首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   7728篇
  免费   682篇
  国内免费   786篇
测绘学   3546篇
大气科学   508篇
地球物理   815篇
地质学   1617篇
海洋学   799篇
天文学   36篇
综合类   699篇
自然地理   1176篇
  2024年   13篇
  2023年   52篇
  2022年   331篇
  2021年   373篇
  2020年   387篇
  2019年   380篇
  2018年   273篇
  2017年   404篇
  2016年   392篇
  2015年   406篇
  2014年   387篇
  2013年   553篇
  2012年   423篇
  2011年   430篇
  2010年   352篇
  2009年   425篇
  2008年   457篇
  2007年   466篇
  2006年   387篇
  2005年   367篇
  2004年   341篇
  2003年   275篇
  2002年   241篇
  2001年   190篇
  2000年   149篇
  1999年   116篇
  1998年   149篇
  1997年   130篇
  1996年   80篇
  1995年   49篇
  1994年   45篇
  1993年   33篇
  1992年   44篇
  1991年   26篇
  1990年   19篇
  1989年   9篇
  1988年   8篇
  1987年   7篇
  1986年   8篇
  1985年   5篇
  1984年   5篇
  1983年   3篇
  1982年   2篇
  1980年   1篇
  1979年   1篇
  1978年   1篇
  1954年   1篇
排序方式: 共有9196条查询结果,搜索用时 15 毫秒
981.
选取中国汾渭平原地区作为研究对象,利用MODIS、OMI和CALIPSO多源卫星遥感资料,同时结合环境监测国控站点污染6要素等逐小时地面环境监测数据以及能见度、霾天气现象记录等地面气象要素资料,综合分析了2013—2018年秋冬季汾渭平原空气质量状况、气溶胶的组分,探讨了卫星遥感气溶胶光学厚度(Aerosol Optical Depth,AOD)与地面污染物浓度的关系,并结合中国气象局化学天气预报系统-EMI评估模式(CUACE-EMI)资料对气象条件和污染减排影响进行评估。结果表明:11个代表城市中有6个城市秋冬季有接近或超过一半的时刻处于污染状态,且污染发生时,各代表城市大多数时刻处于中度及其以上污染级别;三门峡、临汾、运城和西安是霾和重度霾高发的城市,其重度霾爆发频率高达11.63%—14.78%;汾渭平原秋冬季首要污染物为PM2.5和PM10,以污染沙尘、沙漠沙尘和烟尘为主,出现频率分别为36.24%、25.14%和22.96%;MODIS AOD与空气质量指数(Air Quality Index,AQI)、PM2.5、PM10质量浓度之间的相关系数分别为0.72、0.70和0.64;汾渭平原2018年气象条件的变化使PM2.5浓度较2013年、2014年、2015年、2016年和2017年同期上升了17.06%、1.58%、4.34%、11.25%和5.75%,减排措施使PM2.5浓度较2013年、2014年、2015年、2016年和2017年同期分别下降了8.74%、28.01%、4.93%、3.16%和42.62%。  相似文献   
982.
A brief survey is given of the sources, the methods of injection into the atmosphere, and the mechanisms of deposition into bodies of water for various atmospheric pollutants which contribute significantly to the pollution of large bodies of water. Specific examples of the relative importance of atmospheric sources are included where data are available. Programs of the Canadian Atmospheric Environment Service in this field of research are outlined.  相似文献   
983.
基于多源数据的城市热岛效应研究——以石家庄地区为例   总被引:3,自引:1,他引:2  
杨鹏  陈静  侯晓玮  高祺  赵强 《气象》2013,39(10):1304-1313
以石家庄地区为研究对象,选取地面气象观测站数据、区域自动站数据、MODIS 1B以及Landsat7 ETM+卫星遥感影像数据作为主要数据源,从气温和地表温度两个角度考虑,分析城市化进程对城市热岛效应的影响,城市热岛的时间变化规律以及空间分布规律等。研究结果表明:(1)石家庄城市热岛强度随时间逐年增加且呈继续上升趋势,与城市化进程呈同步变化趋势;(2)采用同一时次气温和地表温度资料分别研究石家庄城市热岛空间分布特征,发现夏、秋季白天和四季晚上石家庄城市热岛效应明显,春、冬季白天有时出现“冷岛”效应;(3)与白天相比,在晚上气温和地表温度两者的相关性较好,其相关系数可达0.80;(4)选用Landsat7 ETM+高分辨率卫星遥感影像数据研究不同土地覆盖类型对城市热岛的影响,发现地表温度随着植被指数NDVI的增大而减小,两者之间呈现很好的负相关性,R2=0.72。  相似文献   
984.
土壤有机质是土壤的重要组成部分,对许多土壤属性有重要影响.利用光谱技术进行土壤有机质的快速测定是实现精细农业的基础.近几十年中,高光谱技术的发展为土壤研究提供了新的手段.受土壤有机质质量分数、组成以及土壤水分、土壤质地的影响,室内光谱的估算结果总体较好;受大气、地表植被等影响,航空或卫星的成像光谱估算精度较低.星载成像技术的进步将在一定程度上提高土壤有机质的估算精度,为快速、大范围土壤有机质质量分数的遥感制图提供技术支持.  相似文献   
985.
中国大气气溶胶辐射特性参数的观测与研究进展   总被引:1,自引:1,他引:0  
长期系统的气溶胶辐射特性观测资料是定量研究气溶胶辐射和气候效应的重要基础.本文综合介绍中国大气气溶胶辐射特性观测与研究现状和成果,重点包括以下内容:地面太阳光度计联网观测气溶胶光学厚度、单次散射反照率、尺度谱;从全波段太阳辐射反演气溶胶光学厚度、单次散射反照率;浊度计和黑碳仪测量地面气溶胶散射系数和吸收系数;地基/星载激光雷达观测气溶胶(后向散射系数)垂直分布;极轨/静止卫星遥感反演气溶胶光学特性.  相似文献   
986.
