全文获取类型
收费全文 | 656篇 |
免费 | 35篇 |
国内免费 | 2篇 |
专业分类
测绘学 | 13篇 |
大气科学 | 48篇 |
地球物理 | 165篇 |
地质学 | 202篇 |
海洋学 | 120篇 |
天文学 | 72篇 |
综合类 | 4篇 |
自然地理 | 69篇 |
出版年
2023年 | 2篇 |
2021年 | 7篇 |
2020年 | 10篇 |
2019年 | 13篇 |
2018年 | 16篇 |
2017年 | 18篇 |
2016年 | 25篇 |
2015年 | 24篇 |
2014年 | 21篇 |
2013年 | 41篇 |
2012年 | 20篇 |
2011年 | 30篇 |
2010年 | 34篇 |
2009年 | 38篇 |
2008年 | 56篇 |
2007年 | 39篇 |
2006年 | 23篇 |
2005年 | 20篇 |
2004年 | 25篇 |
2003年 | 14篇 |
2002年 | 27篇 |
2001年 | 12篇 |
2000年 | 16篇 |
1999年 | 10篇 |
1998年 | 10篇 |
1997年 | 4篇 |
1996年 | 9篇 |
1995年 | 11篇 |
1994年 | 10篇 |
1993年 | 9篇 |
1992年 | 2篇 |
1991年 | 4篇 |
1990年 | 5篇 |
1988年 | 4篇 |
1987年 | 5篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 8篇 |
1983年 | 9篇 |
1982年 | 16篇 |
1981年 | 12篇 |
1980年 | 8篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1975年 | 2篇 |
1973年 | 2篇 |
1971年 | 3篇 |
1968年 | 1篇 |
1967年 | 1篇 |
排序方式: 共有693条查询结果,搜索用时 250 毫秒
61.
62.
63.
Palynological results from Julietta Lake currently provide the most direct evidence to support the existence of a glacial refugium for Pinus pumila in mountains of southwestern Beringia. Both percentages and accumulation rates indicate the evergreen shrub survived until at least ∼ 19,000 14C yr BP in the Upper Kolyma region. Percentage data suggest numbers dwindled into the late glaciation, whereas pollen accumulation rates point towards a more rapid demise shortly after ∼ 19,000 14C yr BP. Pinus pumila did not re-establish in any great numbers until ∼ 8100 14C yr BP, despite the local presence ∼ 9800 14C yr BP of Larixdahurica, which shares similar summer temperature requirements. The postglacial thermal maximum (in Beringia ∼ 11,000-9000 14C yr BP) provided Pinus pumila shrubs with equally harsh albeit different conditions for survival than those present during the LGM. Regional records indicate that in this time of maximum warmth Pinus pumila likely sheltered in a second, lower-elevation refugium. Paleoclimatic models and modern ecology suggest that shifts in the nature of seasonal transitions and not only seasonal extremes have played important roles in the history of Pinus pumila over the last ∼ 21,000 14C yr BP. 相似文献
64.
Patricia Carbajales-Dale Dusti Annan-Coultas Anjali Joseph Martie Thompson Roxana Jafarifiroozabadi Susan P. Limber Bonnie Holaday Sahar Mihandoust 《Transactions in GIS》2023,27(4):975-995
Geographic information systems (GIS) have become essential tools in the public health domain, especially when it comes to monitoring and surveillance of disease. The purpose of this article is to describe and explore the benefits of using GIS to improve public health emergency response during a global pandemic and, in particular, how to effectively optimize the allocation of public health resources in a rural setting using a data-driven approach that considers the multifactorial demand for new COVID-19 testing sites. Herein, the authors present their interprofessional project as an example of such efforts to inform applications for practice. The team developed a GIS-based multicriteria decision analysis model for use by decision-makers and public health experts in similar future planning and response scenarios. Focus is placed on rural characteristics (e.g., accessibility), vulnerable populations, and daily changing conditions (e.g., COVID-19 daily case fluctuations) that create additional challenges for public health agencies and policymakers. 相似文献
65.
Admir Créso Targino Patricia Krecl Guilherme Conor Coraiola 《Theoretical and Applied Climatology》2014,117(1-2):73-87
Air temperature was monitored at 13 sites across the urban perimeter of a Brazilian midsize city in winter 2011. In this study, we show that the urban heat island (UHI) develops only at night and under certain weather conditions, and its intensity depends not only on the site's land cover but also on the meteorological setting. The urban heat island intensity was largest (6.6 °C) under lingering high-pressure conditions, milder (3.0 °C) under cold anticyclones and almost vanished (1.0 °C) during the passage of cold fronts. The cooling rates were calculated to monitor the growth and decay of the UHI over each specific synoptic setting. Over four contiguous days under the effect of a lingering high-pressure event, we observed that the onset of cooling was always at about 2 h before sunset. The reference site attained mean cooling rate of ?2.6 °C h?1 at sunset, whilst the maximum urban rate was ?1.2 °C h?1. Under a 3-day cold anticyclone episode, cooling also started about 2 h before sunset, and the difference between maximum rural (?2.0 °C h?1) and urban (?1.0 °C h?1) cooling rates diminished. Under cold-front conditions, the cooling rate was homogeneous for all sites and swang about zero throughout the day. The air temperature has a memory effect under lingering high-pressure conditions which intensified the UHI, in addition to the larger heat storage in the urban area. Cold anticyclone conditions promoted the development of the UHI; however, the cold air pool and relatively light winds smoothed out its intensity. Under the influence of cold fronts, the urban fabric had little effect on the city's air temperature field, and the UHI was imperceptible. 相似文献
66.
67.
