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   2502篇
  免费   507篇
  国内免费   938篇
测绘学   103篇
大气科学   1310篇
地球物理   507篇
地质学   983篇
海洋学   479篇
天文学   30篇
综合类   182篇
自然地理   353篇
  2024年   26篇
  2023年   52篇
  2022年   89篇
  2021年   116篇
  2020年   129篇
  2019年   140篇
  2018年   109篇
  2017年   108篇
  2016年   119篇
  2015年   111篇
  2014年   165篇
  2013年   169篇
  2012年   176篇
  2011年   201篇
  2010年   148篇
  2009年   199篇
  2008年   184篇
  2007年   195篇
  2006年   191篇
  2005年   148篇
  2004年   138篇
  2003年   123篇
  2002年   109篇
  2001年   112篇
  2000年   74篇
  1999年   75篇
  1998年   77篇
  1997年   77篇
  1996年   68篇
  1995年   49篇
  1994年   58篇
  1993年   47篇
  1992年   33篇
  1991年   31篇
  1990年   21篇
  1989年   19篇
  1988年   27篇
  1987年   8篇
  1986年   8篇
  1985年   3篇
  1984年   5篇
  1983年   1篇
  1982年   1篇
  1981年   1篇
  1978年   5篇
  1973年   1篇
  1954年   1篇
排序方式: 共有3947条查询结果,搜索用时 15 毫秒
221.
关于气候系统与地球系统的若干思考   总被引:9,自引:2,他引:9       下载免费PDF全文
分析了地球系统的基本特征和地球系统的能源和运转方式,指出地热涡和地冷涡是地面气候系统的重要成员. 利用全球地磁场资料,研究了地球内部焦耳热的分布和演变规律,结果表明:全球地磁场的变化与全球气温的变化有密切联系,而且焦耳热场的分布与地热带、火山分布、全球最大降水量带有很好的对应. 因此,要对气候变化有一个全面的认识,必须研究整个地球系统.  相似文献   
222.
城市热岛效应下浅层土中混凝土的酸腐蚀试验研究   总被引:1,自引:0,他引:1  
卢毅  施斌  刘瑾  顾凯  唐朝生  高磊 《地震学刊》2011,(3):241-246,274
针对城市热岛效应引起城区浅层土地温场升高这一观测结果,采用室内快速模拟试验方法,对城区酸性土壤腐蚀混凝土材料的温度效应开展了试验研究。试验研究了在温度为5℃、20℃和40℃条件下,混凝土试样在酸浓度分别为0%、5%和10%的沙土介质中放置30d、90d后的抗压强度变化规律,并对混凝土试样在腐蚀过程中的腐蚀系数变化规律和微观机理进行了分析;最后,对南京城区热岛效应环境下浅层土中混凝土材料酸腐蚀的强度变化规律进行了分析。试验结果表明,在同一腐蚀浓度下,混凝土试样抗压强度的下降速度随着温度的升高而不断加快,其腐蚀系数也随着温度的升高而不断增大;腐蚀介质的浓度越大,环境温度对混凝土腐蚀系数的影响就越明显;在温度为40℃、硫酸浓度为10%的沙土中放置30 d后,混凝土的腐蚀系数K达到45.21%,约是在5℃条件下腐蚀系数的2倍,相当于环境温度每上升1℃,混凝土腐蚀系数平均增加0.64%。该研究成果对于减轻城市热岛效应对岩土工程的影响具有重要意义。  相似文献   
223.
