30、sts indicate that a substantial increase in UV radiation could also affect other components of the biosphere.Certain gages, if they exist in sufficiently high concentrations, can be toxic to people, animal and plant life. For example, when ozone occurs in high concentrations, it is toxic to biologic
31、al organisms. This does not happen often, but in heavily polluted localities such as Los Angeles, ozone near the ground sometimes is sufficiently abundant to cause leaf damage to certain plant species. Very large quantities of potentially hazardous gases are introduced into the atmosphere as a resul
32、t of human activities. Air pollutants are emitted from furnaces, factories, refineries, and engines, particularly automobile engines. All these things and others like them burn fossil fuels: coal, oil, gasoline, and kerosene. In the process they emit gases and smoke particles that may spend a great
33、deal of time in the atmosphere reacting with other substances and causing the formation of toxic compounds.The most widespread and potentially hazardous gaseous pollutants are carbon monoxide ,sulfur dioxide, nitrogen oxide, and hydrocarbons. The last of these compounds comes from vaporized gasoline
34、 and other petroleum products. 就像海洋中的鱼一样,人类被局限在大气中一个非常狭窄的层次之内。虽然地球的大气层在水平方向上的不均匀性比在垂直高度上的不均匀性要小得多。但它确确实实在水平和垂直两个方向上都是不均匀的。人们设计了各种各样的标准来划分大气的层次。有的基于垂直温度廓线的性质，有的根据空气在不同高度上的大气成分，有的根据大气在不同高度上对飞机的影响来划分等等。根据空气温度随高度的变化来划分（大气的层次）是气象文献中用得最普遍的一种划分方法。根据1961年世界气象组织大气学委会公布的标准，地球大气被划分为5个主要层次：对流层，平流层，中层，热成层以及外逸层。这
35、些层次之间邻接着4个浅薄的过渡区域：对流层顶，平流层顶，中层顶以及热成层顶。对流层是介于地球表面和对流层顶之间的大气低层在对流层中，温度以平均6.5/km的递减率随高度的增加而降低，其上边界在极地和中纬度地区大约位于812Km的高度，在热带地区大约位于16-18Km的高度。在极地和中纬度，对流层包含了大气层中空气质量的75%左右，然而在热带地区，包含了大约90%。对流层顶是一个中间层次，据观测，其温度是逆温或是等温分布。平流层是位于对流层之上的大气层，在平流层中，温度或是随高度增加，或是几乎保持定常。在平流层的低层（直到地球表面之上大约20Km）温度实际是一个常数（大约-56 ）。然而再向上，
36、大约20Km30Km的高度，温度随高度以1 /km的速度增加，从30Km47Km高度上，以2.8 /km的速率增加。在标准情况下，47Km高度上正常的温度是-2.5摄氏度。温度随高度的增加是由于太阳辐射的紫外线被臭氧分子吸收的缘故。值得一提的是大气层中空气总质量的99%都集中在对流层和平流层中，一直伸展到30-35Km的高度上。平流层顶 位于平流层和中间层的中间层（大约从47-52Km的高度上），平流层顶温度保持定常，约为0摄氏度。 热成层是位于中层顶上的大气层，其温度随高度的增加而增加，到热成层上边界的平均高度，即大约在450Km高度上，达到大约2000摄氏度。该层中温度的增加主要是因为太阳
37、辐射的紫外线被氧分子吸收，分解所致。 外逸层是最远的，也是研究最少的大气的上层部分。它位于大约450Km的高度上。外逸层中空气的密度非常小，以致原子和分子都能逃逸到星际空间。最后，除了以上对大气的划分以外，我们也可以根据大气和地面的相互作用得到另一种分法。根据此原则，大气通常被划分为所谓的边界层（有时也称摩擦层）和自由大气。地球表面和涡度粘滞力对大气边界层（知道1-1.5Km）有相当大的影响；同时，作为一级近似，在自由大气中我们可以忽略涡度粘滞力的影响。 以上这些大气层中，只有对流层（尤其是边界层）中气象参数的垂直分布具有显著的不稳定性的特征。人们观测到该层中存在逆稳和温度随高度超绝热变化。地
38、球大气是气体和气溶胶的混合物，所谓的气溶胶指的是由分布在空气中的微小的液体和固体颗粒组成的系统。空气不是一种特殊的气体，而是由许多气体混合成的。其中一些气体，如，氮，氧，氩，氖等，作为空气的永久组成成分，总是以固定的比例存在于大气中。其中一些成分，如，水汽，二氧化碳和臭氧的含量是随时间和地点的不同而改变。氮在空气中的含量最多，其中主要来源是腐烂的作物残渣，动物尸体及火山爆发。另一方面，大气中的氮又为包括植物和海洋生物在内的生物过程的所消耗。闪电及高温燃烧过程将少量氮气转化为氮化物，再由雨雪带出大气。大气中氮的损耗和产生是平衡的。地球生命必不可少的气体氧气，在大气中已有三千年左右的时间了。氧气由
39、植物释放出来，即通过植物的光合作用吸收二氧化碳，释放氧气，被人类和动物所吸收，人和动物的呼吸系统同植物的呼吸系统恰恰相反。我们吸进氧气呼出二氧化碳，氧气溶进湖，河和海洋中，来维持海洋生物的生存。在同其他物质发生化学反应或是有机物的腐烂过程也要消耗氧气，比如说钢铁生锈就涉及到氧化过程。对人类来说，稀少的，多变的气体是非常重要的。水汽，即水的气态，在大气 的含量是比较稀少的，而且它的生成和消耗是比较快的。因此，水汽在大气中存在的平均时间只有11天，水汽是雨雪得源泉，没有水汽我们就不能生存。众所周知，水汽在大气中的含量是多变的，在沙漠地带，水汽含量非常低，只占空气总量的很少的一部分，相反，在其他地区
