全文获取类型
收费全文 | 3074篇 |
免费 | 378篇 |
国内免费 | 519篇 |
专业分类
测绘学 | 1313篇 |
大气科学 | 653篇 |
地球物理 | 560篇 |
地质学 | 568篇 |
海洋学 | 333篇 |
天文学 | 120篇 |
综合类 | 291篇 |
自然地理 | 133篇 |
出版年
2024年 | 6篇 |
2023年 | 36篇 |
2022年 | 125篇 |
2021年 | 163篇 |
2020年 | 163篇 |
2019年 | 208篇 |
2018年 | 135篇 |
2017年 | 200篇 |
2016年 | 149篇 |
2015年 | 189篇 |
2014年 | 209篇 |
2013年 | 243篇 |
2012年 | 264篇 |
2011年 | 220篇 |
2010年 | 118篇 |
2009年 | 185篇 |
2008年 | 153篇 |
2007年 | 194篇 |
2006年 | 152篇 |
2005年 | 146篇 |
2004年 | 118篇 |
2003年 | 120篇 |
2002年 | 76篇 |
2001年 | 60篇 |
2000年 | 47篇 |
1999年 | 48篇 |
1998年 | 42篇 |
1997年 | 43篇 |
1996年 | 25篇 |
1995年 | 28篇 |
1994年 | 20篇 |
1993年 | 16篇 |
1992年 | 10篇 |
1991年 | 13篇 |
1990年 | 6篇 |
1989年 | 8篇 |
1988年 | 5篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1980年 | 6篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 4篇 |
1976年 | 1篇 |
1954年 | 2篇 |
排序方式: 共有3971条查询结果,搜索用时 15 毫秒
991.
Surface measurements of cloud condensation nuclei (CCN) number concentration (cm−3) are presented for unmodified marine air and for polluted air at Mace Head, for the years 1994 and 1995. The CCN number concentration active at 0.5% supersaturation is found to be approximately log-normal for marine and polluted air at the site. Values of geometric mean, median and arithmetic mean of CCN number concentration (cm−3) for marine air are in the range 124–135, 140–150 and 130–157 for the two years of data. Analysis of CCN number concentration for high wind speed, U, up to 20 m s−1 show enhanced CCN production for U in excess of about 10–12 m s−1. Approximately 7% increase in CCN per 1 m s−1 increase in wind speed is found, up to 17 m s−1. A relationship of the form log10CCN=a+bU is obtained for the periods March 1994 and January, February 1995 for marine air yielding values a of 1.70; 1.90 and b of 0.035 for both periods. 相似文献
992.
Many Large Eddy Simulation (LES) models use the classic Kessler parameterisation either as it is or in a modified form to model the process of cloud water autoconversion into precipitation. The Kessler scheme, being linear, is particularly useful and is computationally straightforward to implement. However, a major limitation with this scheme lies in its inability to predict different autoconversion rates for maritime and continental clouds. In contrast, the Berry formulation overcomes this difficulty, although it is cubic. Due to their different forms, it is difficult to match the two solutions to each other. In this paper we single out the processes of cloud conversion and accretion operating in a deep model cloud and neglect the advection terms for simplicity. This facilitates exact analytical integration and we are able to derive new expressions for the time of onset of precipitation using both the Kessler and Berry formulations. We then discuss the conditions when the two schemes are equivalent. Finally, we also critically examine the process of droplet evaporation within the framework of the classic Kessler scheme. We improve the existing parameterisation with an accurate estimation of the diffusional mass transport of water vapour. We then demonstrate the overall robustness of our calculations by comparing our results with the experimental observations of Beard and Pruppacher, and find excellent agreement. 相似文献
993.
Lanru Jing 《Engineering Geology》1998,49(3-4):371-381
Continuum-based numerical methods have played a leading role in the numerical solution of problems in rock mechanics and engineering geology. However, for fractured rocks, a continuum assumption often leads to difficult parameters to define and over-simplified geometry to be realistic. In such case, discrete representations of fractures and individual blocks must be adopted. In this paper, a newly emerged member in the family of discrete element methods (DEM), the discontinuous deformation analysis (DDA), is presented, including its variational principle, governing equations, solution techniques and contact representation and detection algorithms. Its relative advantages and shortcomings are compared with the explicit distinct element method and the finite element method. An example of the analysis of tunnel stability is provided to demonstrate the capability of this new method. 相似文献
994.
995.
在过去的20年里,探测和勾画岩浆囊的科学有了长足的进步。多种地震技术,包括速度和衰减的层析成像,地震活动性映像,反射和折射,势场技术,如重力,大地测量和电磁观测,都提供了关于在几个火山下面的岩浆囊的大小,形状和物理状态的有用信息。本文对这些技术进行了讨论和综述。我国已在P波波速层析成像方面开展了一系列工作。特别是在云南地区已得到了该省及其邻近地区的三维速度图像。由于现有台网不够密集,获得的速度图像分辨较低。尽管如此,在腾冲地区从20km到450km深度都有低速度结构的显示。要想系统地研究腾冲火山地区的火山构造和地震活动性,以及探测该地区的岩浆囊,必须在该地区布设较密集的地震观测台网。 相似文献
996.
997.
EVALUATION OF GROUND STROKE DENSITY DISTRIBUTION AND LIGHTNING DETECTION EFFICIENCY IN BEIJING-TIANJIN-HEBEI AREA 总被引:8,自引:0,他引:8
It is possible to understand the lightning activities in a specific region and compare test resultsof different apparatus only when a reliable evaluation of detection efficiency distribution pertainingto a particular lightning location system(LLS)is available.Based on the data in 1992.an approximate evaluation of detection efficiency spatial distribu-tion for single-station lightning location system(M-LDARS)and LLP three-station lightning loca-tion system in Beijing-Tianjin-Hebei area is presented in the paper,showing that the average detec-tion efficiencies are smaller than 48% and 46% respectively.In addition,the article offers an eval-uation of spatial distribution of ground stroke density(D_g)and positive stroke percentage,indicat-ing that six high stroke density zones exist along the southeastern sides of the Taihang and Yan-shan Mountains.The stroke density of mountainous region is higher than that of the plain region,in contrast to the case of positive stroke percentage.Also,it is shown that within a radius of 250km,the average of D_g is 1.2(km~(-2) a~(-1))while the average positive stroke percentage is 10.9%.Finally,the paper proposes a possible test method of accurately evaluating the spatial distributionof detection efficiency(A). 相似文献
998.
LOG算子是边缘提取中一种常用算法.着重讨论了应用此种方法进行边缘提取所存在的两个问题,并探讨了解决的方法。 相似文献
999.
影像特征的表示和描述是计算机视觉中的重要内容。本文给出了基于小波变换的影像特征检测算法,从理论上说明了利用不同分辨率上小波变换系数的模值描述特征强度的合理性。 相似文献
1000.
针对航空影像中目标提取的困难,提出了一种基于可变模板的航空影像中建筑物提取方法,并针对几种比较典型的建筑物,详细讨论了模板的设计、目标初始参数的获取及模板最优参数求解等几个关键性的问题。采用这种方法进行航空影像中目标的提取时,可以将影像中的多种信息和人的识别能力进行有效的融合,因而能够达到可靠地提取影像中目标的目的。作为一种有效的优化求解方法,模拟退火被用于目标提取的优化过程。为了验证这种方法提取航空影像中目标的能力,给出了利用可变模板提取航空影像中几种不同类型的建筑物的例子 相似文献