藏南羊卓雍错湖面大气湍流特征观测分析   总被引:1,自引:1,他引:1  
沈鹏珂  张雪芹 《湖泊科学》2019,31(1):243-255
湍流运动是大气边界层的本质特征,是地表与大气之间能量和物质交换的主要方式.本文利用2016和2017年4-10月藏南羊卓雍错湖泊涡动观测资料,分析了湖面大气湍流方差和湍流特征量的统计和变化特征.结果表明:(1)不稳定层结下,三维风速分量和超声虚温、水汽密度、CO2密度的无量纲标准差随稳定度变化符合Monin-Obukhov相似理论的"1/3"或"-1/3"次幂律,垂直风速的拟合效果最好;稳定层结下,除CO2密度无量纲标准差与稳定度无明显关系外,其他量基本上满足相似性规律;中性条件下,以上物理量的无量纲标准差分别趋近常数:3.57、3.93、0.77、20.91、6.35和11.96.(2)水平方向平均湍流强度(0.60和0.58)大于垂直方向(0.13),三维方向湍流强度与平均风速的变化呈显著负相关,相关系数分别为-0.39、-0.42和-0.34.(3)湖面湍流动能随风速呈线性增长,增长率达0.45 m/s;近中性层结时湍流动能最大,层结越稳定或不稳定湍流动能均减小.(4)湖泊下午到傍晚动量输送较强,13:00-22:30时间段平均动量通量达0.091 kg/(m·s2);热量输送以潜热为主,潜热通量日平均值(77.3 W/m2)是感热通量(14.6 W/m2)的5.3倍,感热和潜热通量日变化峰值分别出现在5:30(22.4 W/m2)和16:00(106.6 W/m2).  相似文献   
224.
In the northern Bay of Bengal, the existence of intense temperature inversion during winter is a widely accepted phenomenon. However, occurrences of temperature inversion during other seasons and the spatial distribution within and adjacent to the Bay of Bengal are not well understood. In this study, a higher resolution spatiotemporal variation of temperature inversion and its mechanisms are examined with mixed layer heat and salt budget analysis utilizing long-term Argo(2004 to 2020) and RAMA(2...  相似文献   
225.
本文以二倍体和三倍体虹鳟(Oncorhynchus mykiss)幼鱼为实验对象,探究急性高温胁迫下,不同倍性虹鳟热休克蛋白家族(hsps)基因表达水平。选取二倍体和三倍体虹鳟幼鱼,分别进行常温处理(14℃)和高温胁迫(22℃),并在高温胁迫48h后,将虹鳟置于常温(14℃)恢复48h。分别在实验开始的0,12,24,48,72和96h进行取样,测定实验虹鳟肝脏和肌肉组织中的hsp70s(hsp70a,hsp70b),hsp90s (hsp90a1a,hsp90a2a,hsp90a1b,hsp90a2a,hsp90b1,hsp90b2)mRNA水平。结果表明:hsp70s和hsp90s的各个亚型均能对热胁迫做出反应,并具有应答幅度的组织特异性和时间特异性。在胁迫6和24h后,上述基因mRNA水平达到最高,随后降低。三倍体幼鱼肝脏和肌肉hsps mRNA表达水平的峰值显著高于二倍体幼鱼。研究结果显示,三倍体虹鳟对高温环境更为敏感,需要消耗更多的能量用于适应高温环境。  相似文献   
226.
The results obtained from an Ocean General Circulation Model (OGCM), the Modular Ocean Model 2.2, forced with the National Center for Environmental Prediction/National Center for Atmospheric Research reanalysis data, and observational data have been utilized to document the climatological seasonal cycle of the upper ocean response in the Tropical Indian Ocean. We address the various roles played by the net surface heat flux and the local and remote ocean dynamics for the seasonal variation of near-surface heat budget in the Tropical Indian Ocean. The investigation is based in seven selected boxes in the Arabian Sea, Bay of Bengal and the Equatorial Indian Ocean. The changes of basin-wide heat budget of ocean process in the Arabian Sea and the Western Equatorial Indian Ocean show an annual cycle, whereas those in the Bay of Bengal and the Eastern Equatorial Indian Ocean show a semi-annual cycle. The time tendency of heat budget in the Arabian Sea depends on both the net surface heat flux and ocean dynamics while on the other hand, that in the Bay of Bengal depends mainly on the net surface flux. However, it has been found that the changes of heat budget are very different between western and eastern regional sea areas in the Arabian Sea and the Bay of Bengal, respectively. This difference depends on seasonal variations of the different local wind forcing and the different ocean dynamics associated with ocean eddies and Kelvin and Rossby waves in each regional sea areas. We also discuss the comparison and the connection for the seasonal variation of near-surface heat budget among their regional sea areas. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
227.
