全文获取类型
收费全文 | 57145篇 |
免费 | 6128篇 |
国内免费 | 7977篇 |
专业分类
测绘学 | 3239篇 |
大气科学 | 8449篇 |
地球物理 | 12749篇 |
地质学 | 24025篇 |
海洋学 | 6015篇 |
天文学 | 7601篇 |
综合类 | 3655篇 |
自然地理 | 5517篇 |
出版年
2023年 | 455篇 |
2022年 | 1499篇 |
2021年 | 1802篇 |
2020年 | 1515篇 |
2019年 | 1853篇 |
2018年 | 2335篇 |
2017年 | 2163篇 |
2016年 | 2579篇 |
2015年 | 2025篇 |
2014年 | 2640篇 |
2013年 | 3182篇 |
2012年 | 2644篇 |
2011年 | 2943篇 |
2010年 | 2969篇 |
2009年 | 3148篇 |
2008年 | 2895篇 |
2007年 | 2818篇 |
2006年 | 2422篇 |
2005年 | 2067篇 |
2004年 | 1757篇 |
2003年 | 1684篇 |
2002年 | 1594篇 |
2001年 | 1542篇 |
2000年 | 1716篇 |
1999年 | 2036篇 |
1998年 | 1727篇 |
1997年 | 1637篇 |
1996年 | 1424篇 |
1995年 | 1295篇 |
1994年 | 1189篇 |
1993年 | 1017篇 |
1992年 | 867篇 |
1991年 | 701篇 |
1990年 | 636篇 |
1989年 | 575篇 |
1988年 | 523篇 |
1987年 | 427篇 |
1986年 | 378篇 |
1985年 | 405篇 |
1984年 | 434篇 |
1983年 | 402篇 |
1982年 | 377篇 |
1981年 | 303篇 |
1980年 | 300篇 |
1979年 | 248篇 |
1978年 | 216篇 |
1977年 | 226篇 |
1976年 | 185篇 |
1975年 | 192篇 |
1974年 | 178篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
171.
西南低涡是对流层下层的中尺度扰动。本文在零级近似条件下,采用p坐标系的基本方程组,在边界层内设计两层模式。通过一定的简化方程组后,得到:当Z≤1/2[kL~2/(μ~2Knu~*)] 时,摩擦效应具有二重性,一方面使扰动减弱;另一方面使扰动增强,而前者的影响略大于后者,最终使扰动消失。频率随高度Z的降低而增加。当Z>1/2[kL~2/(μ~2Knμ~*)]时,摩擦作用甚微,几乎可以忽略不计。频率虽然有两个值,但随高度Z的增加,他们趋于一个常值。 相似文献
172.
Summary. Some lavas of Early Carboniferous age from Scotland and Derbyshire have been analysed for palaeofield magnitude by the application of a new correction to data obtained by the Shaw method. Its application has yielded field magnitude values from data which had previously been rejected. A modern pottery specimen and recent lavas from Sicily and Westman Island have also been analysed.
An average virtual dipole moment (VDM, Smith) of 2.5 ± 0.4 × 1022 A m2 was determined for the Early Carboniferous. This is only 30 per cent of the present-day VDM. 相似文献
An average virtual dipole moment (VDM, Smith) of 2.5 ± 0.4 × 10
173.
The instantaneous structure of planetary exospheres is determined by the time history of energy dissipation, chemical, and transport processes operative during a prior time interval set by intrinsic atmospheric time scales. The complex combination of diurnal and magnetospheric activity modulations imposed on the Earth's upper atmosphere no doubt produce an equally complex response, especially in hydrogen, which escapes continuously at exospheric temperatures. Vidal-Madjar and Thomas (1978) have discussed some of the persistent large scale structure which is evident in satellite ultraviolet observations of hydrogen, noting in particular a depletion at high latitudes which is further discussed by Thomas and Vidal-Madjar (1978). The latter authors discussed various causes of the H density depletion, including local neutral temperature enhancements and enhanced escape rates due to polar wind H+ plasma flow or high latitude ion heating followed by charge exchange. We have reexamined the enhancement of neutral escape by plasma effects including the recently observed phenomenon of low altitude transverse ion acceleration. We find that, while significant fluxes of neutral H should be produced by this phenomenon in the auroral zone, this process is probably insufficient to account for the observed polar depletion. Instead, the recent exospheric temperature measurements from the Dynamics Explorer-2 spacecraft suggest that neutral heating in and near the high latitude cusp may be the major contributor to depleted atomic hydrogen densities at high latitudes. 相似文献
174.
