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971.
972.
Pietro Salizzoni Raphaël Van Liefferinge Patrick Mejean Lionel Soulhac Richard J. Perkins 《Boundary-Layer Meteorology》2010,135(3):455-467
The influence of surface roughness on the dispersion of a passive scalar in a rough wall turbulent boundary layer has been
studied using wind-tunnel experiments. The surface roughness was varied using different sizes of roughness elements, and different
spacings between the elements. Vertical profiles of average concentration were measured at different distances downwind of
the source, and the vertical spread of the plume was computed by fitting a double Gaussian profile to the data. An estimate
of the integral length scale is derived from the turbulence characteristics of the boundary layer and is then used to scale
the measured values of plume spread. This scaling reduces the variability in the data, confirming the validity of the model
for the Lagrangian integral time scale, but does not remove it entirely. The scaled plume spreading shows significant differences
from predictions of theoretical models both in the near and in the far field. In the region immediately downwind of the source
this is due to the influence of the wake of the injector for which we have developed a simple model. In the far field we explain
that the differences are mainly due to the absence of large-scale motions. Finally, further downwind of the source the scaled
values of plume spread fall into two distinct groups. It is suggested that the difference between the two groups may be related
to the lack of dynamical similarity between the boundary-layer flows for varying surface roughness or to biased estimates
of the plume spread. 相似文献
973.
974.
The potential role of tropical Pacific forcing in driving the seasonal variability of the Arctic Oscillation (AO) is explored
using both observational data and a simple general circulation model (SGCM). A lead–lag regression technique is first applied
to the monthly averaged sea surface temperature (SST) and the AO index. The AO maximum is found to be related to a negative
SST anomaly over the tropical Pacific three months earlier. A singular value decomposition (SVD) analysis is then performed
on the tropical Pacific SST and the sea level pressure (SLP) over the Northern Hemisphere. An AO-like atmospheric pattern
and its associated SST appear as the second pair of SVD modes. Ensemble integrations are carried out with the SGCM to test
the atmospheric response to different tropical Pacific forcings. The atmospheric response to the linear fit of the model’s
empirical forcing associated with the SST variability in the second SVD modes strongly projects onto the AO. Idealized thermal
forcings are then designed based on the regression of the seasonally averaged tropical Pacific precipitation against the AO
index. Results indicate that forcing anomalies over the western tropical Pacific are more effective in generating an AO-like
response while those over the eastern tropical Pacific tend to produce a Pacific-North American (PNA)-like response. The physical
mechanisms responsible for the energy transport from the tropical Pacific to the extratropical North Atlantic are investigated
using wave activity flux and vorticity forcing formalisms. The energy from the western tropical Pacific forcing tends to propagate
zonally to the North Atlantic because of the jet stream waveguide effect while the transport of the energy from the eastern
tropical Pacific forcing mostly concentrates over the PNA area. The linearized SGCM results show that nonlinear processes
are involved in the generation of the forced AO-like pattern. 相似文献
975.
Meng-Dawn Cheng 《Journal of Atmospheric Chemistry》2018,75(1):1-16
Although a large volume of monitoring and computer simulation data exist for global coverage of HF, study of HF in the troposphere is still limited to industry whose primary interest is the safety and risk assessment of HF release because it is a toxic gas. There is very limited information on atmospheric chemistry, emission sources, and the behavior of HF in the environment. We provide a comprehensive review on the atmospheric chemistry of HF, modeling the reactions and transport of HF in the atmosphere, the removal processes in the vertical layer immediately adjacent to the surface (up to approximately 500 m) and recommend research needed to improve our understanding of atmospheric chemistry of HF in the troposphere. The atmospheric chemistry, emissions, and surface boundary layer transport of hydrogen fluoride (HF) are summarized. Although HF is known to be chemically reactive and highly soluble, both factors affect transport and removal in the atmosphere, the chemistry can be ignored when the HF concentration is at a sufficiently low level (e.g., 10 ppmv). At a low concentration, the capability for HF to react in the atmosphere is diminished and therefore the species can be mathematically treated as inert during the transport. At a sufficiently high concentration of HF (e.g., kg/s release rate and thousands of ppm), however, HF can go through a series of rigorous chemical reactions including polymerization, depolymerization, and reaction with water to form molecular complex. As such, the HF species cannot be considered as inert because the reactions could intimately influence the plume’s thermodynamic properties affecting the changes in plume temperature and density. The atmospheric residence time of HF was found to be less than four (4) days, and deposition (i.e., atmosphere to surface transport) is the dominant mechanism that controls the removal of HF and its oligomers from the atmosphere. The literature data on HF dry deposition velocity was relatively high compared to many commonly found atmospheric species such as ozone, sulfur dioxide, nitrogen oxides, etc. The global average of wet deposition velocity of HF was found to be zero based on one literature source. Uptake of HF by rain drops is limited by the acidity of the rain drops, and atmospheric particulate matter contributes negligibly to HF uptake. Finally, given that the reactivity of HF at a high release rate and elevated mole concentration cannot be ignored, it is important to incorporate the reaction chemistry in the near-field dispersion close to the proximity of the release source, and to incorporate the deposition mechanism in the far-field dispersion away from the release source. In other words, a hybrid computational scheme may be needed to address transport and atmospheric chemistry of HF in a range of applications. The model uncertainty will be limited by the precision of boundary layer parameterization and ability to accurately model the atmospheric turbulence. 相似文献
976.
