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891.
Previous research on climatic change in the Mid-Holocene in China indicates that it was a warm and humid period, accompanied
by stronger summer monsoons, and it is defined as the Megathermal in the Holocene, or the Holocene Optimum period. However,
this conclusion is mainly directed at the monsoonal region in eastern China. In this research, we chose the Gonghe Basin in the
northeastern Qinghai-Tibetan Plateau as the study area. Geochemical analysis of the profiles of paleosols and aeolian sand in the
Santala area in the middle of the Gonghe Basin, along with OSL (optically stimulated luminescence) dating, indicates that the
regional climate has experienced several warm-humid and cold-dry cycles since 11.8 ka. In particular, the Mid-Holocene
(8.1–4.6 ka) was relatively cold and dry as evidenced by drastic fluctuations in chemical weathering degree and humidity, a
higher aridity index, and sparse vegetation, accompanying increased winter monsoonal strength. In order to clarify whether
this is an individual or local signal, we compared our geochemical analysis results with lake and peat records and aeolian deposits
of the monsoonal boundary region. The results indicate that the climate deteriorated widely, with declines in temperature
and moisture, in the Mid-Holocene in the modern monsoonal boundary zone. Furthermore, the duration of climate deterioration
(relatively dry period) generally decreased from west to east in the aforementioned regions. Therefore, this dry phase in
Gonghe Basin may be representative of dry events in Mid-Holocene in northern China. In addition, we discuss the reasons for
this dry climate from several perspectives: (1) it probably can be attributed to a decline in summer monsoonal strength; (2) the
regional evaporation loss (forced by high temperature) was not compensated by regional precipitation; (3) the thermal dynamic
effect of the Qinghai-Tibetan Plateau. 相似文献
892.
The Cassini spacecraft has revealed landforms on the surface of Titan suggested to be viscous cryovolcanic flows and possibly eruptive domes. In order to relate those surface features to the processes and chemistries that produced them, it is necessary to construct flow models, which rely on characterization of the rheological properties of the eruptants. This paper describes our initial exploratory attempts to understand the rheological characteristics of cryogenic slurries, using a 40% methanol-water mixture, as a precursor to more detailed experiments. We have devised a new automated cryogenic rotational viscometer system to more fully characterize cryovolcanic slurry rheologies. A series of measurements were performed, varying first temperature, and then strain rate, which revealed development of yield stress-like behaviors, shear-rate dependence, and thixotropic behavior, even at relatively low crystal fractions, not previously reported. At fixed shear rate our data are fit well by the Andrade equation, with the activation energy modified by a solid volume fraction. At fixed temperature, depending on shearing history, a Cross model could describe our data over a wide shear rate range. A Bingham plastic model appears to be a good constitutive model for the data measured at high shear rates when the shear was global. The yield stress like behavior implies that levee formation on cryolava flows is more likely than would be inferred from the previous studies, and may provide a partial explanation for features interpreted as steep-sided volcanic constructs on Titan. 相似文献
893.
相位差法波前探测是利用焦面和已知像差函数及像差尺度的离焦位置上同时采集的一对短曝光图像,通过最优化方法使代价函数最小化,从而得到目标和波前相位估计值的渡前探测方法.本文通过计算机模拟了像差函数为离焦像差的情况下,经相位差法波前探测成像系统后在焦面和离焦面上采集得到的图像强度分布,采用有限内存拟牛顿法(LFBGS)对模拟图像进行数值求解,实现了渡前相位的重构和对面源目标的重建;由于在实际应用中测量误差和机械加工带来的离焦量偏差是不可避免的,因此在模拟实验中就离焦量尺度不精确的情况进行了分析. 相似文献
894.
895.
飞机人工增雨作业效果分析 总被引:2,自引:0,他引:2
采用2009年12月~2010年1月广西飞机人工增雨作业数据,结合作业期间的相关气象资料,分析飞机人工增雨作业效果。结果表明,在影响广西的天气系统总体偏弱的情况下,实施飞机人工增雨作业后,雷达回波强度出现明显增强,降雨明显增加,降雨实况均比常年同期偏多,偏多程度在1成~2倍以上。 相似文献
896.
897.
To quantitatively investigate the dynamic and thermal responses of the South China Sea (SCS) during and subsequent to the passage of a real typhoon, a three-dimensional, regional coupled air–sea model is developed to study the upper ocean response of the SCS to Typhoon Krovanh (2003). Owing to the scarcity of ocean observations, the three-dimensional numerical modeling with high resolution, as a powerful tool, offers a valuable opportunity to investigate how the air–sea process proceeds under such extreme conditions. The amplitude and distribution of the cold path produced by the coupled model compare reasonably well with the TRMM/TMI-derived data. The maximum SST cooling is 5.3 °C, about 80 km to the right of the typhoon track, which is consistent with the well-documented rightward bias in the SST response to typhoons. In correspondence to the SST cooling, the mixed layer depth exhibits an increase; the increases in the mixed layer depth on the right of typhoon track are significantly higher than those on the left, with maxima of 58 m. This correspondence indicates that the SST cooling is caused mainly by entrainment. Under the influence of typhoon, a cyclonic, near-surface current field is generated in the upper ocean layer, which moves with the typhoon. The typhoon-induced horizontal currents in the wake of the storm have strong near-inertial oscillation, which gradually propagates downward. The unique features of the SCS response to Typhoon Krovanh are also discussed, such as Kuroshio intrusion and coastal subsurface jets. 相似文献
898.
899.
近30年三峡库区洪涝特征及建库前后致涝气候因子的差异 总被引:1,自引:0,他引:1
利用三峡库区1980—2007年逐日雨量观测资料和NCEP/NCAR逐月再分析资料,分析了近30年来三峡库区洪涝变化特征,并对三峡库区洪涝典型年份大气环流形势进行合成分析,进一步研究了典型洪涝年份不同关键区水汽输送特征。近30年来,三峡库区洪涝有明显的年际、年代际差异;近年来,三峡库区年均洪涝和区域性洪涝强度均呈减弱趋势,而区域性洪涝较为频繁;大气环流合成分析表明,乌拉尔山高压脊显著加强,导致经向性环流增强,引导冷空气南下,与低纬北上暖湿气流交汇,有利于三峡库区降水,易于产生洪涝灾害;三峡库区蓄水前后在蒙古和我国东北地区的环流背景合成明显不同,蓄水前典型洪涝年份环流异常更有利于降水;在强盛的偏南气流影响下,蓄水前典型洪涝年份水汽输送指示特征较蓄水后的洪涝年份水汽输送更加明显,更有利于三峡库区降水;典型洪涝年2004、2007年,关键区水汽输送有明显的候变化特征,但在典型洪涝事件中各个关键区水汽输送指示特征并不明显。 相似文献
900.
A set of new parameterizations for the friction velocity and temperature scale over gently
sloped terrain and in calm synoptic conditions are theoretically derived. The friction velocity is
found to be proportional to the product of the square root of the total accumulated heating in the
boundary layer and the sinusoidal function of the slope angle, while the temperature scale is
proportional to the product of the boundary layer depth, the sinusoidal function of the slope angle
and the potential temperature gradient in the free atmosphere. Using the new friction velocity
parameterization, together with a parameterization of eddy diffusivity and an initial potential
temperature profile around sunrise, an improved parameterization for the thermally induced upslope
flow profile is derived by solving the Prandtl equations. The upslope flow profile is found to be
simply proportional to the friction velocity. 相似文献