全文获取类型
收费全文 | 4472篇 |
免费 | 948篇 |
国内免费 | 1326篇 |
专业分类
测绘学 | 367篇 |
大气科学 | 928篇 |
地球物理 | 964篇 |
地质学 | 2445篇 |
海洋学 | 742篇 |
天文学 | 165篇 |
综合类 | 460篇 |
自然地理 | 675篇 |
出版年
2024年 | 27篇 |
2023年 | 85篇 |
2022年 | 249篇 |
2021年 | 251篇 |
2020年 | 237篇 |
2019年 | 273篇 |
2018年 | 305篇 |
2017年 | 251篇 |
2016年 | 303篇 |
2015年 | 271篇 |
2014年 | 314篇 |
2013年 | 283篇 |
2012年 | 294篇 |
2011年 | 311篇 |
2010年 | 318篇 |
2009年 | 332篇 |
2008年 | 290篇 |
2007年 | 237篇 |
2006年 | 193篇 |
2005年 | 179篇 |
2004年 | 145篇 |
2003年 | 137篇 |
2002年 | 153篇 |
2001年 | 147篇 |
2000年 | 144篇 |
1999年 | 168篇 |
1998年 | 129篇 |
1997年 | 121篇 |
1996年 | 95篇 |
1995年 | 91篇 |
1994年 | 68篇 |
1993年 | 75篇 |
1992年 | 52篇 |
1991年 | 39篇 |
1990年 | 27篇 |
1989年 | 34篇 |
1988年 | 21篇 |
1987年 | 23篇 |
1986年 | 19篇 |
1985年 | 7篇 |
1984年 | 9篇 |
1983年 | 12篇 |
1982年 | 10篇 |
1980年 | 6篇 |
1979年 | 4篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1958年 | 2篇 |
1954年 | 1篇 |
排序方式: 共有6746条查询结果,搜索用时 218 毫秒
981.
982.
983.
空中扰动引力快速赋值算法的效能分析 总被引:1,自引:0,他引:1
系统研究了广域多项武算法和有限元算法在扰动引力快速赋值计算中的效能问题.首先分析了算法的基本原理和赋值计算过程,利用改进的扰动引力计算方法构建了准确的扰动场;然后对模拟航迹进行赋值实验,定量分析了赋值精度与运算速度.结果表明:在相同实验条件下,广域多项式方法更有优势;当空间单元为10 ′×10′×20 km(△(φ)午... 相似文献
984.
Observations show that there was change in interannual North Atlantic Oscillation (NAO) variability in the mid-1970s. This change was characterized by an eastward shift of the NAO action centres, a poleward shift of zonal wind anomalies and a downstream extension of climate anomalies associated with the NAO. The NAO interannual variability for the period after the mid-1970s has an annular mode structure that penetrates deeply into the stratosphere, indicating a strengthened relationship between the NAO and the Arctic Oscillation (AO) and strengthened stratosphere-troposphere coupling. In this study we have investigated possible causes of these changes in the NAO by carrying out experiments with an atmospheric GCM. The model is forced either by doubling CO2, or increasing sea surface temperatures (SST), or both. In the case of SST forcing the SST anomaly is derived from a coupled model simulation forced by increasing CO2. Results indicate that SST and CO2 change both force a poleward and eastward shift in the pattern of interannual NAO variability and the associated poleward shift of zonal wind anomalies, similar to the observations. The effect of SST change can be understood in terms of mean changes in the troposphere. The direct effect of CO2 change, in contrast, can not be understood in terms of mean changes in the troposphere. However, there is a significant response in the stratosphere, characterized by a strengthened climatological polar vortex with strongly enhanced interannual variability. In this case, the NAO interannual variability has a strong link with the variability over the North Pacific, as in the annular AO pattern, and is also strongly related to the stratospheric vortex, indicating strengthened stratosphere-troposphere coupling. The similarity of changes in many characteristics of NAO interannual variability between the model response to doubling CO2 and those in observations in the mid-1970s implies that the increase of greenhouse gas concentration in the atmosphere, and the resulting changes in the stratosphere, might have played an important role in the multidecadal change of interannual NAO variability and its associated climate anomalies during the late twentieth century. The weak change in mean westerlies in the troposphere in response to CO2 change implies that enhanced and eastward extended mid-latitude westerlies in the troposphere might not be a necessary condition for the poleward and eastward shift of the NAO action centres in the mid-1970s. 相似文献
985.