利用1971-2005年中国591个气象台站的雾日资料以及逐日最低气温、相对湿度、平均风速资料,分析了35 a来中国各区域年雾日数与这些因子的相关关系,并利用IPCC第四次评估报告所提供的模式数据资料,针对3种不同的排放情景,对21世纪上半叶各区域年平均雾日进行预估。结果表明:对划分的9个雾区的年雾日数的回归方程的拟合效果较好,可以用来进行预估;未来50 a中国大部分地区雾日呈明显减少的变化趋势,在A1B,A2和B1情景下,雾日减少的平均幅度分别为16.2%,13.4%和12.9%。未来50 a中国雾日预估结果的空间分布显示:3种情景下未来中国大部分雾区雾日数都将减少,个别地区雾日数有增加趋势,其中A1B情景下雾日减少区的减少趋势最明显,而B1情景下雾日增加区的增加趋势最明显。 相似文献
977.
To assess the potential impact of climate changes on pasture production in the North Island, New Zealand, eight climate scenarios
of increased temperature and increased (or decreased) rainfall were investigated by integrating a polynomial regression model
for pasture production with a Geographic Information System (GIS). The results indicated that the climate change scenarios
assuming an increase in temperature by 1–2°C and a rainfall change by −20 to +20% would have a very significant impact on
pasture production with a predicted pasture production variation from −46.2 to +51.9% compared with the normal climate from
1961–1990. Increased temperature would generally have a positive effect on pasture production in the south and southeast of
the North Island, and increased rainfall would have a positive effect in the central, south and southeast of the North Island
and a negative effect in the north of the North Island. The interaction of decreased rainfall and increased temperature would
have a negative impact for the whole North Island except some central areas with high rainfall. Relevant management practices
for coping with potential climate change are discussed. 相似文献
978.
979.
普通黑光灯对稻纵卷叶螟和稻飞虱的诱虫量与气象因子的相关分析 总被引:2,自引:0,他引:2
以1991年至2008年广东省恩平市的观测数据,分析了普通黑光灯对稻纵卷叶螟和稻飞虱诱虫量与气象条件的相关性.结果表明:稻纵卷叶螟蛾年总诱量与年总降雨量呈正相关,稻飞虱成虫年诱量与年降雨量呈负相关,稻纵卷叶螟蛾与主要为害的5、6、8月降雨均呈正相关,与7月降雨则呈负相关;稻飞虱与5、7、8月降雨均为负相关,与6月降雨为正相关.此外,热带气旋个数与稻飞虱年诱虫量呈负相关;与稻纵卷叶螟蛾年诱虫量呈正相关.综合分析表明,稻纵卷叶螟蛾与稻飞虱与气象因子比较有较大的差别,对普通黑光灯诱虫量有极大的影响,可间接预测预报两迁飞性害虫的发生量,为准确预测预报两迁害虫为害提供参考. 相似文献
980.
运用常规气象观测资料,从天气环流形势的演变过程及有关物理量变化状况,对2010年3月19—20日发生在乌兰察布地区沙尘天气过程做了客观分析。分析结果表明:造成这次沙尘天气过程的主要影响天气系统是蒙古气旋和地面冷锋。蒙古气旋后部西北强冷空气侵入为起源于蒙古国西南部以及内蒙古西中部周边沙尘的输送提供了动力条件;地面冷锋过境使该地区温度梯度增大,空气对流上升加剧,为上游输送而来的尘土、沙石卷入空中浮悬提供了抬升条件。两者共同作用促使沙尘天气过程持续时间长、范围广,并伴有扬沙和沙尘暴出现。 相似文献