全文获取类型
收费全文 | 43111篇 |
免费 | 741篇 |
国内免费 | 288篇 |
专业分类
测绘学 | 857篇 |
大气科学 | 2962篇 |
地球物理 | 8478篇 |
地质学 | 15469篇 |
海洋学 | 3934篇 |
天文学 | 9516篇 |
综合类 | 98篇 |
自然地理 | 2826篇 |
出版年
2022年 | 268篇 |
2021年 | 481篇 |
2020年 | 547篇 |
2019年 | 636篇 |
2018年 | 1077篇 |
2017年 | 1083篇 |
2016年 | 1176篇 |
2015年 | 684篇 |
2014年 | 1132篇 |
2013年 | 2076篇 |
2012年 | 1326篇 |
2011年 | 1837篇 |
2010年 | 1606篇 |
2009年 | 2038篇 |
2008年 | 1805篇 |
2007年 | 1842篇 |
2006年 | 1698篇 |
2005年 | 1199篇 |
2004年 | 1232篇 |
2003年 | 1260篇 |
2002年 | 1139篇 |
2001年 | 954篇 |
2000年 | 887篇 |
1999年 | 802篇 |
1998年 | 805篇 |
1997年 | 773篇 |
1996年 | 657篇 |
1995年 | 633篇 |
1994年 | 556篇 |
1993年 | 505篇 |
1992年 | 457篇 |
1991年 | 485篇 |
1990年 | 484篇 |
1989年 | 437篇 |
1988年 | 410篇 |
1987年 | 464篇 |
1986年 | 461篇 |
1985年 | 565篇 |
1984年 | 618篇 |
1983年 | 596篇 |
1982年 | 546篇 |
1981年 | 514篇 |
1980年 | 482篇 |
1979年 | 452篇 |
1978年 | 435篇 |
1977年 | 429篇 |
1976年 | 396篇 |
1975年 | 387篇 |
1974年 | 376篇 |
1973年 | 404篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Although emerging technologies like carbon capture and storage and advanced nuclear are expected to play leading roles in greenhouse gas mitigation efforts, many engineering and policy-related uncertainties will influence their deployment. Capital-intensive infrastructure decisions depend on understanding the likelihoods and impacts of uncertainties such as the timing and stringency of climate policy as well as the technological availability of carbon capture systems. This paper demonstrates the utility of stochastic programming approaches to uncertainty analysis within a practical policy setting, using uncertainties in the US electric sector as motivating examples. We describe the potential utility of this framework for energy-environmental decision making and use a modeling example to reinforce these points and to stress the need for new tools to better exploit the full range of benefits the stochastic programming approach can provide. Model results illustrate how this framework can give important insights about hedging strategies to reduce risks associated with high compliance costs for tight CO2 caps and low CCS availability. Metrics for evaluating uncertainties like the expected value of perfect information and the value of the stochastic solution quantify the importance of including uncertainties in capacity planning, of making precautionary low-carbon investments, and of conducting research and gathering information to reduce risk. 相似文献
952.
This essay proposes an innovative institutional strategy for global climate protection, quite distinct from but ultimately complementary to the UNFCCC climate treaty negotiations. Our “building block” strategy relies on a variety of smaller-scale transnational cooperative arrangements, involving not only states, but also subnational jurisdictions, firms, and civil society organizations, to undertake activities whose primary goal is not climate mitigation but which will achieve greenhouse gas reductions as a byproduct. This strategy avoids the problems inherent in developing an enforceable, comprehensive treaty regime by mobilizing other incentives—including economic self-interest, energy security, cleaner air, and furtherance of international development— to motivate a range of actors to cooperate on actions that will also produce climate benefits. The strategy uses three specific models of regime formation (club, linkage, and dominant actor models) which emerge from economics, international relations, and organizational behavior, to develop a variety of transnational regimes that are generally self-enforcing and sustainable, avoiding the free rider and compliance problems endemic in collective action to provide public goods. These regimes will contribute to global climate action not only by achieving emissions reductions in the short term, but also by creating global webs of cooperation and trust, and by linking the building block regimes to the UNFCCC system through greenhouse gas monitoring and reporting systems. We argue that the building blocks regimes would thereby help secure eventual agreement on a comprehensive climate treaty. 相似文献
953.
