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   9226篇
  免费   487篇
  国内免费   65篇
测绘学   231篇
大气科学   887篇
地球物理   2438篇
地质学   3523篇
海洋学   636篇
天文学   1549篇
综合类   40篇
自然地理   474篇
  2023年   39篇
  2022年   53篇
  2021年   156篇
  2020年   168篇
  2019年   145篇
  2018年   378篇
  2017年   397篇
  2016年   553篇
  2015年   383篇
  2014年   477篇
  2013年   704篇
  2012年   552篇
  2011年   518篇
  2010年   479篇
  2009年   518篇
  2008年   372篇
  2007年   335篇
  2006年   272篇
  2005年   244篇
  2004年   270篇
  2003年   204篇
  2002年   185篇
  2001年   155篇
  2000年   138篇
  1999年   125篇
  1998年   117篇
  1997年   135篇
  1996年   86篇
  1995年   102篇
  1994年   102篇
  1993年   67篇
  1992年   55篇
  1991年   56篇
  1990年   78篇
  1989年   48篇
  1988年   41篇
  1987年   65篇
  1986年   56篇
  1985年   59篇
  1984年   72篇
  1983年   53篇
  1982年   65篇
  1981年   54篇
  1980年   34篇
  1979年   42篇
  1978年   41篇
  1977年   36篇
  1975年   44篇
  1974年   48篇
  1973年   43篇
排序方式: 共有9778条查询结果,搜索用时 31 毫秒
991.
992.
993.
994.
995.
996.
The Exoplanet Characterization Observatory (EChO) is a concept of a dedicated space telescope optimized for low-resolution transit and occultation spectroscopy to study the exoplanet diversity through the composition of their atmospheres. The scope of this paper is to answer the following question: Can we schedule a nominal EChO mission, with targets known today (in mid 2013), given the science requirements, realistic performances and operational constraints? We examine this issue from the point of view of duration of the mission and the scheduling restrictions with a sample of exoplanet systems known nowadays. We choose different scheduling algorithms taking into account the science and operational constraints and we verified that it is fairly straightforward to schedule a mission scenario over the lifetime of EChO compliant with the science requirements. We identified agility as a critical constraint that reduces significantly the efficiency of the survey. We conclude that even with known targets today the EChO science objectives can be reached in the 4.5 years duration of the mission. We also show that it is possible to use gaps between exoplanet observations, to fit the required calibration observations, data downlinks and station keeping operations or even to observe more exoplanet targets to be discovered in the coming years.  相似文献   
997.
998.
Abstract

Data assimilation in numerical weather forecasting corrects current forecast values by subtracting a portion of interpolated forecast‐minus‐observation differences at the points of a three‐dimensional grid. Deviations used in updating a forecast data field are forecast errors obtained or derived from observations available at update time. When observations are missing at mandatory levels, construction of full vertical soundings by interpolation introduces extraneous errors. The present paper is concerned with determination of the error in vertical extrapolations of surface winds, and of aircraft and satellite cloud‐tracked winds. In addition it examines the effect on accuracy of using location‐specific statistics compared to averaged statistics as the basis for the interpolation weighting scheme and compares errors of one‐ and two‐variable interpolations.

Interpolation accuracy tests demonstrate the influence of the interpolation scheme on the quality of interpolated information used in forecast updating. The results show that the level of accuracy exceeds the benchmark provided by monthly mean forecast error values only with bivariate interpolation of wind components from off‐level data sources.  相似文献   
999.
Abstract

This study, using a climate change scenario generated by the Canadian Climate Centre (CCC) general circulation model (GCM) examines the impacts of such a climate change on agriculture in southern Quebec. Using a crop model from the Food and Agriculture Organization (FAO), yield responses of a variety of cereals, legumes, oleaginous and special crops to climate change are analysed and discussed.

Results show that under the 2 × CO2 climate scenario the growing season would be longer and accumulation of corn heat units and growing degree days would be more important than under actual climate (1961–1990). One of the more important results of this study is that, on the one hand yield of C3 cereals would be lower and that of C4 cereals higher in most agricultural regions. On the other hand, the direct fertilisation effect of increased CO2 is not considered. It must be cautioned however that we can not generalise results obtained for one legume crop to all legumes.  相似文献   
1000.
Abstract

The lower St Lawrence Estuary is an interesting case amongst estuaries in that it is wide enough to accommodate the development of mesoscale unstable waves and eddies. These features are generated by the runoff‐driven jet along this body's south shore. We present data yielding estimates of the length, time and velocity scales of these unstable disturbances. To relate these quantities to the dynamics we employ a 2‐layer quasigeos‐trophic instability model featuring realistic lateral shear. All model runs show short time and length scales, e‐folding periods of less than 10 days and wavelengths less than 50 km.  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