全文获取类型
收费全文 | 8663篇 |
免费 | 278篇 |
国内免费 | 130篇 |
专业分类
测绘学 | 166篇 |
大气科学 | 709篇 |
地球物理 | 2006篇 |
地质学 | 3073篇 |
海洋学 | 803篇 |
天文学 | 1237篇 |
综合类 | 37篇 |
自然地理 | 1040篇 |
出版年
2021年 | 115篇 |
2020年 | 153篇 |
2019年 | 156篇 |
2018年 | 203篇 |
2017年 | 196篇 |
2016年 | 241篇 |
2015年 | 201篇 |
2014年 | 264篇 |
2013年 | 475篇 |
2012年 | 285篇 |
2011年 | 412篇 |
2010年 | 376篇 |
2009年 | 475篇 |
2008年 | 399篇 |
2007年 | 408篇 |
2006年 | 343篇 |
2005年 | 293篇 |
2004年 | 286篇 |
2003年 | 301篇 |
2002年 | 240篇 |
2001年 | 200篇 |
2000年 | 220篇 |
1999年 | 167篇 |
1998年 | 155篇 |
1997年 | 138篇 |
1996年 | 145篇 |
1995年 | 132篇 |
1994年 | 124篇 |
1993年 | 104篇 |
1992年 | 100篇 |
1991年 | 70篇 |
1990年 | 99篇 |
1989年 | 82篇 |
1988年 | 86篇 |
1987年 | 96篇 |
1986年 | 84篇 |
1985年 | 113篇 |
1984年 | 132篇 |
1983年 | 123篇 |
1982年 | 105篇 |
1981年 | 77篇 |
1980年 | 58篇 |
1979年 | 71篇 |
1978年 | 67篇 |
1977年 | 63篇 |
1976年 | 61篇 |
1975年 | 70篇 |
1974年 | 57篇 |
1973年 | 68篇 |
1972年 | 33篇 |
排序方式: 共有9071条查询结果,搜索用时 15 毫秒
151.
Alvin Seiff David E. Reese Simon C. Sommer Donn B. Kirk Ellis E. Whiting Hasso B. Niemann 《Icarus》1973,18(4):525-563
On June 20, 1971, an instrumented probe designated PAET entered the atmosphere near Bermuda at a velocity of 6.6 km/sec carrying experiments designed for use at planets other than the Earth. Instruments to measure in situ the structure and composition of the atmosphere included accelerometers, pressure and temperature sensors, a mass spectrometer, and a radiometer (to sense characteristic emission from the probe shock layer at high speeds).The experiments were largely successful. The thermal structure of the atmosphere, including two major reversals in gradient, was shown to be well defined to an altitude of 80 km by comparison with more conventional meteorological soundings. The atmospheric mean molecular weight was defined within a percent by the structure experiment. The radiometers defined the bulk composition accurately and the trace quantity of CO2 to one significant figure. The mass spectrometer functioned properly, but failed to give the correct composition because of problems in its sampling system. Oxygen was depleted, apparently by chemical reactions before reaching the spectrometer, and the inlet leak conductance was reduced from its preflight value by an order of magnitude. There is reason to believe that contamination by large molecules from the heat shield was responsible. This experiment should stimulate intensive laboratory work on sampling systems, so that similar problems do not arise in measurements of atmospheric composition at the planets.Results of auxiliary experiments to measure atmospheric water vapor, vehicle dynamics and heating, and communications blackout are also given. 相似文献
152.
Richard G. Klein Kathryn Cruz-Uribe David Halkett Tim Hart John E. Parkington 《Quaternary Research》1999,52(3):393
Boegoeberg 1 (BOG1) is located on the Atlantic coast of South Africa, 850 km north of Cape Town. The site is a shallow rock shelter in the side of a sand-choked gully that was emptied by diamond miners. Abundant coprolites, chewed bones, and partially digested bones implicate hyenas as the bone accumulators. The location of the site, quantity of bones, and composition of the fauna imply it was a brown hyena nursery den. The abundance of Cape fur seal bones shows that the hyenas had ready access to the coast. Radiocarbon dates place the site before 37,000 14C yr ago, while the large average size of the black-backed jackals and the presence of extralimital ungulates imply cool, moist conditions, probably during the early part of the last glaciation (isotope stage 4 or stage 3 before 37,000 14C yr ago) or perhaps during one of the cooler phases (isotope substages 5d or 5b) within the last interglaciation. Comparisons of the BOG1 seal bones to those from regional Middle Stone Age (MSA) and Later Stone Age (LSA) archeological sites suggest (1) that hyena and human seal accumulations can be distinguished by a tendency for vertebrae to be much more common in a hyena accumulation and (2) that hyena and LSA accumulations can be distinguished by a tendency for hyena-accumulated seals to represent a much wider range of individual seal ages. Differences in the way hyenas and people dismember, transport, and consume seal carcasses probably explain the contrast in skeletal part representation, while differences in season of occupation explain the contrast in seal age representation. Like modern brown hyenas, the BOG1 hyenas probably occupied the coast year-round, while the LSA people focused their coastal visits on the August–October interval when nine-to-eleven-month-old seals were abundant. The MSA sample from Klasies River Mouth Cave 1 resembles BOG1 in seal age composition, suggesting that unlike LSA people, MSA people obtained seals more or less throughout the year. 相似文献
153.
