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   1581篇
  免费   65篇
  国内免费   25篇
测绘学   58篇
大气科学   125篇
地球物理   344篇
地质学   540篇
海洋学   117篇
天文学   335篇
综合类   8篇
自然地理   144篇
  2022年   23篇
  2021年   22篇
  2020年   26篇
  2019年   24篇
  2018年   52篇
  2017年   54篇
  2016年   38篇
  2015年   40篇
  2014年   48篇
  2013年   84篇
  2012年   46篇
  2011年   68篇
  2010年   72篇
  2009年   76篇
  2008年   70篇
  2007年   64篇
  2006年   63篇
  2005年   41篇
  2004年   52篇
  2003年   41篇
  2002年   57篇
  2001年   27篇
  2000年   36篇
  1999年   22篇
  1998年   28篇
  1997年   26篇
  1996年   17篇
  1995年   21篇
  1994年   13篇
  1993年   17篇
  1992年   23篇
  1991年   13篇
  1990年   13篇
  1989年   10篇
  1987年   21篇
  1986年   15篇
  1985年   23篇
  1984年   30篇
  1983年   27篇
  1982年   33篇
  1981年   24篇
  1980年   25篇
  1979年   22篇
  1978年   9篇
  1977年   15篇
  1976年   16篇
  1975年   8篇
  1974年   15篇
  1973年   9篇
  1971年   8篇
排序方式: 共有1671条查询结果,搜索用时 15 毫秒
111.
112.
113.
A one-dimensional numerical model of the planetary boundary layer was used to investigate thermal and kinetic energy budgets. The simulation experiments were based on two sets of data. The first set was based on a ‘typical’ June with climatological data extracted for the oceanic region slightly northeast of Barbados. The second set used data from the third phase of project BOMEX, for approximately the same area and time of year as the first set. Comparison with observations of three simulated elements (viz., sea surface temperature and wind and humidity at 6 m) which are important in determining the near-interface energy transports shows that:
  1. the model is capable of realistic simulations of both ‘typical’ conditions, and conditions for a specific four-day period;
  2. the model is capable of realistically simulating the differences between prevailing values of these parameters in the two cases (‘typical’ and specific four-day period).
The simulated interface fluxes are those of incoming and outgoing short- and long-wave radiation; transmitted radiation at -0.5 m in the ocean, sensible heat transfer into the ocean and air, and latent heat flux of evaporation. Comparison with observational analyses shows that the diurnal variations in net radiation and heat storage in the mixed layer are realistically simulated. The simulated values of evaporation are consistent with other estimates for both ‘typical’ conditions and specific conditions during this four-day period. The rate of heat storage varies between +51 and -37 percent of the diurnal maximum incoming radiation, and the evaporation varies between +16% and -13% of this term. The non-dimensional transfer coefficients (C D, CT, Cq) computed from the model show general agreement with the coefficients calculated from observations in the simulated region (Pondet al., 1971). The simulated vertical profiles of temperature are in general agreement with observed profiles, except in the uppermost portions of the atmospheric boundary layer where deviations of approximately 1.5C occur. Simulated vertical profiles of wind speed are generally consistent with observed profiles, with the largest deviations appearing to be of the order of 0.5 m s-1. Simulated vertical profiles of the eddy fluxes of sensible heat, water vapor, and momentum are generally consistent with Bunker's (1970) aircraft-based measurements of these quantities. The time averages of these simulated profiles show regular decreases with height, while simulated profiles for specific hours of the day show intermediate maxima and minima, which are also seen in the measured profiles. The vertically integrated kinetic energy budgets of the modelled atmospheric layer are presented through the four terms of the kinetic energy budget, viz., the upper and the lower boundary drags, dissipation, and potential-to-kinetic conversion. The dominant terms in the atmospheric energy budgets are the production and dissipation terms, with kinetic energy being exported both to the overlying atmospheric layer and to the underlying oceanic layer at rates of about 2 to 6% of the production, respectively. Comparisons between the climatological and BOMEX simulations are presented. The vertically integrated humidity budgets are presented for the two simulation experiments. Under ‘typical’ conditions, the humidity budget reveals an upper boundary flux of about +29% of the lower boundary flux with the vertically integrated advective flux being -59% of the lower flux. For the specific four-day simulation, the upper boundary flux and advection are about +28 and -70%, respectively, of the lower boundary flux.  相似文献   
114.
115.
Joseph A. Burns 《Icarus》1976,28(4):453-458
Mercury, currently rotating very slowly, probably rotated faster in the past. If Mercury's rotation period had been near 8 hours initially, similar to that of most solar system bodies today, it would have been flattened by a few percent. As Mercury was slowed by solar tides, craters which were circular when they were emplaced would have been distorted by the same few percent. Substantial surface stresses, well above the fracture stress, would have been produced unless stress relief occurred; these stresses should have produced tensional fractures near the poles and two intersecting sets of shear planes in equatorial regions. Satellite orbits about the slowly spinning Mercury have been shown to collapse onto its surface: the impact craters resulting from these hypothetical lost satellites should be elongated along the orbit paths, which probably lie near the equator. However, none of these features has been found on the Mariner 10 images. They may be obscured by the effects of tidal heating that should cause an overall internal temperature increase of about 100°K although the increase would be substantially more in certain regions. Radial tides, sometimes called push-pull tides, are important at the present time because Mercury's large orbital eccentricity causes the planet to undergo significant tidal flexing each orbital period; the contemporary tidal heating due to this mechanism is estimated at more than 1016 erg/sec.  相似文献   
116.
Photoelectric observations have been carried out during maximum solar activity in order to investigate the variation of the brightness of the lunar surface with the solar cycle and to detect the possiblity of existing colour anomaly. The brightness data, using a wide-passband filter, do not show any evidence of variation with the solar cycle. No colour anomaly is found for the investigated lunar regions, and the doubt that has been felt by some investigators about the nonexistence of certain colour indices' variation with phase is not confirmed.  相似文献   
117.
A finite element analysis is performed to determine the influence of the choice of a constitutive model for the earth medium upon the response to seismic waves of an embedded rigid structure. The seismic forcing function is characterized by Rayleigh waves with amplitude parameters adjusted to provide identical free-field motion at a surface reference point for one particular sand represented with elastic, plastic and viscoelastic models. Within the limitations of the analysis, the result is that the steady-state rigid body motions of the embedded structure are essentially identical for these constitutive relations and, consequently, it is appropriate to use an elastic representation for the earth medium.  相似文献   
118.
Two schemes are presented in which matter is moved closer to Mars so as to increase the equinoctal precession period of Mars. In such a way the Martian springs, during which Mars is habitable in the long winter model of Sagan (1971), could be extended and, in fact, prolonged indefinitely. The schemes involve using solar energy to move the satellite Phobos and/or material from the asteroid belt. Difficulties exist with the proposals but they are presented to stimulate discussion.  相似文献   
119.
120.
Effects of ice content on the transport of water in frozen soil are studied experimentally and theoretically under isothermal conditions. A physical law, that the flux of water in unsaturated frozen soil is proportional to the gradient of total water content is proposed. Theoretical justification is made by the use of the two-phase flow theory. The experimental results are shown to support the proposed physical law. The results of this study are presented in two parts and this is the second paper describing the theoretical aspects of the study.  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