首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   413篇
  免费   16篇
  国内免费   10篇
测绘学   16篇
大气科学   37篇
地球物理   66篇
地质学   240篇
海洋学   25篇
天文学   41篇
综合类   4篇
自然地理   10篇
  2020年   7篇
  2018年   5篇
  2017年   8篇
  2016年   14篇
  2015年   7篇
  2014年   9篇
  2013年   13篇
  2012年   13篇
  2011年   11篇
  2010年   20篇
  2009年   16篇
  2008年   15篇
  2007年   10篇
  2006年   22篇
  2005年   15篇
  2004年   5篇
  2003年   10篇
  2002年   10篇
  2001年   9篇
  2000年   9篇
  1999年   6篇
  1997年   10篇
  1996年   9篇
  1994年   6篇
  1993年   5篇
  1992年   5篇
  1988年   5篇
  1987年   4篇
  1986年   6篇
  1985年   4篇
  1984年   7篇
  1982年   4篇
  1980年   8篇
  1979年   4篇
  1978年   4篇
  1968年   3篇
  1967年   4篇
  1962年   3篇
  1960年   5篇
  1959年   3篇
  1958年   4篇
  1956年   4篇
  1953年   4篇
  1952年   3篇
  1950年   4篇
  1949年   4篇
  1948年   3篇
  1921年   4篇
  1914年   3篇
  1912年   4篇
排序方式: 共有439条查询结果,搜索用时 62 毫秒
401.
At the atmosphere simulation chamber SAPHIR in Jülich both Laser-Induced Fluorescence Spectroscopy (LIF) and Long-Path Differential Optical Laser Absorption Spectroscopy (DOAS) are operational for the detection of OH radicals at tropospheric levels. The two different spectroscopic techniques were compared within the controlled environment of SAPHIR based on all simultaneous measurements acquired in 2003 (13 days). Hydroxyl radicals were scavenged by added CO during four of these days in order to experimentally check the calculated precisions at the detection limit. LIF measurements have a higher precision (σ= 0.88×106 cm–3) and better time resolution (Δt = 60 s), but the DOAS method (σ= 1.24×106 cm–3, Δt = 135 s) is regarded as primary standard for comparisons because of its good accuracy. A high correlation coefficient of r = 0.95 was found for the whole data set highlighting the advantage of using a simulation chamber. The data set consists of two groups. The first one includes 3 days, where the LIF measurements yield (1 – 2) ×106 cm–3 higher OH concentrations than observed by the DOAS instrument. The experimental conditions during these days are characterized by increased NOx concentration and a small dynamic range in OH. Excellent agreement is found within the other group of 6 days. The regression to the combined data of this large group yields unity slope without a significant offset.  相似文献   
402.
This study applies acoustic sounding to observe coherent structures in the roughness sublayer (RSL) above tall vegetated surfaces. Data were collected on 22 days during two separate field experiments in summer 2003. A quality control scheme was developed to ensure high data quality of the collected time series. The data analysis was done using both discrete and continuous wavelet transform. The flow in the RSL was found to be a superposition of dynamic Kelvin–Helmholtz instabilities and convective mixing. The characteristic time scales for coherent structures resulting from the dynamic instabilities were observed to be approximately 20–30 s while thermal eddies have much larger time scales of 190–210 s. The degree of vertical coherency in the RSL increases with the flow evolving from neutral to near-convective conditions. This increase in the degree of organisation is attributed to the evolution of attached thermal eddies. The coherent structures resulting from instabilities were found to be present throughout the RSL but do not contribute to the increased vertical coherency. An alternative conceptual approach for the definition of the RSL is proposed, which yields its maximum vertical extent to five times the canopy height.  相似文献   
403.
In this study the influence of land-surface parameters on latent heat fluxes simulated with the numerical weather prediction model Lokalmodell (LM) of the German Meteorological Service is investigated. The area of interest is the LITFASS area during the LITFASS-2003 campaign. Based on simulations with varying soil and vegetation properties, we confirm that simulated latent heat fluxes strongly depend on soil moisture and leaf area index. Both parameters are difficult to obtain from in situ measurements with sufficient spatial resolution over heterogeneous land surfaces. Therefore, a procedure is proposed to determine area average values of soil moisture from time domain reflectometer measurements performed at a limited number of sites. The area averages cover the 7 × 7 km2 grid cells of the LM around Lindenberg (south-east of Berlin). Furthermore, satellite inferred plant parameters from NOAA–AVHRR are used to initialise model runs; the derived vegetation parameters show notable differences with those in the standard input of LM. The latent heat fluxes from the LM are compared with the aggregated eddy-covariance-measurements, and while the operational LM shows a strong overestimation of latent heat fluxes, it is demonstrated that the application of land-surface parameters derived from measurements can significantly reduce the deviation between the simulated and measured latent heat fluxes.  相似文献   
404.
