全文获取类型
收费全文 | 37102篇 |
免费 | 1064篇 |
国内免费 | 931篇 |
专业分类
测绘学 | 937篇 |
大气科学 | 2846篇 |
地球物理 | 7610篇 |
地质学 | 13577篇 |
海洋学 | 3327篇 |
天文学 | 8152篇 |
综合类 | 239篇 |
自然地理 | 2409篇 |
出版年
2022年 | 272篇 |
2021年 | 445篇 |
2020年 | 424篇 |
2019年 | 482篇 |
2018年 | 906篇 |
2017年 | 863篇 |
2016年 | 1032篇 |
2015年 | 702篇 |
2014年 | 1029篇 |
2013年 | 1864篇 |
2012年 | 1334篇 |
2011年 | 1776篇 |
2010年 | 1558篇 |
2009年 | 2017篇 |
2008年 | 1695篇 |
2007年 | 1763篇 |
2006年 | 1695篇 |
2005年 | 1221篇 |
2004年 | 1138篇 |
2003年 | 1035篇 |
2002年 | 1000篇 |
2001年 | 845篇 |
2000年 | 825篇 |
1999年 | 671篇 |
1998年 | 715篇 |
1997年 | 689篇 |
1996年 | 570篇 |
1995年 | 560篇 |
1994年 | 479篇 |
1993年 | 418篇 |
1992年 | 416篇 |
1991年 | 385篇 |
1990年 | 455篇 |
1989年 | 372篇 |
1988年 | 354篇 |
1987年 | 437篇 |
1986年 | 343篇 |
1985年 | 430篇 |
1984年 | 531篇 |
1983年 | 451篇 |
1982年 | 452篇 |
1981年 | 401篇 |
1980年 | 418篇 |
1979年 | 358篇 |
1978年 | 344篇 |
1977年 | 340篇 |
1976年 | 309篇 |
1975年 | 296篇 |
1974年 | 312篇 |
1973年 | 340篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
971.
W. R. Skinner M. D. Flannigan B. J. Stocks D. L. Martell B. M. Wotton J. B. Todd J. A. Mason K. A. Logan E. M. Bosch 《Theoretical and Applied Climatology》2002,71(3-4):157-169
Summary
In Canada, the average annual area of burned forest has increased from around 1 million ha in the 1970’s to over 2.5 million
ha in the 1990’s. A previous study has identified the link between anomalous mid-tropospheric circulation at 500 hPa over
northern North America and wildland fire severity activity in various large regions of Canada over the entire May to August
fire season. In this study, a northern North American study region of the hemispheric gridded 5° latitude by 10° longitude
500 hPa dataset is identified and analysed from 1959 to 1996 for a sequence of six monthly periods through the fire season,
beginning in April and ending in September. Synoptic types, or modes of upper air behavior, are determined objectively by
the eigenvector method employing K-means cluster analysis. Monthly burned areas from the Canadian Large Fire Database (LFDB)
for the same period, 1959 to 1996, are analysed in conjunction with the classified monthly 500 hPa synoptic types. Relationships
between common monthly patterns of anomalous upper flow and spatial patterns of large fire occurrence are examined at the
ecozone level. Average occurrence of a monthly synoptic type associated with very large area burned is approximately 18% of
the years from 1959 to 1996. The largest areas burned during the main fire (May to August) season occur in the western Boreal
and Taiga ecozones – the Taiga Plains, Taiga Shield, Boreal West Shield and Boreal Plains. Monthly burned areas are also analysed
temporally in conjunction with a calculated monthly zonal index (Zim) for two separate areas defined to cover western and eastern Canada. In both western and eastern Canada, high area burned
is associated with synoptic types with mid-tropospheric ridging in the proximity of the affected region and low Zim with weak westerlies and strong meridional flow over western Canada.
Received April 3, 2001 Revised July 13, 2001 相似文献
972.
Summary The existence of low-frequency variability in climatic-hydrological-oceanic variables may be useful for long-term forecasting
and climate modelling. By using long time series this paper attempts to identify large-scale quasi-cycles in the precipitation
regimes of Northern Argentina, moisture advection from the Atlantic Ocean and the streamflow of the Paraná River. This work
also shows the presence of coherent waves with long periodicity between the three series. As the three variables are estimated
over different time intervals, the presence of waves in each variable is studied separately, to show they respond to the same
process. The three variables show significant interdecadal variability at low frequency (22–26 years), which might be related
to the ENSO cycle modulation and to the intensification or weakening of the South American Low Level Jet (SALLJ) and South
Atlantic Convergence Zone (SACZ). 相似文献
973.
974.
