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   28929篇
  免费   1192篇
  国内免费   2270篇
测绘学   1808篇
大气科学   2927篇
地球物理   5477篇
地质学   14123篇
海洋学   1767篇
天文学   1790篇
综合类   2578篇
自然地理   1921篇
  2024年   31篇
  2023年   102篇
  2022年   278篇
  2021年   280篇
  2020年   263篇
  2019年   315篇
  2018年   5017篇
  2017年   4258篇
  2016年   2863篇
  2015年   524篇
  2014年   412篇
  2013年   321篇
  2012年   1291篇
  2011年   3044篇
  2010年   2325篇
  2009年   2642篇
  2008年   2179篇
  2007年   2588篇
  2006年   251篇
  2005年   378篇
  2004年   546篇
  2003年   546篇
  2002年   404篇
  2001年   193篇
  2000年   199篇
  1999年   187篇
  1998年   158篇
  1997年   124篇
  1996年   99篇
  1995年   99篇
  1994年   72篇
  1993年   76篇
  1992年   56篇
  1991年   43篇
  1990年   27篇
  1989年   34篇
  1988年   21篇
  1987年   25篇
  1986年   19篇
  1985年   7篇
  1984年   9篇
  1983年   12篇
  1982年   10篇
  1981年   22篇
  1980年   25篇
  1979年   4篇
  1976年   7篇
  1975年   1篇
  1958年   2篇
  1954年   1篇
排序方式: 共有10000条查询结果,搜索用时 312 毫秒
411.
This study provides new evidence for the feedback effects of vegetation cover on summer precipitation in different regions of China by calculating immediate (same season), and one-and two-season lagged correlations between the normalized difference vegetation index (NDVI) and summer precipitation. The results show that the correlation coefficients between NDVI in spring and the previous winter and precipitation in summer are positive in most regions of China, and they show significant difference between regions. The stronger one-and two-season lagged correlations occur in the eastern arid/semi-arid region, Central China,and Southwest China out of the eight climatic regions of China, and this implies that vegetation cover change has more sensitive feedback effects on summer precipitation in the three regions. The three regions are defined as sensitive regions. Spatial analyses of correlations between spring NDVI averaged over each sensitive region and summer precipitation of 160 stations suggest that the vegetation cover strongly affects summer precipitation not only over the sensitive region itself but also over other regions, especially the downstream region.  相似文献   
412.
Recent Rapid Regional Climate Warming on the Antarctic Peninsula   总被引:15,自引:1,他引:15  
The Intergovernmental Panel on Climate Change (IPCC) confirmed that mean global warming was 0.6 ± 0.2 °C during the 20th century and cited anthropogenic increases in greenhouse gases as the likely cause of temperature rise in the last 50 years. But this mean value conceals the substantial complexity of observed climate change, which is seasonally- and diurnally-biased, decadally-variable and geographically patchy. In particular, over the last 50 years three high-latitude areas have undergone recent rapid regional (RRR) warming, which was substantially more rapid than the global mean. However, each RRR warming occupies a different climatic regime and may have an entirely different underlying cause. We discuss the significance of RRR warming in one area, the Antarctic Peninsula. Here warming was much more rapid than in the rest of Antarctica where it was not significantly different to the global mean. We highlight climate proxies that appear to show that RRR warming on the Antarctic Peninsula is unprecedented over the last two millennia, and so unlikely to be a natural mode of variability. So while the station records do not indicate a ubiquitous polar amplification of global warming, the RRR warming on the Antarctic Peninsula might be a regional amplification of such warming. This, however, remains unproven since we cannot yet be sure what mechanism leads to such an amplification. We discuss several possible candidate mechanisms: changing oceanographic or changing atmospheric circulation, or a regional air-sea-ice feedback amplifying greenhouse warming. We can show that atmospheric warming and reduction in sea-ice duration coincide in a small area on the west of the Antarctic Peninsula, but here we cannot yet distinguish cause and effect. Thus for the present we cannot determine which process is the probable cause of RRR warming on the Antarctic Peninsula and until the mechanism initiating and sustaining the RRR warming is understood, and is convincingly reproduced in climate models, we lack a sound basis for predicting climate change in this region over the coming century.  相似文献   
413.
