全文获取类型
收费全文 | 71258篇 |
免费 | 1127篇 |
国内免费 | 666篇 |
专业分类
测绘学 | 1714篇 |
大气科学 | 5030篇 |
地球物理 | 14226篇 |
地质学 | 25249篇 |
海洋学 | 6165篇 |
天文学 | 16332篇 |
综合类 | 208篇 |
自然地理 | 4127篇 |
出版年
2022年 | 396篇 |
2021年 | 704篇 |
2020年 | 778篇 |
2019年 | 826篇 |
2018年 | 1893篇 |
2017年 | 1760篇 |
2016年 | 2236篇 |
2015年 | 1318篇 |
2014年 | 2178篇 |
2013年 | 3771篇 |
2012年 | 2284篇 |
2011年 | 3119篇 |
2010年 | 2605篇 |
2009年 | 3504篇 |
2008年 | 3255篇 |
2007年 | 3045篇 |
2006年 | 2877篇 |
2005年 | 2353篇 |
2004年 | 2253篇 |
2003年 | 2111篇 |
2002年 | 1928篇 |
2001年 | 1800篇 |
2000年 | 1716篇 |
1999年 | 1377篇 |
1998年 | 1470篇 |
1997年 | 1387篇 |
1996年 | 1100篇 |
1995年 | 1142篇 |
1994年 | 962篇 |
1993年 | 872篇 |
1992年 | 855篇 |
1991年 | 759篇 |
1990年 | 856篇 |
1989年 | 717篇 |
1988年 | 650篇 |
1987年 | 815篇 |
1986年 | 663篇 |
1985年 | 856篇 |
1984年 | 922篇 |
1983年 | 862篇 |
1982年 | 832篇 |
1981年 | 707篇 |
1980年 | 665篇 |
1979年 | 606篇 |
1978年 | 604篇 |
1977年 | 553篇 |
1976年 | 539篇 |
1975年 | 499篇 |
1974年 | 508篇 |
1973年 | 469篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Ohne Zusammenfassung 相似文献
992.
The aim of this paper is to present earthquake loss estimations for a portion of downtown Ottawa, Canada, using the HAZUS-MH (Hazards United States Multi-Hazard) software tool. The assessment is performed for a scenario earthquake of moment magnitude 6.5, at an epicentral distance of 15 km, occurring during business hours. A level 2 HAZUS-MH analysis was performed where the building inventory, microzonation studies, and site-specific ground motion hazard maps (2% exceedence probability in 50 years) were all improved based on local information. All collected data were assembled into a set of standard geodatabases that are compatible with the HAZUS-MH software using a GIS-specific procedure. The results indicate that the greatest losses are expected in unreinforced masonry buildings and commercial buildings. Sensitivity studies show that soil classes, the vulnerability of schools, and the spatial scale of loss estimations are also important factors to take into account. 相似文献
993.
994.
M. L. Kaplan J. J. Charney K. T. WaightIII K. M. Lux J. D. Cetola A. W. Huffman A. J. Riordan S. D. Slusser M. T. Kiefer P. S. Suffern Y.-L. Lin 《Meteorology and Atmospheric Physics》2006,94(1-4):235-270
Summary In this paper, we will focus on the real-time prediction of environments that are predisposed to producing moderate-severe
(hazardous) aviation turbulence. We will describe the numerical model and its postprocessing system that is designed for said
prediction of environments predisposed to severe aviation turbulence as well as presenting numerous examples of its utility.
The purpose of this paper is to demonstrate that simple hydrostatic precursor circulations organize regions of preferred wave
breaking and turbulence at the nonhydrostatic scales of motion. This will be demonstrated with a hydrostatic numerical modeling
system, which can be run in real time on a very inexpensive university computer workstation employing simple forecast indices.
The forecast system is designed to efficiently support forecasters who are directing research aircraft to measure the environment
immediately surrounding turbulence.
The numerical model is MASS version 5.13, which is integrated over three different grid matrices in real-time on a university
workstation in support of NASA-Langley’s B-757 turbulence research flight missions. The model horizontal resolutions are 60,
30, and 15 km and the grids are centered over the region of operational NASA-Langley B-757 turbulence flight missions.
The postprocessing system includes several turbulence-related products including four turbulence forecasting indices, winds,
streamlines, turbulence kinetic energy, and Richardson numbers. Additionally there are convective products including precipitation,
cloud height, cloud mass fluxes, lifted index, and K-index. Furthermore, soundings, sounding parameters, and Froude number
plots are also provided. The horizontal cross section plot products are provided from 16,000–46,000 feet in 2,000 feet intervals.
Products are available every three hours at the 60 and 30 km grid interval and every 1.5 hours at the 15 km grid interval.
The model is initialized from the NWS ETA analyses and integrated two times a day. 相似文献
995.