E. P. Veselov 《Russian Meteorology and Hydrology》2008,33(2):98-101
A formula is obtained for calculation of wind speed in tornado, as proportional to the speed of convective updraft. Results are presented of wind speed calculation using the formula for 57 tornadoes observed at different time and in different regions of the world. Possibilities are shown of tornado wind speed forecasting taking into account criteria of tornado danger of the Cb clouds. 相似文献
954.
E. Mahieu C. P. Rinsland R. Zander P. Demoulin L. Delbouille G. Roland 《Journal of Atmospheric Chemistry》1995,20(3):299-310
A set of high-resolution IR solar spectra recorded at the International Scientific Station of the Jungfraujoch, Switzerland, from 84/06 to 93/06, and at the National Solar Observatory McMath-Pierce solar telescope facility on Kitt Peak, Arizona, U.S.A. from 78/05 to 92/07 have been analyzed to determine the vertical column abundances of hydrogen cyanide, HCN, above the two stations. The analysis was based on least-squares curve fitting of calculated spectra to the observations encompassing the P4 and the P8 lines of HCN respectively located at 3299.5273 and 3287.2483 cm–1. The results obtained for the two stations indicate that no significant long-term trend affects either of the two databases; however, this analysis reveals variable increases during springtime of up to a factor of 2 in the HCN total column above the Jungfraujoch and even up to 3 above Kitt Peak. The calculated mean vertical column abundances, excluding the spring observations, are equal to (2.55±0.30)×1015 molec./cm2 (S.D.) and (2.75±0.30)×1015 molec./cm2 respectively above the Jungfraujoch and the Kitt Peak observatories. Based on a realistic volume mixing ratio profile, these columns translate into mean volume mixing ratios equal to 190×10–12 ppv at the respective altitudes of the stations. 相似文献
955.
The stability functions for momentum and heat under a Richardson number formulation are derived from the nondimensional shear functions under a Monin-Obukhov formulation. The Prandtl number is also derived as a function of the Richardson number. Previously, this has been done only in a limited sense. Because the Richardson number formulation is expressed in closed form, iterative techniques are no longer needed in numerical models that use Monin-Obukhov similarity theory. This time-saving approach is made possible by deriving expressions for the friction velocity and temperature in terms of the Richardson-number-dependent stability functions. In addition, the Richardson number approximation in the lowest layer is made to depend explicitly upon the surface roughness. 相似文献
956.
Observations were made of a shallow stratus of upslope origin using an aircraft equipped with insitu probes and with a vertically-pointing radar of 3-mm wavelength. A cloud layer of 300 m thickness was found below the inversion; an additional layer of 100 m thickness was located within the inversion. The coldest temperature within the cloud was -2°C and the cloud contained no ice particles. Drizzle drops up to 180 Am were present in both cloud layers.The observations reveal precipitation and air motion structures of approximately 1 km horizontal dimensions. The origin of this organization appears to be weak convection. In addition, mixing played an important role in forming the cloud droplet and drizzle drop size distributions. 相似文献
957.
H. E. Landsberg 《Climatic change》1986,9(1-2):123-128
There is a general lack of meteorological data in arid and semiarid regions, despite the need for such information for the proper exploitation of these areas. Drought is a natural part of the climate of these regions, just like anywhere else. Drought indices are surveyed and the statistical nature of drought is stressed. The correlation between average temperature and precipitation is explored, as well as the role of wind in creating conditions favorable for the intensification of desert conditions. 相似文献
958.
Summary Some of the vortices which develop over the Tibetan plateau during summer give rise to severe weather over eastern China. These weather developments can be difficult to forecast, but have been simulated successfully in a recently developed numerical model. It will be shown that different vortices respond differently to topographic effects, to nonlinear processes and to sensible and latent heating in their formation, maintenance, and motion. Elevated terrain appears to enhance the influence of thermodynamic processes, block airflow, and increase frictional dissipation.Sensible heating sometimes collaborates with topography in blocking cold air intrusions and is not only a function of terrain elevation but also of the synoptic situation. Without the input of latent heating, vortices over the eastern part of the Tibetan plateau tend to degenerate.