利用1985~2008年OAflux3、NCEP\NCAR再分析资料与中国大陆东部108个站点的降水资料,应用回归和合成方法,分析了中国东部夏季降水的年际变化与同期东海及邻近海域潜热通量变异的关系。结果表明:东海及邻近海域(以下称东海)夏季潜热通量年际变化显著的区域位于东海区域,为与同期中国东部降水密切相关的关键区域。当东海的潜热通量偏高(低)时,中国东部长江以南地区上空盛行偏东北(西南)风异常,这将不(有)利于水汽由南向北的输送,从而可能使到达长江中下游流域及以北地区的水汽偏少(多);并且,长江中、下游流域为下沉(上升)气流和低层水汽辐散(辐合)正异常,对应降水偏少(偏多);华南地区为上升(下沉)气流和低层水汽辐合(辐散)正异常,对应降水偏多(偏少)。分析结果还表明,东海的潜热通量可通过影响东亚大气环流而成为引起中国东部夏季汛期降水年际异常的重要原因之一。  相似文献   
228.
Fry of grass carp, Ctenopharyngodon idella (Val.), were air‐freighted from Hong Kong to New Zealand in October 1971. The fish were at first kept in tanks with a recirculating water supply, where they soon showed symptoms of parasitic infestation and suffered heavy mortality.

Attempts were made to remove parasites with chemotherapeutic treatment. Tripartiella sp., Dactylogyrus ctenopharyngodonis, and Gyrodactylus clenopharyngodontis were eliminated by prolonged bathing of the infected fish with quinine. Chemical treatment failed to eliminate Ichthyophthirius mullifiliis from the body surface and the gills, but the rate of spontaneous recovery of the fish from this disease was accelerated by keeping them in clean, running water. Experiments to remove the cestode Bothriocephalus gowkongensis from the intestine of the fish with tetravalent tin compounds and to control the intermediate host (Cyclops) in the water resulted in the eradiction of the cestode.

When parasitic infestations had been brought under control the fish were transferred to outdoor ponds, where they began to feed on aquatic and terrestrial vegetation and rapidly increased in size.  相似文献   
229.
Synoptic features in/around thermal fronts and cross-frontal heat fluxes in the southern Huanghai./Yellow Sea and East China Sea (HES) were examined using the data collected from four airborne expendable bathythermograph surveys with horizontal approxmately 35 km and vertical 1 m(from the surface to 400 m deep) spacings. Since the fronts are strongly affected by HES current system, the synoptic thermal features in/around them represent the interaction of currents with surrounding water masses. These features can not be obtained from climatological data. The identified thermal features are listed as follows : ( 1 ) multiple boundaries of cold water, asymmetric thermocline intrusion, locally-split front by homogeneous water of approxmately 18 ℃, and mergence of the front by the Taiwan Warm Current in/around summertime southern Cheju - Changjiang/Yangtze front and Tsushima front; (2) springtime frontal eddy-like feature around Tsushima front; (3) year-round cyclonic meandering and summertime temperature-inversion at the bottom of the surface mixed layer in Cheju - Tsushima front; and (4) multistructure of Kuroshio front. In the Kuroshio front the mean variance of vertical temperature gradient is an order of degree smaller than that in other HES fronts. The southern Cheju- Changjiang front and Cheju -Tsushima front are connected with each other in the summer with comparable cross-frontal temperature gradient. However, cross-frontal heat flux and lateral eddy diffusivity are stronger in the southern Cheju - Changjiang front. The cross-frontal heat exchange is the largest in the mixing zone between the modified Huanghai Sea bottom cold water and the Tsushima Warm Current, which is attributable to enhanced thermocline intrusions.  相似文献   
230.
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