江南造山带雪峰隆起区元古宙浊积岩的地球化学组成良好地反映了沉积盆地的板块构造性质,并为解释区域大地构造的性质和演化提供了有用的信息。中元古代冷家溪群和新元古代板溪群(包括下部马底驿组和上部五强溪组)地层在主元素成分上无明显区别,总体上以中等SiO2含量和K2O/Na2O比值以及较高的(Fe2O3+MgO)含量为特征。稀土元素球粒陨石标准化曲线与典型的后太古宙页岩(如PAAS或NASC)和上陆壳相似,以轻稀土富集、显著的铕负异常和重稀土平坦为特征。大离子亲石元素和铁镁族元素的含量中等。主元素和微量元素特征指示了研究区元古宙浊积岩很可能代表了一套活动大陆边缘弧后沉积盆地的产物。陆内裂谷盆地模式对于解释新元古代华南地区的构造性质和演化可能并不合适。华夏陆块与扬子陆块的拼接以及Rodinia超大陆的最终形成可能发生在约0.8Ga左右的晋宁(雪峰)造山运动期,而不是全球Grenville造山期末的四堡(东安)运动期。  相似文献   
405.
Basement-Cover Sequences within the UHP unit of the Dabie Shan   总被引:5,自引:0,他引:5  
Field data define two lithologically distinct basement‐cover sequences within the ultrahigh‐pressure metamorphic (UHPM) unit of the Dabie Shan, eastern China. One of the cover units, the Changpu unit, comprises calc‐arenitic metasediments in stratigraphic contact with a basement consisting of gneisses of the Yangtze craton. The second cover unit, the Ganghe unit, consists of felsic‐intermediate metavolcanics and clastic metasediments. Basement exposure of the Ganghe unit is not known. Fold axes in the Ganghe unit are oblique to those of the Changpu unit, which are parallel to those of the Yangtze gneisses. Preservation of primary textures in some volcanic rocks, and tectonic separation from the Yangtze gneisses by a greenschist facies mylonite, support an interpretation of the Ganghe unit as a low‐strain domain. Protolith associations in the Ganghe and Changpu units are compatible with deposition in a rift setting and along a passive continental margin, respectively. A U–Pb single zircon age of 761 ± 33 Ma for volcanoclastic rocks of the Ganghe unit demonstrates a Neoproterozoic deposition age, concordant with inferred rifting at that time. Eclogite facies parageneses in the gneisses and both cover units, along with P–T data demonstrate regional UHPM in the Dabie Shan.  相似文献   
406.
Vertical heat fluxes induced by mesoscale thermally driven circulations maycontribute significantly to the subgrid-scale fluxes in large-scale models (e.g.,general circulation models). However, they are not considered in these modelsyet. To gain insight into the importance and possible parameterisation of themesoscale flux associated with slope winds, an analytical (conceptual) modelis developed to describe the relationship between the mesoscale heat flux andatmospheric and land-surface characteristics. The analytical model allows usto evaluate the mesoscale flux induced by slope winds from only a few profilemeasurements within a domain. To validate the analytical model the resultingheat flux profiles are compared to profiles of highly resolved wind and temperaturefields obtained by simulations with a mesoscale numerical model.With no or moderate synoptic wind the mesoscale heat flux generated by the slopewind circulation may be as large as, or even larger than, the turbulent fluxes at thesame height. At altitudes lower than the crest of the hills the mesoscale flux is alwayspositive (upward). Generally it causes cooling within the boundary layer and heatingabove. Despite the simplifications made to derive the analytical model, it reproducesthe profiles of the mesoscale flux quite well. According to the analytical model, themesoscale heat flux is governed by the temperature deviation at the slope surface, thedepth of the slope-wind layer, the large-scale lapse rate, and the wavelength of thetopographical features.  相似文献   
407.
408.