董守玉  万迪坤 《地震研究》1997,20(2):178-184
“平面图形演化方法”是利用无量刚的异常信息量来反映震前异常空间分布及其随时间的演化,从而预报发震时间和地点的一种方法,这种方法μ值等值线图法,μ值异常平面图法组成,它是通过绘制不同的时间上述二种平面图,通过分析水位,水化异常于震前在时间上,空间上的演化规律,寻找μ值异常集中区或高值区,异常丛集图像分布区,条带图像分布带及条带交叉图像地区,并考虑异常分布图像与活动构造带和地震带的关系,从而对强震发生  相似文献   
414.
本文考虑液固耦联及底板几何非线性与土壤弹性,建立了液-固-土耦合的储液罐翘离分析力学模型。算例表明,恢复力矩与翘离角之间的关系可以表达为分段线性,翘离将使地震响应增大。  相似文献   
415.
井间电阻率层析成象的某些进展   总被引:7,自引:2,他引:7  
用有限元法作电阻率层析成象的核心问题是Jacobi矩阵的求取和反演成象算法的研究。本文主要论述了这两方面的最新进展,即求取Jacobi矩阵的格林函数法,用于反演成象计算的平滑度约束反演。文中通过理论模型的成象计算,比较了两种反演算法的成象效果。  相似文献   
416.
Two semi-asymmetric flow patterns of typhoons are chosen to qualitatively determine the effect of exchange of horizontal momentum between inflow and outflow layers and the environment on the motion of typhoons. The results show that only the asymmetric flow component (residual after azimuthal mean flow has been removed) could cause a net momentum input into or output from a typhoon and therefore contribute to the changes in speed and direction of the typhoon movement. A typhoon with major inflow and/or outflow channels on its right (left) side would tend to accelerate and turn left (decelerate and turn right); On the other hand, a typhoon with major inflow and/or outflow channels in the rear (front) semicircle would tend to accelerate and turn right (decelerate and turn left).  相似文献   
417.
董亚萍 《盐湖研究》1997,5(2):57-63
1,4—二溴苯,4,4’—二溴联苯、4,4’—二溴三联和亚磷酸三乙酯在催化剂无氯化镍催化下,干燥氮气保护.反应得到相应的有机二磷酸四乙酯。  相似文献   
418.
分析了甘肃陇南山区农业气候资源的分布特征及其水平、垂直差异,提出了合理开发利用的农业决策建议,指出应利用资源优势建立名特优小商品生产基地。  相似文献   
419.
红层地区砂泥岩互层状斜坡岩体流变特性研究   总被引:8,自引:0,他引:8  
本文在对侏罗系砂溪庙组砂,泥岩进行的系统流变试验的基础上,详细研究了砂,泥岩的流变特性及其相应的长期强度,以及砂,泥岩互层状组成的斜坡碉体流变特征,特别是模拟研究了风化深度较厚的泥岩产生的粘塑性流动对斜坡应力场及斜坡变形破坏所起的不可忽视的作用。  相似文献   
420.
长江三峡坝区断裂构造的形成和演变的最新研究   总被引:2,自引:0,他引:2  
基底岩体内存在4组断裂构造。这些断裂、裂隙并非岩体的原生构造,而属于构造成因。根据断裂内岩脉侵入、岩石蚀变作用、结构面特征和构造岩类型的分析研究,认为断裂具有多期活动性的特点。对比基底岩体和沉积盖层的构造,确认岩体中的断裂构造在晚元古代晋宁期就已基本形成,以后在燕山期黄陵背斜出现时,喜马拉雅期断块构造产生时,断裂多次重新活动,演变为新的断裂系统。  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