全文获取类型
收费全文 | 107278篇 |
免费 | 1626篇 |
国内免费 | 956篇 |
专业分类
测绘学 | 2841篇 |
大气科学 | 8006篇 |
地球物理 | 21346篇 |
地质学 | 36951篇 |
海洋学 | 9525篇 |
天文学 | 24249篇 |
综合类 | 311篇 |
自然地理 | 6631篇 |
出版年
2021年 | 892篇 |
2020年 | 1046篇 |
2019年 | 1108篇 |
2018年 | 2284篇 |
2017年 | 2217篇 |
2016年 | 2927篇 |
2015年 | 1780篇 |
2014年 | 2871篇 |
2013年 | 5552篇 |
2012年 | 2991篇 |
2011年 | 4100篇 |
2010年 | 3610篇 |
2009年 | 4895篇 |
2008年 | 4332篇 |
2007年 | 4145篇 |
2006年 | 4069篇 |
2005年 | 3215篇 |
2004年 | 3336篇 |
2003年 | 3140篇 |
2002年 | 2983篇 |
2001年 | 2654篇 |
2000年 | 2583篇 |
1999年 | 2189篇 |
1998年 | 2222篇 |
1997年 | 2119篇 |
1996年 | 1891篇 |
1995年 | 1762篇 |
1994年 | 1585篇 |
1993年 | 1454篇 |
1992年 | 1397篇 |
1991年 | 1275篇 |
1990年 | 1469篇 |
1989年 | 1234篇 |
1988年 | 1158篇 |
1987年 | 1350篇 |
1986年 | 1195篇 |
1985年 | 1486篇 |
1984年 | 1698篇 |
1983年 | 1551篇 |
1982年 | 1453篇 |
1981年 | 1360篇 |
1980年 | 1210篇 |
1979年 | 1188篇 |
1978年 | 1234篇 |
1977年 | 1076篇 |
1976年 | 1031篇 |
1975年 | 986篇 |
1974年 | 943篇 |
1973年 | 982篇 |
1971年 | 608篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
Measurements of the activation energy of electrical conductance and desiccation rates on subtidal marine algae from Florida
were compared to similar data from the Bay of Fundy, Nova Scotia, on intertidal marine algae frequently subjected to long
periods of exposure to air. We have developed a method for calculating the reaction rate constant of desiccation of fully
hydrated marine algae. Values of activation energies and desiccation rate constants are consistent with the requirements for
survival of these algae under widely different environmental conditions. 相似文献
993.
The electron cyclotron waves that originate at low altitudes (<0.5 RE) and observed by ground facilities have been studied in the presence of a weak parallel electric field in auroral magnetoplasma consisting of trapped energetic auroral electrons and cold background electrons of ionospheric origin. The model distribution for auroral trapped electrons is taken as Maxwellian ring distribution. An expression for the growth rate has been obtained in the presence of parallel electric field assuming that the real frequency in the whistler mode is not affected by the presence of the electric field. The results show that waves grow (or damp) in amplitude for a parallel (or antiparallel) electric field. The influence of the electric field is more pronounced at a shorter wavelength spectrum. An increase in population of energetic electrons increases the growth rate and thus, plays a significant role in the wave excitation process in the auroral regions. 相似文献
994.
995.
Study of sporadic-E clouds by backscatter radar 总被引:1,自引:0,他引:1
It is shown that swept-frequency backscatter ionograms covering a range of azimuths can be used to study the dynamics of sporadic-E clouds. A simple technique based on analytic ray tracing can be used to simulate the observed narrow traces associated with Es patches. This enables the location and extent of the sporadic-E clouds to be determined. The motion of clouds can then be determined from a time sequence of records. In order to demonstrate the method, results are presented from an initial study of 5 days of backscatter ionograms from the Jindalee Stage B data base obtained during March-April 1990. Usually 2–3 clouds were observed each day, mainly during the evening and up to midnight. The clouds lasted from 1–4 h and extended between 30°–80° in azimuth and 150–800 km in range. The clouds were mostly stationary or drifted generally westward with velocities of up to 80 ms–1. Only one cloud was observed moving eastward. 相似文献
996.
B. Rajagopalan U. Lall D. G. Tarboton D. S. Bowles 《Stochastic Environmental Research and Risk Assessment (SERRA)》1997,11(1):65-93
A nonparametric resampling technique for generating daily weather variables at a site is presented. The method samples the
original data with replacement while smoothing the empirical conditional distribution function. The technique can be thought
of as a smoothed conditional Bootstrap and is equivalent to simulation from a kernel density estimate of the multivariate
conditional probability density function. This improves on the classical Bootstrap technique by generating values that have
not occurred exactly in the original sample and by alleviating the reproduction of fine spurious details in the data. Precipitation
is generated from the nonparametric wet/dry spell model as described in Lall et al. [1995]. A vector of other variables (solar
radiation, maximum temperature, minimum temperature, average dew point temperature, and average wind speed) is then simulated
by conditioning on the vector of these variables on the preceding day and the precipitation amount on the day of interest.
An application of the resampling scheme with 30 years of daily weather data at Salt Lake City, Utah, USA, is provided. 相似文献
997.
In this contribution we discuss the geometry-free GPS single baseline model and show how the least-squares ambiguities are affected by changes in the stochastic model. We particularly pay attention to the effect of time correlation, cross-correlation and satellite elevation dependence. We also differentiate between the impact on the location of the ambiguity search space and the impact on the size and shape of the search space. The analysis is carried out for both the model in which the ionospheric delays are assumed absent, and for the model in which they are assumed present. The former model is applicable to short baselines only. 相似文献
998.
999.
1000.
Byproduct Formation During the Reduction of TCE by Zero-Valence Iron and Palladized Iron 总被引:4,自引:0,他引:4
Liyuan Liang Nic Korte J.D. Goodlaxson Jay Clausen Quintus Fernando Rosy Muftikian 《Ground Water Monitoring & Remediation》1997,17(1):122-127
Trichloroethene (TCE) was reduced with zero-valence iron and palladized iron in zero-head-space extractors. Progress of the reaction in these batch studies was monitored with purge-and-trap gas chromatography and a flame ionization detector. When a 5 ppm initial concentration of TCF. reacts with zero-valence iron, approximately 140 ppb of vinyl chloride persists for as long as 73 days. The concentration of vinyl chloride (approximately If) ppb) remaining with palladized iron is approximately an order of magnitude less than when zero-valence iron is the reductant. These data suggest that volatile byproducts may be under-represented in oilier published data regarding reduction with zero-valence metals. These results also demonstrate that the reduction of TCE with palladized iron (0.05 percent palladium) is more than an order of magnitude faster than with zero-valence iron. Wilh a 5:1 solution-to-solid ratio the TCE half-life with zero-valence iron is 7.41 hours. but is only 0.59 hours with the palladized iron. 相似文献