全文获取类型
收费全文 | 1049篇 |
免费 | 26篇 |
国内免费 | 6篇 |
专业分类
测绘学 | 20篇 |
大气科学 | 56篇 |
地球物理 | 205篇 |
地质学 | 391篇 |
海洋学 | 65篇 |
天文学 | 240篇 |
综合类 | 2篇 |
自然地理 | 102篇 |
出版年
2021年 | 10篇 |
2019年 | 10篇 |
2018年 | 15篇 |
2017年 | 21篇 |
2016年 | 21篇 |
2015年 | 15篇 |
2014年 | 21篇 |
2013年 | 47篇 |
2012年 | 30篇 |
2011年 | 31篇 |
2010年 | 42篇 |
2009年 | 37篇 |
2008年 | 44篇 |
2007年 | 42篇 |
2006年 | 33篇 |
2005年 | 23篇 |
2004年 | 37篇 |
2003年 | 28篇 |
2002年 | 31篇 |
2001年 | 15篇 |
2000年 | 23篇 |
1999年 | 26篇 |
1998年 | 24篇 |
1997年 | 9篇 |
1996年 | 16篇 |
1995年 | 22篇 |
1994年 | 13篇 |
1993年 | 11篇 |
1992年 | 15篇 |
1991年 | 20篇 |
1990年 | 11篇 |
1989年 | 15篇 |
1988年 | 14篇 |
1987年 | 14篇 |
1986年 | 18篇 |
1985年 | 24篇 |
1984年 | 18篇 |
1983年 | 12篇 |
1982年 | 20篇 |
1981年 | 9篇 |
1980年 | 13篇 |
1979年 | 13篇 |
1978年 | 12篇 |
1977年 | 14篇 |
1976年 | 15篇 |
1975年 | 21篇 |
1974年 | 18篇 |
1973年 | 14篇 |
1972年 | 10篇 |
1970年 | 7篇 |
排序方式: 共有1081条查询结果,搜索用时 15 毫秒
21.
22.
Investigations were carried out as to the feasibility of using a semiconductor source in the design of a new rapid response, open-path hygrometer. A single-beam instrument was constructed employing an infrared light emitting diode (LED) as a source instead of the usual high energy, wideband filament. The spectral emission envelope encompassed the 1.87 m water absorption band. Electronic modulation and thermoelectrical cooling of the diode eliminated the conventional chopper wheel and stabilized the peak wavelength emission. Path length was 200 mm. Over a water vapour concentration range of 0–16 g m–3, absorption varied by 2% in a linear fashion. At 10 Hz, the noise level was 0.1 g m–3 rms. Hygrometer resolution and stability are constrained by the detector noise level, the small source emission in the absorption band and low frequency drift in the optical filter. Despite these problems, the new instrument showed comparable performance characteristics to a commercial Lyman- hygrometer. Latent heat fluxes measured with both instruments and a Kaijo-Denki, 3-D sonic anemometer agreed to within 4% over a range 0–350 W m–2. Further improvements in performance can be anticipated with advances in detector and LED technology. 相似文献
23.
Green AE 《International journal of population geography : IJPG》1995,1(1):29-50
"Amongst the key dimensions of population and labour market change in most parts of western Europe and other similar economies in the 1990s are low fertility rates, rising numbers of smaller households, increasing female participation rates, growth in higher level non-manual occupations and the spread of flexible employment patterns. This paper explores how these trends have led to an increase in the number and significance of one particular subgroup: dual career households. An overview from available secondary sources of the geography and socio-economic characteristics of such households in Britain is presented, and directions for possible future research on dual career households are outlined." 相似文献
24.
An axisymmetric model for approximate solution of the magnetospheric Alfvén wave problem at latitudes above the plasmapause is proposed, in which a realistic dipole geometry is combined with finite anisotropic ionosphere conductivities, thus bringing together various ideas of previous authors. It is confirmed that the axisymmetric toroidal and poloidal modes interact via the ionospheric Hall effect, and an approximate method of solution is suggested using previously derived closed solutions of the uncoupled wave equations.A solution for zero Hall conductivity is obtained, which consists of sets of independent shell oscillations, regardless of the magnitude of the Pedersen conductivity. One set reduces to the classical solutions for infinite Pedersen conductivity, while another predicts a new set of harmonics of a quarter-wave fundamental, with longer eigenperiods than the classical solutions for a given L-shell. 相似文献
25.