In September 2011 and March 2012, benthic nutrient fluxes were measured in the San Francisco Bay Delta, across a gradient from above the confluence of the Sacramento and San Joaquin Rivers to Suisun Bay. Dark and illuminated core incubation techniques were used to measure rates of denitrification, nutrient fluxes (phosphate, ammonium, nitrate), and oxygen fluxes. While benthic nutrient fluxes have been assessed at several sites in northern San Francisco Bay, such data across a Delta–Bay transect have not previously been determined. Average September rates of DIN (nitrate, nitrite, ammonium) flux were net positive across all sites, while March DIN flux indicated net uptake of DIN at some sites. Denitrification rates based on the N2/Ar ratio approach were between 0.6 and 1.0 mmol m?2 day?1, similar to other mesotrophic estuarine sediments. Coupled nitrification–denitrification was the dominant denitrification pathway in September, with higher overlying water nitrate concentrations in March resulting in denitrification driven by nitrate flux into the sediments. Estimated benthic microalgal productivity was variable and surprisingly high in Delta sediments and may represent a major source of labile carbon to this ecosystem. Variable N/P stoichiometry was observed in these sediments, with deviations from Redfield driven by processes such as denitrification, variable light/dark uptake of nutrients by microalgae, and adsorption of soluble reactive phosphorus. 相似文献
68.
Manuel Bollo Manent José Ramón Hernández Santana Ana Patricia Méndez Linares 《Central European Journal of Geosciences》2014,6(2):219-228
The present work evaluates the state of the environment in Mexico based on indicators of the present status of the country’s natural resource management, social and economical conditions and anthropogenic modifications. The Mexican environment is interpreted as a spatially open system having a historical character that is essentially determined by the continual interaction between nature, society and economy. The landscape approach is followed, considering as units of territorial analysis each one of the 145 biophysical environmental units included in the national physiographic regionalization. The assessment of 16 indicators for each biophysical environmental unit was made considering their regional environmental integrity problems, the degree of disarticulation of their structure and function, and the alteration of their territorial structure, all of which determine whether or not they accomplish their environmental functions and achieve environmental stability. The classification of the state of the environment included 5 categories in 8 combinations represented in the map of the state of the environment in Mexico for the year 2008. The map shows that nearly 47.10% of the country’s surface has an environmental status ranging between unstable and critical, the problematic areas being mostly concentrated in the southeast and center of the national territory. 相似文献
69.
Statistical studies of hard X-ray flares position on the solar disk have shown that the more energetic hard X-rays have a
tendency to be more concentrated near the limb rather than at disk center, whereas lower-energy hard X-ray emission seems
isotropic. Since the high-frequency radio emission is believed to be produced by the same energetic electron population responsible
for the high-energy hard X-rays, we searched the microwave/millimeter emitting bursts for center-to-limb variation in their
emission. A total of 499 bursts observed by the radio telescopes in Bern at the frequencies of 3.1, 5.2, 8.4, 11.8, 19.6,
35.0, and 50.0 GHz were analyzed. Simultaneous Hα flares were used for determination of the radio burst position on the solar
disk. For each of the 7 frequencies, the peak flux and duration were studied as a function of heliocentric position. For 312
bursts, spectral parameters such as spectral index, peak frequency, and flux at spectral maximum were analyzed. For a subset
of 43 bursts with emission at all frequencies, the emission and spectral parameters were analyzed. Center-to-limb variations
of the spectral parameters for all bursts were sought. In order to interpret the observational results, we have performed
a numerical simulation of gyrosynchrotron spectra. We find that high-frequency events, which are also the more energetic ones,
have larger center-to-limb variations in their parameters than do the overall flares. Moreover, this behavior agrees with
theoretical predictions. 相似文献
70.
Doug W. Johnson Simon Osborne Robert Wood Karsten Suhre Patricia K. Quinn Tim Bates M. O. Andreae Kevin J. Noone Paul Glantz Brian Bandy J. Rudolph Colin O'Dowd 《Tellus. Series B, Chemical and physical meteorology》2000,52(2):348-374
During the 1st Lagrangian experiment of the North Atlantic Regional Aerosol Characterisation Experiment (ACE‐2), a parcel of air was tagged by releasing a smart, constant level balloon into it from the Research Vessel Vodyanitskiy . The Meteorological Research Flight's C‐130 aircraft then followed this parcel over a period of 30 h characterising the marine boundary layer (MBL), the cloud and the physical and chemical aerosol evolution. The air mass had originated over the northern North Atlantic and thus was clean and had low aerosol concentrations. At the beginning of the experiment the MBL was over 1500 m deep and made up of a surface mixed layer (SML) underlying a layer containing cloud beneath a subsidence inversion. Subsidence in the free troposphere caused the depth of the MBL to almost halve during the experiment and, after 26 h, the MBL became well mixed throughout its whole depth. Salt particle mass in the MBL increased as the surface wind speed increased from 8 m s−1 to 16 m s−1 and the accumulation mode (0.1μm to 3.0 μm) aerosol concentrations quadrupled from 50 cm−3 to 200 cm−3 . However, at the same time the total condensation nuclei (>3 nm) decreased from over 1000 cm−3 to 750 cm−3 . The changes in the accumulation mode aerosol concentrations had a significant effect on the observed cloud microphysics. Observational evidence suggests that the important processes in controlling the Aitken mode concentration which, dominated the total CN concentration, included, scavenging of interstitial aerosol by cloud droplets, enhanced coagulation of Aitken mode aerosol and accumulation mode aerosol due to the increased sea salt aerosol surface area, and dilution of the MBL by free tropospheric air. 相似文献