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   59805篇
  免费   407篇
  国内免费   1044篇
测绘学   2051篇
大气科学   4009篇
地球物理   10887篇
地质学   26346篇
海洋学   4185篇
天文学   9311篇
综合类   2207篇
自然地理   2260篇
  2022年   264篇
  2021年   426篇
  2020年   398篇
  2019年   467篇
  2018年   7142篇
  2017年   6358篇
  2016年   4287篇
  2015年   646篇
  2014年   852篇
  2013年   1316篇
  2012年   2484篇
  2011年   5283篇
  2010年   4560篇
  2009年   4941篇
  2008年   4048篇
  2007年   5071篇
  2006年   1154篇
  2005年   1115篇
  2004年   1183篇
  2003年   1250篇
  2002年   985篇
  2001年   576篇
  2000年   491篇
  1999年   348篇
  1998年   344篇
  1997年   328篇
  1996年   255篇
  1995年   263篇
  1994年   236篇
  1993年   182篇
  1992年   207篇
  1991年   181篇
  1990年   195篇
  1989年   187篇
  1988年   149篇
  1987年   174篇
  1986年   167篇
  1985年   201篇
  1984年   193篇
  1983年   192篇
  1982年   186篇
  1981年   186篇
  1980年   183篇
  1979年   175篇
  1978年   157篇
  1977年   140篇
  1976年   135篇
  1975年   135篇
  1974年   124篇
  1973年   164篇
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
1.
2.
This work provides a comprehensive physically based framework for the interpretation of the north Australian rainfall stable isotope record (δ18O and δ2H). Until now, interpretations mainly relied on statistical relationships between rainfall amount and isotopic values on monthly timescales. Here, we use multiseason daily rainfall stable isotope and high resolution (10 min) ground‐based C‐band polarimetric radar data and show that the five weather types (monsoon regimes) that constitute the Australian wet season each have a characteristic isotope ratio. The data suggest that this is not only due to changes in regional rainfall amount during these regimes but, more importantly, is due to different rain and cloud types that are associated with the large scale circulation regimes. Negative (positive) isotope anomalies occurred when stratiform rainfall fractions were large (small) and the horizontal extent of raining areas were largest (smallest). Intense, yet isolated, convective conditions were associated with enriched isotope values whereas more depleted isotope values were observed when convection was widespread but less intense. This means that isotopic proxy records may record the frequency of which these typical wet season regimes occur. Positive anomalies in paleoclimatic records are most likely associated with periods where continental convection dominates and convection is sea‐breeze forced. Negative anomalies may be interpreted as periods when the monsoon trough is active, convection is of the oceanic type, less electric, and stratiform areas are wide spread. This connection between variability of rainfall isotope anomalies and the intrinsic properties of convection and its large‐scale environment has important implications for all fields of research that use rainfall stable isotopes.  相似文献   
3.
4.
We present our spectroscopic observations of the novae V1425, V1493, and V1494 Aql carried out with the 125-cm telescope at the Crimean Station of the Sternberg Astronomical Institute in the wavelength range 4000–11000 Å. We measured the emission-line intensities, determined the nova shell expansion velocities from the line profile FWHMs and components, and estimated the interstellar reddening from the first members of the Balmer series. The chemical composition of the nova shells is analyzed. Nitrogen and oxygen were found to be overabundant in V1425 and V1494 Aql; the helium abundance turned out to be normal in the two stars.  相似文献   
5.
The magnetic-field distribution outside a flat, infinitely conductive unbounded disk in the field of a point magnetic dipole is determined. A relationship is established between the problem of magnetic-field determination and the problem of the flow of an ideal incompressible fluid around an infinitely thin disk.  相似文献   
6.
The effects of natural fish oil,DHA oil and soybean lecithin in microparticulate diets on stress tolerance of larval gilthead seabream(Sparus aurata)were investigated after 15 days feeding trials.The tolerance of larval gilthead seabream to various stress factors such as exposure to air(lack of dissolved oxygen),changes in water temperature(low)and salinity(high) were determined.This study showed that microparticulate diet with natural fish oil and soybean lecithin was the most effective for in-creasing the tolerance of larval gilthead seabream to various stresses,and that microparticulate diet with natural fish oil and palmitic acid(16:0)was more effective than microparticulate diet with DHA oil and soybean lecithin.  相似文献   
7.
8.
Subglacial and subaqueous sediments deposited near the margin of a Late-glacial ice-dammed lake near Achnasheen, northern Scotland, are described and interpreted. The subglacial sediments consist of deformation tills and glacitectonites derived from pre-existing glaciolacustrine deposits, and the subaqueous sediments consist of ice-proximal outwash and sediment flow deposits, and distal turbidites. Sediment was delivered from the glacier to the lake by two main processes: (1) subglacial till deformation, which fed debris flows at the grounding line; and (2) meltwater transport, which fed sediment-gravity flows on prograding outwash fans. Beyond the ice-marginal environment, deposition was from turbidity currents, ice-rafting and settling of suspended sediments. The exposures support the conclusion that the presence of a subglacial deforming layer can exert an important influence on sedimentation at the grounding lines of calving glaciers.  相似文献   
9.
Nowadays, the research works on landscape at fine scales using high-resolution images are uncommon.This research is based on the analysis of the combination of remote sensing data (1KONOS imagery acquired in 2002 and historical aerial photo taken in 1942). In the paper, the ecotopes in Qiujiadou and Xishao villages in Yixing City of Jiangsu Province in 1942 and 2002 were compared and landscape changes as well as the causes of the considerable changes were analyzed. It was found that the ecotope changes were at greater level in some aspects such as water surface and perennial vegetation coverage etc. This study at fine scale is globally significant for the rural areas, especially for the subsistence agricultural land, which occupies larger percentage in the earth. And it analyzes the structure of landscape based on a new landscape classification system--stratifications method.  相似文献   
10.
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