全文获取类型
收费全文 | 9399篇 |
免费 | 1659篇 |
国内免费 | 2216篇 |
专业分类
测绘学 | 467篇 |
大气科学 | 1907篇 |
地球物理 | 2547篇 |
地质学 | 4766篇 |
海洋学 | 940篇 |
天文学 | 522篇 |
综合类 | 926篇 |
自然地理 | 1199篇 |
出版年
2024年 | 52篇 |
2023年 | 164篇 |
2022年 | 417篇 |
2021年 | 474篇 |
2020年 | 418篇 |
2019年 | 509篇 |
2018年 | 566篇 |
2017年 | 484篇 |
2016年 | 569篇 |
2015年 | 462篇 |
2014年 | 605篇 |
2013年 | 518篇 |
2012年 | 485篇 |
2011年 | 569篇 |
2010年 | 540篇 |
2009年 | 497篇 |
2008年 | 457篇 |
2007年 | 474篇 |
2006年 | 354篇 |
2005年 | 290篇 |
2004年 | 281篇 |
2003年 | 279篇 |
2002年 | 292篇 |
2001年 | 260篇 |
2000年 | 302篇 |
1999年 | 387篇 |
1998年 | 332篇 |
1997年 | 335篇 |
1996年 | 278篇 |
1995年 | 241篇 |
1994年 | 288篇 |
1993年 | 219篇 |
1992年 | 167篇 |
1991年 | 129篇 |
1990年 | 104篇 |
1989年 | 77篇 |
1988年 | 98篇 |
1987年 | 50篇 |
1986年 | 45篇 |
1985年 | 41篇 |
1984年 | 36篇 |
1983年 | 33篇 |
1982年 | 35篇 |
1981年 | 27篇 |
1980年 | 7篇 |
1979年 | 10篇 |
1978年 | 5篇 |
1975年 | 2篇 |
1958年 | 7篇 |
1954年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 24 毫秒
111.
The Upstream “Strong Signals” of the Water Vapor Transport over the Tibetan Plateau during a Heavy Rainfall Event in the Yangtze River Basin 总被引:2,自引:0,他引:2
A heavy rainfall event that occurred over the middle and lower reaches of the Yangtze River Basin(YRB) during July11–13 2000 is explored in this study. The potential/stream function is used to analyze the upstream "strong signals" of the water vapor transport in the Tibetan Plateau(TP). The studied time period covers from 2000 LST 5 July to 2000 LST 15 July(temporal resolution: 6 hours). By analyzing the three-dimensional structure of the water vapor flux, vorticity and divergence prior to and during the heavy rainfall event, the upstream "strong signals" related to this heavy rainfall event are revealed. A strong correlation exists between the heavy rainfall event in the YRB and the convective clouds over the TP. The "convergence zone" of the water vapor transport is also identified, based on correlation analysis of the water vapor flux two days and one day prior to, and on the day of, the heavy rainfall. And this "convergence zone" coincides with the migration of the maximum rainfall over the YRB. This specific coupled structure actually plays a key role in generating heavy rainfall over the YRB. The eastward movement of the coupled system with a divergence/convergence center of the potential function at the upper/lower level resembles the spatiotemporal evolution of the heavy rainfall event over the YRB. These upstream "strong signals" are clearly traced in this study through analyzing the three-dimensional structure of the potential/stream function of upstream water vapor transport. 相似文献
112.
Water pipe cooling has been widely used for the temperature control and crack prevention of massive concrete structures such as high dams. Because both under‐cooling and over‐cooling may reduce the efficiency of crack prevention, or even lead to great harm to structures, we need an accurate and robust numerical tool for the prediction of cooling effect. Here, a 3D discrete FEM Iterative Algorithm is introduced, which can simulate the concrete temperature gradient near the pipes, as well as the water temperature rising along the pipes. On the basis of the heat balance between water and concrete, the whole temperature field of the problem can be computed exactly within a few iteration steps. Providing the pipe meshing tool for building the FE model, this algorithm can take account of the water pipe distribution, the variation of water flow, water temperature, and other factors, while the traditional equivalent algorithm based on semi‐theoretical solutions can only solve problems with constant water flow and water temperature. The validation and convergence are proved by comparing the simulated results and analytical solutions of two standard second‐stage cooling problems. Then, a practical concrete block with different cooling schemes is analyzed and the influences of cooling factors are investigated. In the end, detailed guidance for pipe system optimization is provided. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
113.
