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321.
Summary Unseasonably cold weather episodes have the potential to cause dislocation to many aspects of society, regardless of the
season in which they occur. In this work we devise a method for quantitatively identifying extreme cold events in such a way
that it is not biased to the winter season (as is usual in most other studies). We have applied this method to the daily maximum
temperatures (over the period January 1972 to June 1991) in the southern Australian cities of Melbourne and Perth. We identify
10 cold events in winter and summer for the cities.
Analyses were performed to determine the synoptic environment in which these events occurred. The most common synoptic type
in these samples was the ‘classic’, which is characterised by, amongst other factors, the passage of a cold front over the
city on the day of the outbreak, and the transport of air from subantarctic latitudes. Melbourne recorded five such events
in summer and six in winter, while seven and eight occurred in the two seasons for Perth. The circulation features and characteristics
of other synoptic types identified with these episodes is also examined.
The mean synoptic anomalies which are coincident with these cold events are analysed. For both cities and seasons there is
a ‘high-low’ anomalous dipole in the regional MSLP pattern, with the high located in the ‘upstream’ quadrant from the anomalous
cyclone. Having said this, the relative importance of the two features of the dipole in being associated with the cold event
strongly depended on the city and season under consideration. The research shows that the regional structures associated with
cold events in Melbourne and Perth bear some similarity, but also display a number of significant differences. These differences
are associated partly with the different climatological and synoptic settings in which these cities find themselves, and the
nature of their seasonality.
Received October 10, 1999/Revised April 7, 2000 相似文献
322.
Intensive agricultural land use can have detrimental effects on landscape properties, greatly accelerating soil erosion, with consequent fertility loss and reduced agricultural potential. To quantify the effects of such erosional processes on hillslope morphology and gain insight into the underlying dynamics, we use a twofold approach. First, a statistical analysis of topographical features is conducted, with a focus on slope and gradient distributions. The accelerated soil erosion is shown to be fingerprinted in the distribution tails, which provide a clear statistical signature of this human-induced land modification. Theoretical solutions are then derived for the hillslope morphology and the associated creep and runoff erosion fluxes, allowing us to distinguish between the main erosional mechanisms operating in disturbed and undisturbed areas. We focus our application on the landscape at the Calhoun Critical Zone Observatory in the US Southern Piedmont, where severe soil erosion followed intensive cotton cultivation, resulting in highly eroded and gullied hillslopes. The observed differences in hillslope morphologies in disturbed and undisturbed areas are shown to be related to the disruption of the natural balance between soil creep and runoff erosion. The relaxation time required for the disturbed hillslopes to reach a quasi-equilibrium condition is also investigated. © 2019 John Wiley & Sons, Ltd. 相似文献
323.
324.
According to statistical informations the orbital period of this cataclysmic binary of Z Camelopardalis type should be in the range between 0d.10 and 0d.19. A sole deep minimum was found photoelectrically. It lasted about 40 minutes and had an amplitude of nearly 2 mag in U. For reasons described in detail it can hardly be interpreted as an eclipsing one. Possibly there is no contribution to the light variation of AM Cas from orbital motion. 相似文献
325.
326.
N. Richter 《Astronomische Nachrichten》1980,301(6):301-304
Aus dem Tautenburger Katalog von 745 kompakten Galaxien in 4 Feldern um M3 wurden blaue Objekte selektiert. Dies geschah mit Hilfe des Farbenindex U – B ≧ −0.40 mag und durch spektroskopische Inspektion auf Objektivprismen-aufnahmen. Es wurden 45 Objekte als blau oder als emissionsverdächtig befunden. 相似文献
327.
The fate of moderately volatile elements in impact events—Lithium connection between the Ries sediments and central European tektites
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Zuzana Rodovská Tomáš Magna Karel Žák Roman Skála Tomasz Brachaniec Channon Visscher 《Meteoritics & planetary science》2016,51(12):2403-2415
Lithium abundances and isotope compositions are presented for a suite of sediments from the surroundings of the Ries Impact structure, paralleled by new Li data for central European tektites (moldavites) from several substrewn fields (South Bohemia, Moravia, Cheb Basin, Lusatia), including a specimen from the newly discovered substrewn field in Poland. The data set was supplemented by three clay fractions isolated from sedimentary samples. Moldavites measured in this study show a very narrow range in δ7Li values (?0.6 to 0.3‰ relative to L‐SVEC) and Li contents (23.9–48.1 ppm). This contrasts with sediments from the Ries area which show remarkable range in Li isotope compositions (from ?6.9 to 13.4‰) and Li contents (0.6–256 ppm). The OSM sediments which, based on chemical similarity, formed the major part of moldavites, show a range in δ7Li values from ?2.0 to 7.9‰ and Li contents from 5.8 to 78.9 ppm. Therefore, the formation of moldavites was apparently accompanied by large‐scale mixing, paralleled by chemical and isotope homogenization of their parent matter. The proposed Li mixing model indicates that sands, clayey sediments, and low volumes of carbonates are the major components for tektite formation whereas residual paleokarst sediments could have been a minor but important component for a subset of moldavites. Striking homogenization of Li in tektites, combined with limited Li loss during impacts, may suggest that moderately volatile elements are not scavenged and isotopically fractionated during large‐scale collisions, which is consistent with recent models. In general, whether homogenization of bodies with distinct Li isotope systematics takes place, or collision of bodies with similar Li systematics operates cannot be resolved at present stage but Li isotope homogeneity of solar system planets and asteroidal bodies tentatively implies the latter. 相似文献
328.
329.
This paper explains that the terms"horizontal and vertical scales"are not appropriate in map projections theory. Instead, the authors suggest using the term"sca... 相似文献
330.
High-Frequency and Low-Frequency Variability in Stochastic Daily Weather Generator and Its Effect on Agricultural and Hydrologic Modelling 总被引:1,自引:0,他引:1
The high-frequency and low-frequency variabilities, which are often misreproduced by the daily weather generators, have a significant effect on modelling weather-dependent processes. Three modifications are suggested to improve the reproduction of the both variabilities in a four-variate daily weather generator Met&Roll: (i) inclusion of the annual cycle of lag-0 and lag-1 correlations among solar radiation, maximum temperature and minimum temperature, (ii) use of the 3rd order Markov chain to model precipitation occurrence, (iii) applying the monthly generator (based on a first-order autoregressive model) to fit the low-frequency variability. The tests are made to examine the effects of the three new features on (i) a stochastic structure of the synthetic series, and on (ii) outputs from CERES-Wheat crop model (crop yields) and SAC-SMA rainfall-runoff model (monthly streamflow characteristics, distribution of 5-day streamflow) fed by the synthetic weather series. The results are compared with those obtained with the observed weather series.Results: (i) The inclusion of the annual cycle of the correlations has rather ambiguous effect on the temporal structure of the weather characteristics simulated by the generator and only insignificant effect on the output from either simulation model. (ii) Increased order of the Markov chain improves modelling of precipitation occurrence series (especially long dry spells), and correspondingly improves reliability of the output from either simulation model. (iii) Conditioning the daily generator on monthly generator has the most positive effect, especially on the output from the hydrological model: Variability of the monthly streamflow characteristics and the frequency of extreme streamflows are better simulated. (iv) Of the two simulation models, the improvements related to the three modifications are more pronounced in the hydrological simulations. This may be also due to the fact that the crop growth simulations were less affected by the imperfections of the unmodified version of Met&Roll. 相似文献