The system Ca2Al3Si3O11(O/OH)-Ca2Al2FeSi3O11(O/OH), with emphasis on the Al-rich portion, was investigated by synthesis experiments at 0.5 and 2.0 GPa, 500-800 °C, using the technique of producing overgrowths on natural seed crystals. Electron microprobe analyses of overgrowths up to >100 µm wide have located the phase transition from clinozoisite to zoisite as a function of P-T-Xps and a miscibility gap in the clinozoisite solid solution. The experiments confirm a narrow, steep zoisite-clinozoisite two-phase loop in T-Xps section. Maximum and minimum iron contents in coexisting zoisite and clinozoisite are given by Xpszo (max) = 1.9*10 - 4 T+ 3.1*10 - 2 P - 5.36*10 - 2{\rm X}_{{\rm ps}}^{{\rm zo}} {\rm (max) = 1}{\rm .9*10}^{ - 4} T{\rm + 3}{\rm .1*10}^{ - 2} P - {\rm 5}{\rm .36*10}^{ - 2} and Xpsczo (min) = (4.6 * 10 - 4 - 4 * 10 - 5 P)T + 3.82 * 10 - 2 P - 8.76 * 10 - 2{\rm X}_{{\rm ps}}^{{\rm czo}} {\rm (min)} = {\rm (4}{\rm .6} * {\rm 10}^{ - {\rm 4}} - 4 * {\rm 10}^{ - {\rm 5}} P{\rm )}T + {\rm 3}{\rm .82} * {\rm 10}^{ - {\rm 2}} P - {\rm 8}{\rm .76} * {\rm 10}^{ - {\rm 2}} (P in GPa, T in °C). The iron-free end member reaction clinozoisite = zoisite has equilibrium temperatures of 185ᇆ °C at 0.5 GPa and 0ᇆ °C at 2.0 GPa, with (Hr0=2.8ǃ.3 kJ/mol and (Sr0=4.5ǃ.4 J/mol2K. At 0.5 GPa, two clinozoisite modifications exist, which have compositions of clinozoisite I ~0.15 to 0.25 Xps and clinozoisite II >0.55 Xps. The upper thermal stability of clinozoisite I at 0.5 GPa lies slightly above 600 °C, whereas Fe-rich clinozoisite II is stable at 650 °C. The schematic phase relations between epidote minerals, grossular-andradite solid solutions and other phases in the system CaO-Al2O3-Fe2O3-SiO2-H2O are shown.  相似文献   
409.
Zusammenfassung Im ersten Abschnitt werden Einwände gegen die bisherigen Untersuchungen entkräftet. Im zweiten Abschnitt werden neue Belege für die doppelte Schwankung des Großwetters in der gemäßigten Zone während des Sonnenfleckenzyklus erbracht. Es wird gezeigt, daß die Sommer in Neuengland, die 3 bis 1 1/2 Jahre vor einem Fleckenminimum liegen, fast durchweg wärmer als normal sind und alle Hochsommer, die 2,8 bis 1,9 Jahre vor einem Fleckenextrem (Minimum oder Maximum) liegen und in denen weitere solare Bedingungen erfüllt sind, in Mitteleuropa trockener und wärmer als normal sind und übernormalen Luftdruck in Mittel- und Westeuropa aufweisen. Das gleiche gilt in den meisten dieser Jahre auch für den ganzen Sommer und auch für die südlicheren Teile Nordeuropas. Die relative Häufigkeit des Zusammentreffens dieser Großwettererscheinungen mit bestimmten solaren Bedingungen wird als überzufällig erwiesen; die Zusammenhänge werden physikalisch gedeutet.Im dritten Abschnitt wird ein überzufälliger Zusammenhang des Verlaufs der Sonnenflecken vom Vorwinter bis zum Herbst mit dem Temperaturgepräge des Folgewinters in Mitteleuropa dargestellt und erklärt.Im vierten Abschnitt wird aus der zur Erklärung der Doppelschwankung des Großwetters innerhalb des Sonnenfleckenzyklus aufgestellten Hypothese eine logische Folgerung gezogen, die sich auf die Stärke der zonalen Zirkulation in den gemäßigten Breiten in der Höhe der 500 mb-Flächevor einer länger anhaltenden Vermehrung der Sonnenflecken bis zur nächsten Sonnenrotation bezieht. Es wird festgestellt, daß diese rein theoretisch gezogene Schlußfolgerung in der Wirklichkeit mindestens im Winterhalbjahr tatsächlich erfüllt ist, womit ein neues Argument für die Richtigkeit der aufgestellten Theorie des Einflusses der von der Sonne ausgehenden Strahlungsschwankungen auf das Großwetter gewonnen wird. Die Beeinflussung des Großwetters geschieht nicht durch die korpuskularen Strahlungen der Sonne, die auf ionosphärische Vorgänge und dadurch auch auf den Erdmagnetismus wirken, sondern durch länger anhaltende Intensitätsänderungen der Wellenstrahlung, vor allem der ultravioletten Strahlung der Sonne.