154.
Natural Hazards - This article aims to provide a quantitative study of immediate food supplies based on a three-stage analysis. Firstly, a numerical autoregressive integrative moving average... 相似文献
155.
While there are numerous thick loess–palaeosol sequences preserved across the Carpathian Basin, well dated sites that provide terrestrial palaeoenvironmental records extending beyond last glacial–interglacial cycle are scarce. Robust chronologies are essential for correlations of loess with other long-term Quaternary records and to further understanding of the palaeoenvironment and climate of this important region beyond the last 125 ka. Here a new geochronology based on 13 post-infrared infrared stimulated luminescence ages focused on the lower part of the loess–palaeosol sequence at Erdut is presented. The results show that the lower part of the Erdut profile spans the penultimate glacial cycle (MIS 7 to MIS 5). The considerable sediments overlaying the investigated part of the profile suggest that this section spans two glacial cycles, rather than the previously suggested one. The most likely source of the discrepancy is the use of uncorrected infrared stimulated luminescence signal, which can cause age underestimation if not accounted for. This study demonstrates the need to revisit sites such as Erdut, re-date them using updated measurement protocols, and update existing palaeoenvironmental interpretations. 相似文献
156.
157.
LUO Kai-li LI Bao-sheng ZHU Yi-zhi JIN He-ling ZHANG David Dian YAN Man-cun LI Hou-xin YAO Chun-xia ZHANG Yu-hongf . Department of Geography South China Normal University Guangzhou P. R. China . State Key Labor 《中国地理科学(英文版)》2001,11(4):336-342
About 70years ago,Frenc卜卜alaeohdoglst回LL sc卜dars al卜ome and a卜road卜ave successively con-HARD de Chardin P.et al.Initiated the Quaternary ducted large amount ofwork on the Later Quaternarygeologlcal research In the >alawusu River Basin of strata(TEILHARD,1924; YUAN,1978; LI,1987;desert region of Northern China and established th,ZHENG,1989; SUN et al,1996; LI et al,1993),Salawusu Formation门EILHARD,1924).Sine,then,palaeobiology… 相似文献
158.
David J. Briggs 《Geoforum》1981,12(1):99-106
Using data on crop yields and soil conditions derived from reports of the Soil Survey of England and Wales, and incorporating a variety of climatic and topographic data, relationships were identified between the yield of spring barley and edaphic and climatic conditions. Yields are highest on fine loamy and silty soils and on freely to imperfectly drained soils. Multiple regression analysis indicated that yields are related to sand contents, drainage conditions and available water capacity of the soil, potential summer soil moisture deficit and annual accumulated temperature. 相似文献
159.
Relative sea level (RSL) data derived from isolation basins at Innaarsuit, a site on the south shores of the large marine embayment of Disko Bugt, West Greenland, record rapid RSL fall from the marine limit (ca. 108 m) at 10,300-9900 cal yr B.P. to reach the present sea level at 3500 cal yr B.P. Since 2000 cal yr B.P., RSL rose ca. 3 m to the present. When compared with data from elsewhere in Disko Bugt, our results suggest that the embayment was deglaciated later and more quickly than previously thought, at or slightly before 10,300 cal yr B.P. The northern part of Disko Bugt experienced less rebound (ca. 10 m at 6000 cal yr B.P.) compared with areas to the south. Submergence during the late Holocene supports a model of crustal down-warping as a result of renewed ice-sheet growth during the neoglacial. There is little evidence for west to east differences in crustal rebound across the southern shores of Disko Bugt. 相似文献
160.
The knowledge of soil moisture spatio-temporal variability is highly relevant for water resources management. This paper reports an analysis of the spatial–temporal variability of soil moisture data for a small to medium-scale soil-sensors network in a coastal wetland of southwestern Spain. Measurements were taken from five sites located in the Doñana National Park over the time-period of one hydrological year from September 2017 to September 2018. The total area of the soil-sensors network shows an extension about 25 × 3 km. Soil moisture data was separated into time invariant (the temporal mean of the whole period at each site) and time-variant terms (the deviations of soil moisture from the mean, or anomalies). The time-invariant component was generally the main contributor to the total spatial variance of soil moisture and it was mostly controlled by the groundwater levels in the area. Nevertheless, the time variant terms have a huge effect on soil moisture variability in very dry states. Characteristic convex time-dependent patterns for this field site were found between spatially averaged soil moisture and its variability. This information could be used for the up and downscaling of soil moisture from satellite data. Those patterns of relation between spatial mean and variability of soil moisture were only affected by heavy rainfalls giving rise to hysteretic behaviour. This study shows that even though groundwater level is a time-variant variable, it significantly affects soil moisture's time-variant but also time-invariant terms due to the different average groundwater level depths at the different sites. 相似文献