G. L. Geernaert 《Boundary-Layer Meteorology》2002,105(3):433-450
In order to apply Monin–Obukhov similarity theory to estimatethe profiles of mean quantities and surface fluxes from bulk meteorological parameters, the assumptions of homogeneityand stationarity must be valid. Unfortunately, in coastal zones as well as many other regions of interest, theseassumptions are often violated. In this paper, an extension to the theory is presented that considers systematically varyingstate variables. Along-wind variations of windspeed, atmospheric stratification, and roughness are examined with respectto their relative importance to momentum flux divergence, and the drag coefficient is shown to be systematicallylower in coastal zones. For profiles of scalars, an analysis of the set of quasi-homogeneous terms is only speculated,and the relative importance for the terms will strongly depend on which scalar is of interest. 相似文献
975.
W. M. L. Meijninger A. E. Green O. K. Hartogensis W. Kohsiek J. C. B. Hoedjes R. M. Zuurbier H. A. R. De Bruin 《Boundary-Layer Meteorology》2002,105(1):63-83
A large aperture scintillometer (LAS) andradio wave scintillometer (RWS)were installed over a heterogeneous areato test the applicability of the scintillation method.The heterogeneity in the area, whichconsisted of many plots, was mainly caused bydifferences in thermal properties ofthe crops; the variations in theaerodynamic roughness lengthwere small. The water vapour fluxesderived from the combined LAS-RWSsystem, also known as the two-wavelengthmethod, agreed fairly well with the aggregatedwater vapour fluxes derived from in-situeddy covariance measurements. The water vapourfluxes derived from a stand-alone LASare also presented. It was found that a single LASand an estimate of the area averagedavailable energy (using a simple parameterisationscheme) can provide also reasonablearea-averaged water vapour fluxes. 相似文献
976.
Summary The Betts-Miller and the Kain-Fritsch schemes are two of the many approaches to convective parameterization available to
modelers. In the case of hurricane Irene (1999), the choice of parameterization markedly impacted the modeled track and structure
of the hurricane and its subsequent extratropical transition. Specifically, in model runs using Betts-Miller, Irene recurved
too early, causing the storm to weaken over the cool open ocean, delaying its transition, and changing the character of the
storm. The Kain-Fritsch scheme more accurately reproduced the track of Irene and, hence, its interaction with upper-level
features that caused extratropical transition and post-transition intensification. The two parameterizations produce different
characteristic vertical warming profiles; the differences in warming are related to the structural differences in the simulated
storm, affecting the hurricane response to its environment.
Received October 13, 2001 Revised December 23, 2001 相似文献
977.
978.
This paper describes an extension to the Combined Hydrology And Stability Model (CHASM) to fully include the effects of vegetation and slope plan topography on slope stability. The resultant physically based numerical model is designed to be applied to site‐specific slopes in which a detailed assessment of unsaturated and saturated hydrology is required in relation to vegetation, topography and slope stability. Applications are made to the Hawke's Bay region in New Zealand where shallow‐seated instability is strongly associated with spatial and temporal trends in vegetation cover types, and the Mid‐Levels region in Hong Kong, an area subject to a variety of landslide mechanisms, some of which may be subject to strong topographic control. An improved understanding of process mechanism, afforded by the model, is critical for reliable and appropriate design of slope stabilization and remedial measures. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
979.
F. J. Molina G. Verzeletti G. Magonette Ph. Buchet V. Renda M. Geradin A. Parducci M. Mezzi A. Pacchiarotti L. Federici S. Mascelloni 《地震工程与结构动力学》2002,31(8):1563-1582
A pseudodynamic testing procedure has been applied by which the seismic response of a base‐isolated building is obtained by using as specimen the isolators, while the superstructure is numerically simulated. The procedure also takes advantage of the continuous pseudodynamic testing capabilities of the ELSA laboratory, which increase the accuracy of the results and reduce the strain‐rate effect of the rubber bearings. A simple proportional correction of the measured forces compensates the remaining strain‐rate effect due to the unrealistic speed of the test. The correction factor is obtained by means of a characterizing test on the specific rubber isolators. The developed method has been successfully applied to the prediction of the seismic response of a base‐isolated four‐storey building submitted to several specified accelerograms. The results for those earthquakes as well as the effects of some changes of the parameters of the system are discussed. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
980.
Semi‐active stiffness damper (SASD) is one of many semi‐active control systems with the capability to mitigate the dynamic response using only a small amount of external power. The system consists of a hydraulic damper connected to the bracing frame in a selected story unit. In this paper, study of a SASD in two building models of five‐stories under four benchmark earthquake records is reported. The purpose of this study is to evaluate the effectiveness of the control system against structure type and varying earthquake inputs. Various control laws are chosen to work with SASD, such as: resetting control, switching control, linear quadratic regulator (LQR) and modified LQR, and the results are compared with no control and passive control cases. Numerical results show that the use of a SASD is effective in reducing seismic responses. Control effectiveness is dependent on the type of structure and earthquake excitation. Passive control is less effective than other control cases as expected. Resetting control, switching control and LQR generally perform similarly in response reduction. While modified LQR is more efficient and robust compared with other control algorithms. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献