A. A. M. Holtslag G. J. Steeneveld B. J. H. van de Wiel 《Boundary-Layer Meteorology》2007,125(2):361-376
At present a variety of boundary-layer schemes is in use in numerical models and often a large variation of model results
is found. This is clear from model intercomparisons, such as organized within the GEWEX Atmospheric Boundary Layer Study (GABLS).
In this paper we analyze how the specification of the land-surface temperature affects the results of a boundary-layer scheme,
in particular for stable conditions. As such we use a well established column model of the boundary layer and we vary relevant
parameters in the turbulence scheme for stable conditions. By doing so, we can reproduce the outcome for a variety of boundary-layer
models. This is illustrated with the original set-up of the second GABLS intercomparison study using prescribed geostrophic
winds and land-surface temperatures as inspired by (but not identical to) observations of CASES-99 for a period of more than
two diurnal cycles. The model runs are repeated using a surface temperature that is calculated with a simple land-surface
scheme. In the latter case, it is found that the range of model results in stable conditions is reduced for the sensible heat
fluxes, and the profiles of potential temperature and wind speed. However, in the latter case the modelled surface temperatures
are rather different than with the original set-up, which also impacts on near-surface air temperature and wind speed. As
such it appears that the model results in stable conditions are strongly influenced by non-linear feedbacks in which the magnitude
of the geostrophic wind speed and the related land-surface temperature play an important role. 相似文献
996.
冕洞的研究在近二十多年里取得了丰硕的成果。本文回顾了冕洞的发现及观测历史,系统阐述了冕洞的结构特征、形成及演化规律,讨论了冕洞对日地空间产生的影响,冕洞与超级活动区的关系以及冕洞在太阳活动预报中所起的作用,在此基础上利用1970—1995年的冕洞资料对冕洞的时空分布和磁极性演化规律与太阳活动周的关系,以及冕洞与太阳风速度、地磁扰动等方面进行分析研究,得出以下结论:(1)冕洞在南北半球的分布在形态上基本是对称的,但在冕洞数量上北半球稍占优势;(2)冕洞的盛衰演化呈周期性,表现为赤道冕洞周期与黑子周期是完全一致的,极冕洞周期与黑子周期相位相差180°;(3)赤道冕洞的纬度分布随太阳活动周上升而上升,当太阳活动周达到极大值时,它也达到极大,然后再随太阳活动周下降而下降,极冕洞的纬度延伸方向演化与赤道冕洞相反;(4)极冕洞的极场呈11年周期性,并且极场反转出现在太阳活动峰年期间;(5)太阳风和地磁扰动与冕洞的演化有着密切的关系 相似文献
997.
Z. B. Li L. M. Song J. L. Qu Y. J. Lei J. Y. Nie C. M. Zhang 《Astrophysics and Space Science》2012,341(2):383-393
In this paper, the rms-flux (root mean square-flux) relation along the Z-track of the bright Z-Source Cyg X-2 is analyzed using the observational data of Rossi X-ray Timing Explorer (RXTE). Three types of rms-flux relations, i.e. positive, negative, and ‘arch’-like correlations are found in different branches. The rms is positively correlated with flux in normal branch (NB), but anti-correlated in the vertical horizontal branch (VHB). The rms–flux relation shows an ‘arch’-like shape in the horizontal branch (HB). We also try to explain this phenomenon using existing models. 相似文献
998.
999.
1000.
We are investigating the effects of in vivo exposure of prototypical enzyme inducing agents on hepatic biotransformation enzyme expression in largemouth bass (Micropterus salmoides), a predatory game fish found throughout the United States and Canada. The current study targeted those genes involved in biotransformation and oxidative stress that may be regulated by Ah-receptor-dependent pathways. Exposure of bass to beta-naphthoflavone (beta-NF, 66 mg/kg, i.p.) elicited a 7-9-fold increase in hepatic microsomal cytochrome P4501A-dependent ethoxyresorufin O-deethylase (EROD) activities, but did not affect cytosolic GST catalytic activities toward 1-chloro-2,4-dinitrobenzene (CDNB) or 5-androstene-3,17-dione (ADI). Glutathione S-transferase A (GST-A) mRNA expression exhibited a transient, but non-significant increase following exposure to beta-NF, and generally tracked the minimal changes observed in GST-CDNB activities. Expression of the mRNA encoding glutamate-cysteine ligase catalytic subunit (GCLC), the rate-limiting enzyme in glutathione (GSH) biosynthesis, was increased 1.7-fold by beta-NF. Changes in GCLC mRNA expression were paralleled by increases in intracellular GSH. In summary, largemouth bass hepatic CYP1A-dependent and GSH biosynthetic pathways, and to a lesser extent GST, are responsive to exposure to beta-NF. 相似文献