北京快速更新循环预报系统(BJ-RUC)要素预报质量评估 总被引:4,自引:2,他引:2
使用自动站观测资料对BJ-RUC系统预报的北京15个观测站的逐时2 m温度、相对湿度、1小时降水量和风速等地面要素质量进行检验评估。结果表明:BJ-RUC系统预报的2 m温度、相对湿度和风速与实况具有一致的变化趋势,但各要素的预报性能不尽相同。结果表明:(1)对于2 m温度的预报,±1℃误差的准确率为0.2~0.5,±2℃误差的准确率更高;预报性能与预报时效无明显关系;温度预报准确性与季节和日变化有关,春、秋季优于冬、夏季,夜间优于白天。(2)对相对湿度的预报整体偏低,夏季白天偏低、冬季白天偏高。(3)对1小时降水的预报,无论全年还是夏季,晴雨预报准确率很高;对0.1 mm降水的TS评分在0.2左右。(4)对不同站点的预报性能存在明显差异。对平原地区,即海拔高度较低的站点预报性能相对较好,15个站点中,对北京的代表站点(南郊观象台)的预报最为准确;对山区站点的预报性能相对较差,对海拔最高的延庆站的预报性能最差。总体来说,BJ RUC对站点要素预报的变化趋势较好,预报准确率较高,具有很好的参考价值,但要素预报与季节、日变化及海拔高度等因素有关.具体应用时还需做适当订正。 相似文献
986.
987.
针对目前"九五"仪器数据采集工作中遇到的问题,应用Visual Basic编制通用型地震前兆通讯控制软件.其主要特点是,微机不需安装通讯控制卡,RS-232串口或网口均可,网络对时服务,并结合日常工作要求新增部分功能.结果发现,效果良好. 相似文献
988.
Research on seismic performance of shear walls with concrete filled steel tube columns and concealed steel trusses 总被引:1,自引:1,他引:0
In order to further improve the seismic performance of RC shear walls, a new composite shear wall with concrete filled steel
tube (CFT) columns and concealed steel trusses is proposed. This new shear wall is a double composite shear wall; the first
composite being the use of three different force systems, CFT, steel truss and shear wall, and the second the use of two different
materials, steel and concrete. Three 1/5 scaled experimental specimens: a traditional RC shear wall, a shear wall with CFT
columns, and a shear wall with CFT columns and concealed steel trusses, were tested under cyclic loading and the seismic performance
indices of the shear walls were comparatively analyzed. Based on the data from these experiments, a thorough elastic-plastic
finite element analysis and parametric analysis of the new shear walls were carried out using ABAQUS software. The finite
element results of deformation, stress distribution, and the evolution of cracks in each phase were compared with the experimental
results and showed good agreement. A mechanical model was also established for calculating the load-carrying capacity of the
new composite shear walls. The results show that this new type of shear wall has improved seismic performance over the other
two types of shear walls tested. 相似文献
989.
Late Jurassic-Early Cretaceous Northern Qaidam Basin, NW China: Implications for the earliest Cretaceous intracontinental tectonism 总被引:4,自引:0,他引:4
Lei Wu Ancheng Xiao Liqun WangZhongyan Shen Suping ZhouYuanzhong Chen Liang Wang Dong LiuJunya Guan 《Cretaceous Research》2011,32(4):552-564
Formation of Mesozoic western China, which was dominated by tectonic amalgamation along its southern margin and associated intracontinental tectonisms, holds a key for interpreting the succedent Cenozoic evolution. This paper presents new data including lithology, sedimentary facies, stratigraphic contact, seismic interpretation and paleo-structures within the Upper Jurassic-Lower Cretaceous strata in the northern Qaidam Basin, NW China. These data all account for a contractional tectonic deformation in the earliest Cretaceous. The South Qilian Shan, according to the sedimentary features and provenance analysis, reactivated and exhumated during the deformation, controlling the deposition of the Lower Cretaceous sequences. A simplified model for the Late Jurassic-Early Cretaceous paleogeography and tectonics of the northern Qaidam Basin is accordingly proposed. The results also support a ∼25° clockwise rotation of the Qaidam Basin since the Early Cretaceous and a more accurate Mesozoic evolution process for the basin. This earliest Cretaceous deformation, associated with the reactivation of the South Qilian Shan at the time, are part of the intracontinental tectonisms in central Asia during the Mesozoic, and probably driven by both the closure of the Mongol-Okhostk Ocean to the north and the collision of the Lhasa and the Qiangtang blocks to the south. 相似文献
990.
出露于雅鲁藏布江缝合带北侧尼木县的变质岩主要由石榴黑云片麻岩和黑云斜长角闪角岩组成.研究表明岩石变质程度达到角闪角岩相-辉石角岩相;石榴子石变斑晶具有生长环带,角闪石均为钙质角闪石,黑云母大多为铁质黑云母和铁叶云母,长石多为更长石和中长石,少量为正长石.利用石榴子石-黑云母温度计、石榴子石-黑云母-斜长石-石英压力计和角闪石-斜长石温度及压力计计算获得石榴黑云片麻岩和黑云斜长角闪角岩的变质温度分别为619 ~661℃,695 ~ 702℃,压力范围分别为1.86~1.94kbar和3.69~4.56kbar.野外和室内研究认为岩石原岩为冈底斯带南缘叶巴组火山岩及其上部沉积岩,岩石经历了高温低压的接触变质作用.结合已有冈底斯带陆缘岩浆活动特征,对变质岩的形成环境和过程进行了反演. 相似文献