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   8734篇
  免费   1747篇
  国内免费   2501篇
测绘学   929篇
大气科学   1904篇
地球物理   2152篇
地质学   4515篇
海洋学   1217篇
天文学   339篇
综合类   966篇
自然地理   960篇
  2024年   48篇
  2023年   158篇
  2022年   463篇
  2021年   555篇
  2020年   438篇
  2019年   558篇
  2018年   497篇
  2017年   520篇
  2016年   573篇
  2015年   570篇
  2014年   627篇
  2013年   597篇
  2012年   578篇
  2011年   625篇
  2010年   574篇
  2009年   589篇
  2008年   528篇
  2007年   485篇
  2006年   411篇
  2005年   385篇
  2004年   255篇
  2003年   223篇
  2002年   254篇
  2001年   237篇
  2000年   239篇
  1999年   289篇
  1998年   253篇
  1997年   210篇
  1996年   234篇
  1995年   176篇
  1994年   169篇
  1993年   169篇
  1992年   140篇
  1991年   75篇
  1990年   65篇
  1989年   50篇
  1988年   45篇
  1987年   24篇
  1986年   27篇
  1985年   13篇
  1984年   11篇
  1983年   8篇
  1982年   9篇
  1981年   8篇
  1980年   3篇
  1979年   3篇
  1978年   2篇
  1958年   7篇
  1957年   1篇
  1954年   2篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
981.
江王记 《现代测绘》2007,30(6):37-38
在通道中利用“阂值”命令创建选择X-255。在通道混合中创建任意色阶区间选择X1~X2。利用通道混合“减去”模式中“缩放”“补偿值”实现深度自由选择。  相似文献   
982.
基于距离变换的提取和识别点状符号的方法   总被引:1,自引:0,他引:1  
提出了一种基于距离变换提取和识别点状符号的新方法。首先对分版图的提取结果进行优化处理,然后根据弯曲密度和线划密度等特征,进一步过滤掉不满足点状符号特征的线划。为了更准确地度量图像间的相似性,提出了加权距离函数,并用该函数值为度量标准进行模板匹配以识别符号。该算法较好地解决了字线粘连、背景复杂等情况下的点状符号提取和识别问题,识别准确率较高。  相似文献   
983.
费一雄 《四川气象》2007,27(3):21-22
本文回顾了四川盆地近五十多年来气温的时段性变化。从1997年开始的盆地性变暖,尚属国内气温变暖时期迟、变暖程度低的少数地区之一,随着时间的推移,还将进一步变暖,从而增加极端性天气的发生频率,需要认真应对。  相似文献   
984.
The studies on the effect of atmospheric aerosol on climate and environment are hot issues in the current circle of international science and technology. In recent years the pollution of aerosol is getting worse and worse over the Pearl River Delta. The clouds of aerosol occur all year round, with heavy pollution area located at the western side at the mouth of Pearl River. The haze weather mainly occurs from October to April next year, resulting in visibility deterioration. From the beginning of 1980s, visibility dramatically deteriorated, obviously increasing haze weather, in which there are three big fluctuations, showing the periods of pollutions of dust, sulphate and dust, fine particle from photochemical process and sulphate and dust accompanying with the development of economy respectively. The long-term tendency of visibility caused by fog and light fog does not show a tendency due to human activities or economic development, which mainly shows the interannual and interdecadal variation of climate. The deterioration of visibility has close relation to the fine particles over Pearl River Delta, with half of PM10 overpass the limited value set by national second graded standard (150μg m-3), meanwhile, all values of PM2.5 overpass the day-mean limited value of American national standard (65μg m-3), especially from October to January next year, monthly mean values of PM2.5 almost reach two times of standard value, indicating the fine particle concentration is very high. The ratio of PM2.5 to PM10 is also very high, reaching 58%-77%, higher especially in dry season than in rainy season. Thus it is the fine particle pollution in aerosol pollution over the Pearl River Delta. Compared with the data of 15 years ago, the ratio of fine particle to aerosol has obviously increased.  相似文献   
985.
气象条件对大气波导的影响   总被引:3,自引:0,他引:3  
在介绍大气折射基本类型及其对电磁波影响的基础上,概述了三种类型大气波导的形成机制,分析了不同天气系统、气象要素和天气现象对大气波导的影响,总结了易于出现大气波导现象的天气条件。研究气象条件对大气波导的影响,对于研究和预测电磁波在大气中的传播特性以及在军事上的应用都具有十分重要的意义。  相似文献   
986.
探讨地图集电子版的设计原则,从《江苏省地图集》电子版的设计与实现入手,分析地图集电子版的设计思路和技术方案.阐述多媒体电子地图集的解决方案.  相似文献   
987.
黑龙江老柞山金矿床是佳木斯地块中北部的大型热液叠加夕卡岩型矿床.对矿区东矿带赋矿片麻状花岗岩进行了LA-ICP-MS的U-Pb定年和Hf同位素分析,共获得3组年龄,分别为697.1±8.0、703.0±8.8和484.7±2.7 Ma.其中前2组为捕获的锆石年龄,暗示佳木斯地块南缘可能存在新元古代的结晶基底.第三组锆石年龄代表片麻状花岗岩结晶年龄.锆石原位Hf同位素测试显示,片麻状花岗岩的εHft)值为-2.4~+3.6,TDM2为1620~1390 Ma,指示片麻状花岗岩原岩可能是中元古代陆壳物质重熔.结合区域构造演化,认为片麻状花岗岩可能形成于晚泛非-早加里东期的碰撞造山作用,是麻山群深熔岩浆结晶作用的产物.  相似文献   