T. Grabińska 《Astrophysics and Space Science》1985,115(2):369-376
This paper presents arguments relating to the phenomenon of scantily populated regions with intergalactic obscuration. 相似文献
175.
176.
177.
Three continuous marine fish cell lines of FG (i. e., Hounder Gill) from flounder (Paralichthys olivaceus) gill, SPH (i. e. , Sea Perch Heart) from sea perch (Lateolabrax japonicus) heart and RSBF (i.e., Red Sea Bream Fin) from red sea bream (Pagrosomus major) fin, were characterized by lactate dehydrogenase (LDH) isozyme and morphological analysis. The LDH isozyme patterns of these three cell lines and their corresponding tissues of origin were investigated and compared. The results showed: (1) No difference was found in the LDH isozyme patterns of FG and flounder gill tissue. However, the LDH isozyme patterns of SPH and RSBF were significantly different from their corresponding tissues of origin; (2) LDH isozyme patterns of FG, SPH and RSBF were markedly different from each other and could serve as genetic markers for species identification and detection of cross contamination. Morphological change analysis of these three cell lines in comparison to their original tissues indicated that FG cells still appeared epithelioid without morphological transformation. However, morphological changes were found in SPH and RSBF compared to their original tissues. Therefore, the cellular morphology was still plastic in the relatively stable culture conditions, and it was possible that change of LDH patterns wasrelated to morphological changes of fish cells in vitro. 相似文献
178.
Jennifer M. Galloway Christine T. Doherty R. Timothy Patterson Helen M. Roe 《第四纪科学杂志》2009,24(4):322-335
A pollen‐based study from Tiny Lake in the Seymour‐Belize Inlet Complex of central coastal British Columbia, Canada, permits an evaluation of the dynamic response of coastal temperate rainforests to postglacial climate change. Open Pinus parklands grew at the site during the early Lateglacial when the climate was cool and dry, but more humid conditions in the later phases of the Lateglacial permitted mesophytic conifers to colonise the region. Early Holocene conditions were warmer than present and a successional mosaic of Tsuga heterophylla and Alnus occurred at Tiny Lake. Climate cooling and moistening at 8740 ± 70 14C a BP initiated the development of closed, late successional T. heterophylla–Cupressaceae forests, which achieved modern character after 6860 ± 50 14C a BP, when a temperate and very wet climate became established. The onset of early Holocene climate cooling and moistening at Tiny Lake may have preceded change at more southern locations, including within the Seymour‐Belize Inlet Complex, on a meso‐ to synoptic scale. This would suggest that an early Holocene intensification of the Aleutian Low pressure system was an important influence on forest dynamics in the Seymour‐Belize Inlet Complex and that the study region was located near the southern extent of immediate influence of this semi‐permanent air mass. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
179.
Technique for Controlling Spread of Limnotic Oncomelania 总被引:1,自引:0,他引:1
Schistosomiasis is a parasitic disease mostly found in areas along the Changjiang River of China. The disease is spread solely through an intermediary named oncomelania, so its spread of schistosomiasis can be controlled by properly designing water intakes which prevent oncomelania from entering farming land or residential areas. This paper reports a successful design process and a new oncomelania-free intake device. The design of the new intake is based on a sound research program in which extensive experimental studies were carried out to gain knowledge of oncomelania eco-hydraulic behavior and detailed flow field information through CFD simulation. 相似文献
180.