40、，特别是潮湿的热的海面上，或者赤道洋面上，水汽可以占到大气总量的5%左右。尽管大气中的水汽的含量随时间和地点的不同有很大的变化，但它在整个地球上的总量实际上是不变的。而二氧化碳却不是这样的。在过去的一百年里，这种稀少的但却很重要的气体的含量一直在上升腐烂的植物，土壤中的腐殖物以及燃烧煤，石油，天然气，等化石燃料都能向大气释放二氧化碳。二氧化碳主要的汇是海洋和植物，植物需要二氧化碳进行光合作用，二十世纪八十年代中期，大气化学家就森林的燃烧，砍伐对大气中二氧化碳产生的影响进行了争论。海洋吸收了大量的二氧化碳，大约占化石燃料燃烧释放的二氧化碳总量的一半。人们预测，往后的几十年，这部分吸收量将会逐步减
41、少，而释放的二氧化碳的总量则会增加，这种情况至少会延续到下个世纪初。二十世纪八十年代，大气中的二氧化碳，每年以空气的百万分之一的速度增加。但是，据预测，这种增加速度在未来的几十年中还要更快一些。1983年，其含量平均为空气的百万分之340。另一种重要多变的气体 -臭氧（O3），多存在于高层大气中，但在交通拥挤，有许多工厂或是充足阳光照射的地区也有臭氧存在。像洛杉矶这样的城市，O3最多时可超过0.1ppm。大气中臭氧的含量大多在1.010ppm之间。它们随高度，季节，时间，天气状况的不同而不同。高层大气中的O3 层是由光化学反应造成的。臭氧层之所以重要，是因为它通过吸收上层大气紫外线辐射，从而减
42、少紫外线到达地面的总量，接受紫外线照射的时间越长，就越容易造成严重的灼伤，从而增加皮肤癌的危险。生物学家指出，紫外线过度的增加还会影响生物圈中的其它组成成分。 有些气体，如果浓度过高，对人，动物及植物生命可能有害，比如，O3浓度过高会对生物有机体造成危害。虽然这种情况不常发生。但洛杉矶这样污染严重的城市，有时接近地面的O3含量变得足以毁坏某些植物表面的叶片。人类活动将大量具有潜在危险的气体带入大气。大型锅炉，工厂，炼油厂和发动机，特别是汽油发动机排放出空气污染物。所有这些场所以及其它特别类似场所：燃烧煤，石油，汽油，煤油等化石燃料，在燃烧过程中，它们向空气中排放气体或烟尘颗粒，这些颗粒经过相当
43、长的时间又和空气中其他成分发生反应，形成有毒的化合物。 分布最广泛的，而且潜在危险最大的气体污染物是一氧化碳，二氧化硫，氧化氮和碳氢化合物。这些化合物最终来源于蒸发的汽油和其他石油产物。 第一课 大气的结构和组成 Unit Two: Frontogenesis and frontal characteristics第二课 锋生和锋的特征The first real advance in our detailed understanding of mid-latitude weather variations was made with the discover that many of the
44、 day-to-day changes are associated with the formation and movement of boundaries, or fronts, between different air masses.Observations of the temperature, wind directions, humidity and other physical phenomena during unsettled periods showed that discontinuities often persist between impinging air m
45、asses of differing characteristics. The term “front”, for these surfaces of air mass conflict, was a logical one proposed during the First World War by a group of meteorologists working in Norway, and their ideas are still an integral part of most weather analysis and forecasting particularly in mid
46、dle and high latitudes. 1. Frontal wavesIt was observed that the typical geometry of the air mass interface, or front, resembles a wave from. Similar wave patterns are, in fact, found to occur on the interface between many different media, for example, waves on sea surface, ripples on beach sand, ae
47、olian sand dunes, etc. Unlike these wave forms, however, the frontal waves in the atmosphere are commonly unstable: that is, they suddenly originate, increase in size, and then gradually dissipate. Numerical model calculations show that, in middle latitudes waves in a baroclinic atmosphere are unsta
48、ble if their wavelength exceeds a few thousand kilometers. Frontal wave cyclones are typically 1500-3000 km in wavelength. The initially attractive analogy between atmospheric wave systems and waves formed on interface of other media is, therefore, an insufficient-basis on which to develop explanations of frontal waves. In particular, the circulation of the upper troposphere plays a key role in providing appropriate conditions for their development and growth, as will be shown below.