988.
The loess-paleosol sequences of the last 1.2 Ma in China have recorded two kinds of climate extremes: the strongly developed S4, S5-1 and S5-3 soils (corresponding to the marine δ18O stages 11, 13, and 15, respectively) as evidence of three episodes of great warmth and two coarse-grained loess units (L9 and L15, corresponding to the marine δ18O stages 22, 23, 24 and 38, respectively) which indicate severest glacial conditions. The climatic and geographical significance of these events are still unclear, and their cause remains a puzzle.Paleopedological, geochemical and magnetic susceptibility data from three loess sections (Xifeng, Changwu and Weinan) suggest that the S4, S5-1 and S5-3 soils were formed under sub-tropical semi-humid climates with a tentatively estimated mean annual temperature (MAT) of at least 4–6°C higher and a mean annual precipitation (MAP) of 200–300 mm higher than for the present-day, indicating a much strengthened summer monsoon. The annual rainfall was particularly accentuated for the southern-most part of the Loess Plateau, suggesting that the monsoon rain belt (the contact of the monsoonal northward warm-humid air mass with the dry-cold southward one) might have stood at the southern part of the Plateau for a relatively long period each year. The loess units L9 and L15 were deposited under semi-desertic environments with a tentatively estimated MAT and MAP of only about 1.5–3°C and 150–250 mm, indicating a much strengthened winter monsoon, and that the summer monsoon front could rarely penetrate into the Loess Plateau region.Correlation with marine carbon isotope records suggests that these climate extremes have large regional, even global, significance rather than being local phenomena in China. They match the periods with greatest/smallest Atlantic–Pacific δ13C gradients, respectively, indicating their relationships with the strength of Deep Water (NADW) production in the North Atlantic. These results suggest that the monsoon climate in the Loess Plateau region was significantly linked with the North Atlantic thermohaline circulation on timescales of 104 years.  相似文献   
989.
INTRODUCTION~hon0ffornjcelmtSwnthebasisofthestudyonthecrustaldefonnationanddeVel0prnent,thedistributionofndnereIs,andthendnerogeneticlawr.ThegeornagneticfieklisthecomP0siterefledionoftheformati0nanddefortnationofthecrust.Sornagnct-icsLLrVeydatacomPriseirnPortantbasisfordelincatingthearchiteCtonicelements,dCtendningtheirboundarylines,andanalopngthesanndarystrudriineachelernent,whicharetthernaintasks0fgoophySicalworkers.Thegoornagneticdelineati0nofanareahadforlongbodonequalitativeybyobs…  相似文献   
990.
Xi-hua  Wang  Song  Qin  Xin-ping  Li  Peng  Jiang  Cheng-kui  Zeng  Mei  Qin 《中国海洋湖沼学报》1998,16(1):62-66

This study on the effects of ultrasonic treatment on female gametophytes ofLaminaria japonica showed that:

  1. 1.

    Ultrasonic treatment had shortening effect on filaments of female gametophytes. Within certain period of time, the average length of filamentous female gametophytes was shortened.

  2. 2.

    Ultrasonic treatment had emptying effect on cells. The number of empty cells increased with time of treatment. Ultrasonic treatment had harmful effect on cells.

  3. 3.

    Ultrasonic treatment could break down cell walls. The combination of frequency of 20 kHz, output of 15 W, 40 s and 60 s of treatment was best for this purpose. After ultrasonic treatment, the regeneration of female gametophytes into sporophytes was effected. Female ganetophytes could not recover after too long period of treatment.

  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