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   326篇
  免费   43篇
  国内免费   54篇
测绘学   32篇
大气科学   37篇
地球物理   41篇
地质学   142篇
海洋学   56篇
天文学   67篇
综合类   15篇
自然地理   33篇
  2024年   1篇
  2023年   4篇
  2022年   13篇
  2021年   12篇
  2020年   4篇
  2019年   26篇
  2018年   15篇
  2017年   12篇
  2016年   17篇
  2015年   20篇
  2014年   16篇
  2013年   24篇
  2012年   14篇
  2011年   22篇
  2010年   14篇
  2009年   7篇
  2008年   9篇
  2007年   10篇
  2006年   11篇
  2005年   14篇
  2004年   8篇
  2003年   21篇
  2002年   31篇
  2001年   14篇
  2000年   13篇
  1999年   11篇
  1998年   7篇
  1997年   5篇
  1996年   6篇
  1995年   4篇
  1994年   7篇
  1993年   4篇
  1992年   1篇
  1991年   3篇
  1990年   7篇
  1989年   2篇
  1988年   2篇
  1987年   3篇
  1986年   2篇
  1985年   2篇
  1984年   1篇
  1983年   1篇
  1979年   1篇
  1958年   2篇
排序方式: 共有423条查询结果,搜索用时 62 毫秒
71.
The present work reports on a methodology to assess the climatic severity of a particular geographic region as compared to specific information available in the current regulations. The viability for each of the 387 municipalities in the Autonomous Community of Extremadura (Spain) is analysed, making a distinction between those with reliable climate reports and those for which no such information is available. In the case study, although the weather conditions in Extremadura are quite homogeneous according to the Spanish Technical Building Code (STBC 2015) classification and most areas are associated to zone C4 (soft winters and hot summers), the southern area in the region is associated to zone D1, similar to the north of Spain, where winters and summers are cool, which does not coincide with the actual climate in the south of Extremadura. The general climatic homogeneity in Extremadura was also highlighted with the new procedure, predominating zone C4, but unexpected or unreal climatic zoning was not generated, giving place to a consistent spatial distribution of zones throughout the region. Consequently, the proposed method allows a more accurate climatic zoning of any region in agreement with the Spanish legislation on energy efficiency in buildings, which would enhance the setting of thermal demand rates according to the actual climatic characterisation of the area in which a particular municipality is located.  相似文献   
72.
Measurements of the Sun in the near-infrared He i 10830 Å absorption line were performed using the echelle spectrograph with a dispersion of 6.71 mÅ per pixel at the Vacuum Tower Telescope (German Solar Telescopes, Teide Observatory, Izaña, Tenerife, Spain) on May 26, 1993. These measurements were compared with full-disc soft X-ray images of the Sun (Japanese solar satellite Yohkoh), full-disc solar images in H (Big Bear Solar Observatory), full-disc solar images in the He i 10830 Å line (National Solar Observatory, Kitt Peak) and with full-disc microwave solar maps at 37 GHz (Metsähovi Radio Research Station). In the He 10830 Å line the Sun displays a limb darkening similar to that in the visible part of the spectrum. Active regions and H filaments show a strong absorption in the He 10830 Å line, whereas the absorption is weak in coronal holes.  相似文献   
73.
There is considerable concern that isoflavones, such as genistein in fish feed composed of soybean protein, aff ects somatic growth in fish. Our previous works demonstrated that 30 and 300 μg/g dietary genistein had no significant eff ect on growth performance in Nile tilapia (Oreochromis niloticus), but the higher level of genistein (3 000 μg/g) significantly depressed growth. This study was conducted to further examine the eff ects of dietary genistein on the endocrine disruption on growth hormone/insulin-like growth factor-I (GH/IGF-I) axis in Nile tilapia (O. niloticus). Juvenile fish were fed by hand twice daily to satiation with one of four isonitrogenous and isoenergetic diets, each containing either 0, 30, 300 or 3 000 μg/g genistein. Following an 8-week feeding period, plasma GH and IGF-I levels were investigated by radioimmunoassay and gene expression levels of gh, ghrelin, gnrhs, ghr, npy, npyrs, pacap, ghrs, igf-I, igf-Ir, and igfbp3 were examined by real-time PCR. The results show that no significant change in plasma GH and IGF-I levels in fish fed with diets containing 30 μg/g and 300 μg/g genistein. mRNA expression of genes along the GH/IGF-I axis remained unaff ected, except for igf-Ir, which was stimulated by the 300 μg/g genistein diet. While in fish fed the 3 000 μg/g genistein diet, the plasma GH and IGF-I levels decreased, and mRNA expression of gh, ghr2, npyr1, igf-I, and igf-Ir were also significantly depressed. In contrast, npy and igfbp3 mRNA expression were enhanced. This study provides convincing evidence for growth impediment by genistein by disturbing the GH/IGF-I axis in Nile tilapia O. niloticus.  相似文献   
74.