Carbon isotopic fractionation between CO2 vapour,silicate and carbonate melts: an experimental study to 30 kbar 总被引:5,自引:0,他引:5
David P. Mattey W. R. Taylor D. H. Green C. T. Pillinger 《Contributions to Mineralogy and Petrology》1990,104(4):492-505
The carbon isotopic fractionation between CO2 vapour and sodamelilite (NaCaAlSi2O7) melt over a range of pressures and temperatures has been investigated using solid-media piston-cylinder high pressure apparatus. Ag2C2O4 was the source of CO2 and experimental oxygen fugacity was buffered at hematite-magnetite by the double capsule technique. The abundance and isotopic composition of carbon dissolved in sodamelilite (SM) glass were determined by stepped heating and the 13C of coexisting vapour was determined directly by capsule piercing. CO2 solubility in SM displays a complex behavior with temperature. At pressures up to 10 kbars CO2 dissolves in SM to form carbonate ion complexes and the solubility data suggest slight negative temperature dependence. Above 20 kbars CO2 reacts with SM to form immiscible Na-rich silicate and Ca-rich carbonate melts and CO2 solubility in Na-enriched silicate melt rises with increasing temperature above the liquidus. Measured values for carbon isotopic fractionation between CO2 vapour and carbonate ions dissoived in sodamelilite melt at 1200°–1400° C and 5–30 kbars average 2.4±0.2, favouring13C enrichment in CO2 vapour. The results are maxima and are independent of pressure and temperature. Similar values of 2 are obtained for the carbon isotopic fractionation between CO2 vapour and carbonate melts at 1300°–1400° C and 20–30 kbars. 相似文献
26.
C.A. Green 《Planetary and Space Science》1978,26(10):955-967
An analysis was made of a complex large amplitude Pc 4 micropulsation, of four hours duration around local noon, observed at five ground stations in the United Kingdom (2.4? L ?3.8). The final pulsation waveform was shown to be the results of the superposition of wave packets of different periods. The meridional variation of the amplitude of the different period wavepackets was consistent with their being fundamental “toroidal” field line resonances within the plasmasphere, rotated through 90° in their transmission through the ionosphere in accordance with recent theoretical predictions. Other predicted ionospheric effects, such as the loss of the sense-of-polarization reversal across the amplitude maximum, were apparent in the meridional variation of the polarization characteristics. 相似文献
27.
R.J. Allan 《Chemical Geology》1976,18(3):245-250
28.
Carbon isotope fractionation in wood during carbonization 总被引:1,自引:0,他引:1
A significant uncertainty exists as to whether δ13C values in charcoal meaningfully represent the stable isotopic content of the original material, with studies suggesting variable responses to both natural and laboratory heating. An extensive study was undertaken using fully homogenised samples of wood taken from Eucalyptus spp., Quercus robur and Pinus radiata. The results demonstrate that the duration of heating had no tangible effect on the final composition of the charred material, with the δ13C and carbon content of wood fixed after 30 min of heating. Furthermore, all three wood types become progressively depleted in 13C with increasing temperature. The results demonstrate that even at temperatures commonly reached in natural fires (<450 °C) isotopic fractionation of up to 1.3‰ can take place indicating that the absolute values obtained from charcoal extracted for paleoenvironmental reconstruction must be interpreted with caution. 相似文献
29.
30.
W. M. L. Meijninger A. E. Green O. K. Hartogensis W. Kohsiek J. C. B. Hoedjes R. M. Zuurbier H. A. R. De Bruin 《Boundary-Layer Meteorology》2002,105(1):63-83
A large aperture scintillometer (LAS) andradio wave scintillometer (RWS)were installed over a heterogeneous areato test the applicability of the scintillation method.The heterogeneity in the area, whichconsisted of many plots, was mainly caused bydifferences in thermal properties ofthe crops; the variations in theaerodynamic roughness lengthwere small. The water vapour fluxesderived from the combined LAS-RWSsystem, also known as the two-wavelengthmethod, agreed fairly well with the aggregatedwater vapour fluxes derived from in-situeddy covariance measurements. The water vapourfluxes derived from a stand-alone LASare also presented. It was found that a single LASand an estimate of the area averagedavailable energy (using a simple parameterisationscheme) can provide also reasonablearea-averaged water vapour fluxes. 相似文献