Summary In the first part objections against the former investigations are refuted. In the second part new arguments are given for the double variation of the large scale weather conditions (Großwetter) in the temperate zone during the sun spot cycle. It is shown that the summers in New England which lie 3 to 1 1/2 years before a sun spot minimum are almost without exception warmer than normal, and all midsummers which lie 2,8 to 1,9 years before a sun spot extremum (minimum or maximum) and in which further solar conditions are satisfied are drier and warmer than normal in Central Europe and show supernormal atmospheric pressure in Central and Western Europe. The same is true during most of the years also for the whole summer and also for the more southern parts of Northern Europe. The relative frequency of meeting of these large scale occurences with certain solar conditions are proved to be beyond chance; the relations are physically interpreted.In the third part a relation beyond chance of the course of the sun spots from the foregoing winter until autumn with the temperature-character of the following winter in Central Europe is described and explained.In the fourth part a logical conclusion is drawn from the hypothesis for explaining the double oscillation of the Großwetter within the sun spot cycle. This conclusion regards to the strength of the zonal circulation in the middle latitudes in the height of the 500 mb-area before a longer increasing of the sun spots until the next rotation of the sun. It is stated that this conclusion is fulfilled in the reality. This is a new argument for the correctness of the used theory concerning the influence of the variations of the sun's radiation upon the Großwetter. This influence is not caused by the corpuscular radiations of the sun which have an effect on the ionospheric occurences and by these also on the terrestrial magnetism, but by assiduous changes of the intensity of the undulatory radiation, especially of the ultra-violet radiation of the sun.

Résumé La première partie de l'article réfute des objections soulevées par les études précédentes. La deuxième partie apporte de nouvelles preuves de la double variation du temps (à grande échelle) de la zone tempérée pendant le cycle des taches solaires. On montre qu'en Nouvelle-Angleterre les étés précédant de 3 à 1,5 ans un minimum de taches sont presque tous trop chauds et que tous les pleins-étés précédant de 2,8 à 1,9 ans un extremum de taches (minimum ou maximum) et pour lesquels d'autres conditions solaires sont remplies sont en Europe centrale trop secs et trop chauds avec pression supra-normale sur le centre et l'Ouest de l'Europe. La même relation existe dans la plupart de ces années également pour tout l'été et dans les régions plus méridionales de l'Europe septentrionale. La fréquence relative de ces conjonctures s'avère comme supérieure à celle du hasard pur; l'auteur en expose les bases physiques.La troisième partie traite d'une relation signifiante entre l'évolution des taches solaires du début de l'hiver à l'automne et l'allure de la température de l'hiver suivant en Europe centrale.Dans la quatrième partie, l'auteur tire de l'hypothèse expliquant la variation double du temps dans l'espace du cycle solaire une conséquence logique se rapportant à l'intensité de la circulation zonale aux latitudes moyennes, à l'altitude de la surface de 500 mb, avant l'augmentation durable des taches solaires jusqu'à la rotation suivante du soleil. Il montre que cette conséquence purement théorique se réalise en fait, au moins pendant le semestre hivernal, et qu'elle fournit ainsi un nouvel argument en faveur de la théorie qui prétend une influence des variations du rayonnement solaire sur la situation météorologique générale. Cette situation n'est pas influencée par le rayonnement corpusculaire du soleil qui intervient dans les phénomènes ionosphériques et dans le magnétisme terrestre, mais bien par les variations à longue période du rayonnement ondulatoire, et avant tout de l'ultra-violet solaire.


Mit 3 Textabbildungen.  相似文献   
410.