创新地理学研究的几个理论问题   总被引:10,自引:4,他引:6  
吕拉昌  黄茹  廖倩 《地理科学》2016,36(5):653-661
创新地理学是研究人类创新活动与地理环境关系的地域系统,是一门独立的人文地理分支学科,具有交叉学科的性质。其研究的“人类创新活动”是人类活动的最为重要的方面,对智慧的人地关系系统建设具有重要的意义。创新地理学与其他人文地理学的分支学科具有密切的联系,也与政治学、管理学、经济学、政策学、城市规划等学科有关,创新地理学面临的主要任务是:创新地理学基本理论的研究;创新要素(人才、资本、技术等)在空间的地域分布与组合规律的研究;创新环境、创新生态及评价研究;创新地理测度、创新空间格局与效应的研究;创新联系、创新网络及创新集群的研究;多尺度的创新体系的研究;创新、城市发展与规划的研究。  相似文献   
75.
94-95斫养殖对虾病毒爆发流行期间,对患病中国对虾肝胰上皮细胞和胃肌细胞进行了电镜检测,发现一种核包涵体型杆状病毒(140-160nm×280-330nm),对其装配机质的超微结构研究表明,核包涵 体型杆状病毒以核内装配为主,部分病毒基质转入胞质进行了装配。  相似文献   
76.
基于小波变换和“改变了的”零树法编解码原理 ,结合水声信道的特点 ,构建了一个以 DSP为核心的图像处理系统。在不同压缩比的情况下 ,完成了对一幅 6 4× 6 4点阵的静态彩色图像数据的压缩和解压缩。当压缩比与传统的 JPEG算法的压缩比相近的情况下 ,解压缩后图像的总体观感效果有明显的改善 ,从而对在水声信道中传输彩色图像作出了积极的尝试  相似文献   
77.
楚科奇海与白令海表层沉积中的钙质和硅质微体化石研究   总被引:13,自引:4,他引:13  
通过对北冰洋楚科奇海和令海41个表层沉积样品中的有孔虫、介形类等钙质微体化石和硅藻、放射虫、海绵骨针等硅质微体化石的定量分析,发现表层沉积中浮游有孔虫几乎缺失,这可能与该区表层生产力相对低、碳酸盐溶解作用较强有关,而底栖有孔虫和硅质微体化石的丰度分布则明显受表层沉积物类型、表层初级生物生产力和碳酸盐溶解作用所控制。其中,北冰洋楚科奇海陆架区有孔虫丰度和分异度低,含少量浅水介形类,放射虫在陆架浅水区缺失,但含有较多硅藻和海绵骨针等其它硅质微体化石,反映该区由于海冰、表层海水温度较冷而导致表歧初级生产力相对低。白令海陆坡区底栖有孔虫丰度比较科奇海高一个数量级,底栖有孔虫分异度也相对高,硅藻、放射虫、海绵骨针等硅质微体化石的丰度与钙质化石一样,其丰度比楚科奇海明显高,反映表层初级生产力相对高。根据白令海陆坡区底栖有孔虫和硅质微体化石丰度、底栖有孔虫胶结质壳比值的水深变化,推测该区碳酸盐溶跃层和补偿深度(CCD)相对浅,分别位于水深2000m和3800m处。  相似文献   
78.
The internal structure of prominences appearing as twisted tubes was studied. The sample embraced 15 stable and 13 eruptive prominences, exposing patterns which possibly reflect a helical configuration. The equivalent pitch angles () of twisted fine structure features were measured. In some cases the evolution of the internal structure was followed and 49 independent measurements of the parameter were performed in total. The results are presented in the plane relating the parameter and the normalized prominence height. The eruptive prominences occupy the region characterized by > 50° and h > 0.8d, where h and d are the prominence height and the footpoint half-separation, respectively. All prominences characterized by h < 0.6d or by < 35° were stable. Such a result is in good agreement with an order of magnitude treatment of the forces acting in a curved magnetic tube, anchored at both ends in the photosphere.  相似文献   
79.
We studied 24 spotless flares of Ha importance 1 which occurred during the 21st cycle of solar activity. The spotless flares could be grouped in three categories according to their location and time history of the associated active region. Our association of the flares with radio events was based on relative timing and on the flare importances. Weak microwave gradual rise and fall events were frequently recorded during the occurrence of the spotless flares. A few flares from our sample could be associated with impulsive and complex microwave bursts. Only in one case an association of a spotless flare with a significant metric type II/IV event seems to be justified.Proceedings of the Second CESRA Workshop on Particle Acceleration and Trapping in Solar Flares, held at Aubigny-sur-Nère (France), 23–26 June, 1986.  相似文献   
80.
Abstract

In this paper, we introduce the cyclostationary processes into climate analysis and undertake a systematic study of the cyclic spectra of surface temperature fluctuations. The technique is adapted from cyclostationarity theory in signal processing. To demonstrate the usefulness of this technique, a very simple cyclostationary stochastic climate model is constructed. Our results show that the seasonal cycle strongly modulates the amplitudes of the covariance and the spectrum. The technique was also applied to the surface temperature fluctuations in a fifteen‐year seasonal run of the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM2, R15) using a zonally symmetric all‐land surface as the lower boundary. The results indicate that intraseasonal oscillations localized according to time of year are still present even after the surface temperature fields have been normalized using the commonly used procedure. Both examples suggest that the “annual cycle” cannot be “removed” by simply using a normalization procedure. The climate is not as completely represented when modelled as stationary processes.  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