对内蒙古三个典型大骨节病区<15岁人群发中21个元素和病情进行了三年连续定期观察,统计分析结果表明:(1)病区食物对人体元素含量的影响可以分为二种情况,一是元素在食物链中含量过低,食物的变化虽可使机体该元素发生变化,但变幅甚小,致使其代谢处于缺乏状态;另一类是机体内这些元素的含量因食物的改变而发生较大变化,显示这些元素与病情无内在联系.(2)对发中硒等2l个元素与病情分别作单相关和多元逐步回归分析,结果证明,硒缺乏是导致大骨节病早期病变的基本因素,而其它元素与病情的关系具有不确定性. 相似文献
114.
115.
海洋在天气、气候及大气环流变化过程中起着重要作用,海洋热源的异常分布是造成长期天气异常的重要原因。海洋对大气的作用主要是通过海气界面的热量交换来实现的,因此,直接运用海气热量交换来考虑海洋对大气的加热作用,分析海洋加热异常对长期天气、气候及大气环流的影响,具有更明确的物理意义。
西北太平洋副热带高压是造成我国旱、涝及天气变化的主要副热带天气系统,对我国夏季气温、雨带位置及强度影响很大。西北太平洋中、低纬副热带地区是海洋向大气提供热量和水汽较多的地区,也是海气相互作用最强烈的海域,这一地区的海洋加热对西北太平洋副热带高压的变化将有直接影响。本文分析了冬季西北太平洋副热带海域海洋加热主要场的分布特征,指出了海洋加热与西北太平洋副热带高压的关系。结果表明,冬季该海域海洋加热较常年偏多时,冬、春至初夏副高偏弱,位置偏东、南(6月副高脊线位置除外);反之,海洋加热较常年偏少时,副高偏强,位置偏西、北(6月副高脊线位置除外)。该海域海洋加热对副热带地区大气环流,尤其是副热带高压环流的变化有着重要作用。本结果对预测西北太平洋副热带高压的变化,研究中纬度海洋加热对西北太平洋副热带高压及其大气环流的影响有十分重要的意义。 相似文献
116.
117.
本文主要论述吉林省延边地区屯田营组时代。通过对屯田营组研究历史的回顾,从火山岩喷发—沉积序列和生物地层资料指出屯田营组时代为晚侏罗世,屯田营组与金沟岭组同是晚侏罗世两个火山旋回。 相似文献
118.
K. S. Dwarakanath W. M. Goss J. H. Zhao C. C. Lang 《Journal of Astrophysics and Astronomy》2004,25(3-4):129-141
We have imaged a region of ∼ 5′ extent surrounding Sgr A* in the HI 21 cm-line absorption using the Very Large Array. A Gaussian
decomposition of the optical depth spectra at positions within ∼ 2′ (∼ 5 pc at 8.5 kpc) of Sgr A* detects a wide line underlying
the many narrow absorption lines. The wide line has a mean peak optical depth of 0.32 ± 0.12 centered at a mean velocity of
V1sr = −4 ± 15 km s{−1}. The mean full width at half maximum is 119 ± 42 km s−1. Such a wide line is absent in the spectra at positions beyond ∼ 2′ from Sgr A*. The position-velocity diagrams in optical
depth reveal that the wide line originates in various components of the circumnuclear disk (radius ∼ 1.3′ ) surrounding Sgr
A*. These components contribute to the optical depth of the wide line in different velocity ranges. The position-velocity
diagrams do not reveal any diffuse feature which could be attributed to a large number of HI clouds along the line of sight
to Sgr A*. Consequently, the wide line has no implications either to a global population of shocked HI clouds in the Galaxy
or to the energetics of the interstellar medium as was earlier thought. 相似文献
119.
120.
近年来的一些研究表明,由于地震前孕震区地下介质的电性结构的变化,将导致地磁短周期转换函数发生异常变化。通过对地震前后的地磁短周期转换函数的参数的计算、分析和研究,有可能提取出可靠的地震短临前兆异常(曾小苹等,1990)。为了进一步开展对该方法研究工作并使其在我区地震预测预报研究中得到应用,我们使用了静海台的地磁记录资料,分析计算了该台地磁短周期转换函数在地震前后的变化情况,确立了该方法应用天津地区地震预报的短临异常特征及指标,取得了较好的效果。 相似文献