Inundation patterns in two of the largest savanna floodplains of South America were studied by analysis of the 37‐GHz polarization difference observed by the Scanning Multichannel Microwave Radiometer (Nimbus‐7 satellite). Flooded area was estimated at monthly intervals for January 1979 through to August 1987 using mixing models that account for the major landscape units with distinctive microwave emission characteristics. Results are presented separately for five subregions in each of the two floodplain regions to show the spatial as well as temporal variability in inundation patterns. The total area inundated during the 9 years varied between 2069 and 78 460 km2 in the Llanos de Moxos (also spelled as Mojos; median area, 23 383 km2) and 1278 and 105 454 km2 in the Llanos del Orinoco (median, 25 374 km2), not including the open‐water area of permanent lakes and river channels. The correlation between flooded area and river stage was used to extend the inundation records over a 30‐year period in the Moxos (1967–97) and a 58‐year period (1927–85) in the Orinoco. Interannual variability in inundation is greater in the Moxos than the Orinoco. Comparison of these data, however, with a previously published analysis of the Pantanal wetland shows that inundation patterns in these two floodplain regions are not as variable across years as they are in the Pantanal. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   
987.
Observations from 560 weather stations in China show that sand–dust storms occur most frequently in April in north China. The region consists of Sub-dry Mid Temperate, Dry Mid Temperate, Sub-dry South Temperate and Dry South Temperate Zones and much of the land surface is desert or semi-desert: it is relatively dry with minimal rainfall and a high annual mean temperature. In most regions of China, the annual mean frequency of sand–dust events decreased sharply between 1980 and 1997 and then increased from 1997 to 2000. Statistical analyses demonstrate that the frequency of sand–dust storms correlates highly with wind speed, which in turn is strongly related to land surface features; on the other hand, a significant correlation between storm events and other atmospheric quantities such as precipitation and temperature was not observed. Accordingly, land surface cover characteristics (vegetation, snowfall and soil texture) may play a significant role in determining the occurrence of sand–dust storms in China. Analysis of Normalized Difference Vegetation Index derived from National Oceanic and Atmospheric Administration and Empirical Orthogonal Function show that since 1995 surface vegetation cover in large areas of Northern China has significantly deteriorated. Moreover, a high correlation is shown to exist among the annual occurrence of sand–dust storms, surface vegetation cover and snowfall. This suggests that the deterioration of surface vegetation cover may strongly influence the occurrence of sand–dust storms in China. Soils with coarse and medium textures are found to be more associated with sand–dust storms than other soils.  相似文献   
988.
Land use has a large impact on ecosystem functioning, though evidences of these impacts at the regional scale are scarce. The objective of this paper was to analyze the impacts of agricultural land use on ecosystem functioning (radiation interception and carbon uptake) in temperate areas of North and South America. From land cover maps generated using high-resolution satellite images we selected sites dominated by row crops (RC), small grain crops (SG), pastures (PA), and rangelands (RA) in the Central Plains of USA and the Pampas of Argentina. These two regions share climatic characteristics and the agricultural conditions (crop types) are also very similar. Both areas were originally dominated by temperate grasslands. In these sites we extracted the temporal series of the normalized difference vegetation index (NDVI) from the NOAA satellites for the period 1989–1998 and calculated the mean seasonal NDVI curve for each site. Additionally, we calculated the mean annual NDVI, the maximum NDVI, the date of the year when the max NDVI was recorded and the interannual variability of these three attributes. We compared the mean values of each NDVI-derived attribute between land cover types and between continents. The NDVI seasonal patterns for each land cover type were roughly similar between the Central Plains and the Pampas during the growing season. The largest differences were observed during the winter and spring, when the NDVI of all land cover types in the Central Plains remained at lower values than in the Pampas. This was probably caused by the high annual thermal amplitude in the Central Plains that results in a much more restricted growing season. As a result of these differences in the shape of the NDVI curve, the mean annual NDVI in the Central Plains was lower than in the Pampas for all land cover types but the maximum NDVI did not differ importantly. In both regions, row crops delayed the date of the NDVI peak, small grain crops advanced it and pastures did not change it importantly, compared with rangelands. The interannual variability of the NDVI attributes was higher for small grains than for row crops in both regions. However, small grains crops were consistently more variable between years in the Central Plains than in the Pampas. The opposite occurred with pastures and rangelands, which were more variable in the Pampas than in the Central Plains. This paper confirms and generalizes previous findings that showed important imprints of land use on ecosystem functioning in temperate ecosystems. Our results support the idea that the changes in land cover that have occurred in the Central Plains and the Pampas leaded to similar changes in the way that ecosystems absorb solar radiation and in the patterns of carbon uptake.  相似文献   
989.
1 IN T R O D U C T IO NZhaoyuan C ity, located in northeast of ShandongProvince,iscalled "G old C ity"and itisone ofim portantgold production bases in C hina.The exploitation ofthegold broughtoutthe extensive vacantareas,leading tolarge-scale ground subsi…  相似文献   
990.
遥感图像是地表地物表征的集合,构成影像景观单元。遥感解译属于人脑意识活动的范畴,是利用表层信息由表及里、由此及彼的逻辑推理过程。探求因果联系的科学归纳法,探求等同关系的类比推理法,探求真实性的科学论证法是遥感解译过程中应遵循的思想方法。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号