With 22 Figures 相似文献
Numerische Simulation von Wirbelbildungen über dem Hochland von Tibet
Zusammenfassung Einige der sich im Sommer über dem Hochland von Tibet entwickelnden Wirbel verursachen Unwetter in Ostchina. Die Vorhersage dieser Entwicklung kann schwierig sein, sie werden aber von einem kürzlich vorgestellten numerischen Modell erfolgreich simuliert. Es wird gezeigt, daß verschiedene Wirbel verschieden auf die Orographie, auf nichtlineare Vorgänge sowie auf fühlbare und latente Wärme in ihrer Entstehung, Erhaltung und Bewegung reagieren. Ein Hochland scheint den Einfluß thermodynamischer Vorgänge zu verstärken, Strömungen zu blockieren und die Reibungsdissipation zu steigern.Die fühlbare Wärme verhindert manchmal gemeinsam mit der Topographie Kaltlufteinbrüche, hängt aber nicht nur von der Hochland-sondern auch von der synoptischen Situation ab. Ohne latente Wärme neigen Wirbel über dem Ostteil des Hochlandes von Tibet zur Degeneration.
With 22 Figures 相似文献
959.
960.
Storm rainfall conditions for floods and debris flows from recently burned areas in southwestern Colorado and southern California 总被引:8,自引:1,他引:8
Susan H. Cannon Joseph E. Gartner Raymond C. Wilson James C. Bowers Jayme L. Laber 《Geomorphology》2008,96(3-4):250
Debris flows generated during rain storms on recently burned areas have destroyed lives and property throughout the Western U.S. Field evidence indicate that unlike landslide-triggered debris flows, these events have no identifiable initiation source and can occur with little or no antecedent moisture. Using rain gage and response data from five fires in Colorado and southern California, we document the rainfall conditions that have triggered post-fire debris flows and develop empirical rainfall intensity–duration thresholds for the occurrence of debris flows and floods following wildfires in these settings. This information can provide guidance for warning systems and planning for emergency response in similar settings.Debris flows were produced from 25 recently burned basins in Colorado in response to 13 short-duration, high-intensity convective storms. Debris flows were triggered after as little as six to 10 min of storm rainfall. About 80% of the storms that generated debris flows lasted less than 3 h, with most of the rain falling in less than 1 h. The storms triggering debris flows ranged in average intensity between 1.0 and 32.0 mm/h, and had recurrence intervals of two years or less. Threshold rainfall conditions for floods and debris flows sufficiently large to pose threats to life and property from recently burned areas in south-central, and southwestern, Colorado are defined by: I = 6.5D− 0.7 and I = 9.5D− 0.7, respectively, where I = rainfall intensity (in mm/h) and D = duration (in hours).Debris flows were generated from 68 recently burned areas in southern California in response to long-duration frontal storms. The flows occurred after as little as two hours, and up to 16 h, of low-intensity (2–10 mm/h) rainfall. The storms lasted between 5.5 and 33 h, with average intensities between 1.3 and 20.4 mm/h, and had recurrence intervals of two years or less. Threshold rainfall conditions for life- and property-threatening floods and debris flows during the first winter season following fires in Ventura County, and in the San Bernardino, San Gabriel and San Jacinto Mountains of southern California are defined by I = 12.5D−0.4, and I = 7.2D−0.4, respectively. A threshold defined for flood and debris-flow conditions following a year of vegetative recovery and sediment removal for the San Bernardino, San Gabriel and San Jacinto Mountains of I = 14.0D−0.5 is approximately 25 mm/h higher than that developed for the first year following fires.The thresholds defined here are significantly lower than most identified for unburned settings, perhaps because of the difference between extremely rapid, runoff-dominated processes acting in burned areas and longer-term, infiltration-dominated processes on unburned hillslopes. 相似文献