Zusammenfassung Das Verhältnis der meßbaren Intensitäten in einem Lichtmeßgerät flacher Bauart dicht unter und über Wasser wird Taucheffekt genannt. Dieses Verhältnis weicht von der wahren Relation der Lichtintensitäten innerhalb weiter Grenzen ab. Entsprechend den verschiedenen optisch-geometrischen Bedingungen bei Opal-, Klarglas- und Mattscheiben gibt es drei verschiedenartige Taucheffekte.Opal- und Mattscheiben verhalten sich gegensätzlich, ihre Taucheffektbereiche liegen unterhalb, bzw. oberhalb des gut definierbaren Klarglaswertes. Dieser bildet für Opalglas die obere, für Mattglas die untere Grenze. Die untere Grenze für Opalglas ist annähernd bestimmt, die obere Grenze für Mattglas nicht. Kunststoffgläser mit ihren niedrigeren Brechungszahlen ergeben noch breitere Taucheffektbereiche als Silikatgläser.Das Verhältnis der tatsächlichen Intensitäten liegt im Bereich des Opalglas-Taucheffekts. Es muß daher möglich sein, ein Opalglas herzustellen, mit dem ein Lichtmeßgerät über und unter Wasser richtig zeigt.Opal- und Mattgläser weisen einen Oberflächeneffekt, d. h. einen Totalreflexionsfehler in der Nähe der Wasseroberfläche auf, dessen Zusammenhang mit dem Taucheffekt geklärt wird.Es wird eine einfache Vorrichtung angegeben, um bei Taucheffekt-Bestimmungen den Totalreflexionsfehler zu vermeiden.
Summary The ratio of light intensities just below and above a water surface as measured by a light meter of disk-shaped design is called immersion effect. This ratio differs within a wide margin from the ratio of the true intensities. According to the different optical-geometrical conditions of light transmission through opal, clear and ground glass three different types of the immersion effect can be distinguished.The immersion effect of opal glass is lower, and that of ground glass higher than the (well defined) value for clear glass. The latter is the upper margin for opal glass, and the lower margin for ground glass. The lower margin for opal glass is approximately defined while the upper margin for ground glass is indefinite. Plastic glasses with their low index of refraction cause an even wider range of the immersion effect than silicate glasses.The true ratio of light intensities lies within the range of the immersion effect of opal glass. Therefore, it should be possible to design a light meter with an opal filter which gives correct readings above and below the water surface.When placed closely beneath the water surface opal and ground glass show an additional surface effect. It is caused by total reflection at the water surface. Its relation to the immersion effect is discussed.A simple device is described which eliminates the surface effect and permits accurate determinations of the immersion effect.

Résumé Le rapport entre les intensités mesurées par un photomètre plat directement sous la surface de l'eau et au-dessus de celle-ci est appelé «effet de plongée»; il diffère de la relation véritable des intensités lumineuses dans de larges limites. Par suite des propriétés optiques différentes des disques de verre opalin, transparent ou mat, il existe trois effets de plongée différents.Les disques opalins et mats se comportent de façon contraire; leurs domaines d'effet de plongée se trouvent au-dessous et au-dessus de la valeur bien définie correspondant au verre transparent. Celle-ci constitute pour le verre opalin la limite supérieure, pour le verre mat la limite inférieure. La limite inférieure pour le verre opalin est à peu près définie, la limite supérieure pour le verre mat ne l'est pas. Les verres synthétiques à indices bas ont des domaines d'effet de plongée encore plus larges que les verres à silicates.Le rapport véritable des intensités se trouve dans le domaine de l'effet de plongée du verre opalin. Il doit donc être possible de préparer un verre opalin à l'aide duquel un photomètre fonctionne correctement dans et sur l'eau.Les verres opalins et mats présentent un «effet de surface», c'est-àdire une erreur de réflexion totale au voisinage de la surface de l'eau, dont on explique la relation avec l'effet de plongée.On donne un dispositif simple permettant d'éviter l'erreur de réflexion totale.


Mit 11 Textabbildungen

Diese Arbeit ist dem Andenken anFranz Sauberer gewidmet. Sie enthält manche Antwort auf methodische Grundfragen, um deren KlärungSauberer stets gewissenhaft bemüht war. Dem lieben, unvergessenen Kollegen sei damit wenigstens in memoriam der Dank erstattet, den ihm der Verfasser für Anregung und Kritik schuldet.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号